Plasma diagnostic tools for ECR ion sources : What can we learn from these experiments for the next generation sources

International audience; The order-of-magnitude performance leaps of ECR ion sources over the past decades result from improvements to the magnetic plasma confinement, increases in the microwave heating frequency, and techniques to stabilize the plasma at high densities. Parallel to the technical development of the ion sources themselves, significant effort has been directed into the development of their plasma diagnostic tools. We review the recent results of Electron Cyclotron Resonance Ion Source (ECRIS) plasma diagnostics highlighting a number of selected examples of plasma density, electron energy distribution, and ion confinement time measurements, obtained mostly with the second-gener…

Transverse distribution of beam current oscillations of a 14 GHz electron cyclotron resonance ion source

The temporal stability of oxygen ion beams has been studied with the 14 GHz A-ECR at JYFL (University of Jyvaskyla, Department of Physics). A sector Faraday cup was employed to measure the distribution of the beam current oscillations across the beam profile. The spatial and temporal characteristics of two different oscillation “modes” often observed with the JYFL 14 GHz ECRIS are discussed. It was observed that the low frequency oscillations below 200 Hz are distributed almost uniformly. In the high frequency oscillation “mode,” with frequencies >300 Hz at the core of the beam, carrying most of the current, oscillates with smaller amplitude than the peripheral parts of the beam. The result…

Inner shell ionization of argon in ECRIS plasma

Abstract The volumetric K α emission rate of argon emitted from the electron cyclotron resonance (ECR) heated plasmas of the JYFL (University of Jyvaskyla, Department of Physics) 14 GHz ECR ion source (ECRIS) and the 14.5 GHz Grenoble Test Source (GTS) at iThemba Laboratory for Accelerator Based Sciences have been measured to gain an understanding of the influence of the ion source tune parameters on the absolute inner shell ionization rate. It was observed that the behaviour of the ionization rate and the extracted ion beam currents react differently, depending the parametric sweep performed. The neutral gas pressure and incident microwave power was found to have the strongest influence on…

Photoelectron Emission from Metal Surfaces Induced by Radiation Emitted by a 14 GHz Electron Cyclotron Resonance Ion Source

Photoelectron emission measurements have been performed using a room-temperature 14 GHz ECR ion source. It is shown that the photoelectron emission from Al, Cu, and stainless steel (SAE 304) surfaces, which are common plasma chamber materials, is predominantly caused by radiation emitted from plasma with energies between 8 eV and 1 keV. Characteristic X-ray emission and bremsstrahlung from plasma have a negligible contribution to the photoelectron emission. It is estimated from the measured data that the maximum conceivable photoelectron flux from plasma chamber walls is on the order of 10% of the estimated total electron losses from the plasma. peerReviewed

Limitations of electron cyclotron resonance ion source performances set by kinetic plasma instabilities.

Electron cyclotron resonance ion source (ECRIS) plasmas are prone to kinetic instabilities due to anisotropy of the electron energy distribution function stemming from the resonant nature of the electron heating process. Electron cyclotron plasma instabilities are related to non-linear interaction between plasma waves and energetic electrons resulting to strong microwave emission and a burst of energetic electrons escaping the plasma, and explain the periodic oscillations of the extracted beam currents observed in several laboratories. It is demonstrated with a minimum-B 14 GHz ECRIS operating on helium, oxygen, and argon plasmas that kinetic instabilities restrict the parameter space avail…

The relationship between visible light emission and species fraction of the hydrogen ion beams extracted from 2.45 GHz microwave discharge.

The relationship between Balmer-α and Fulcher-band emissions with extracted H + , H + 2 , and H + 3 ions is demonstrated for a 2.45 GHz microwave discharge. Ion mass spectra and optical measurements of Balmer-α and Fulcher-band emissions have been obtained with a Wien Filter having an optical viewport on the plasma chamber axis. The beam of approximately 1 mA is analyzed for different plasma conditions simultaneously with the measurement of light emissions both with temporal resolution. The use of visible light emissions as a valuable diagnostic tool for monitoring the species fraction of the extracted beams is proposed. peerReviewed

Integrated Modeling of the Beam Formation and Extraction in the Linac4 Hydrogen Negative Ion Source

In order to make the predictive simulation tools for the extracted beam current and emittance in the Linac4 H− ion source, we have launched the development of the integrated model for the H− ion beam formation and extraction process. More specifically, our 3D KEIO-Beam Formation and eXtraction (3D KEIO-BFX) is coupled with the NINJA, IBSIMU and TRAVEL. The extracted H− ion beam current and co-extracted electron current obtained by the integrated model have shown reasonable agreement with the experiments. peerReviewed

Central Region Upgrade for the Jyväskylä K130 Cyclotron

The Jyväskylä K130 cyclotron has been in operation for more than 25 years providing beams from H to Au with energies ranging from 1 to 80 MeV/u for nuclear physics research and applications. At the typical energies around 5 MeV/u used for the nuclear physics program the injection voltage used is about 10 kV. The low voltage limits the beam intensity especially from the 18 GHz ECRIS HIISI. To increase the beam intensities the central region of the K130 cyclotron is being upgraded by increasing the injection voltage by a factor of 2. The new central region with spiral inflectors for harmonics 1-3 has been designed. The new central region shows better transmission in simulations than the origi…

Experimental evidence on microwave induced electron losses from ECRIS plasma

The balance between warm and hot (>1 keV) electron density and their losses from the magnetic confinement system of an Electron Cyclotron Resonance Ion Source (ECRIS) plasma is considered to be one of the main factors determining the rate of the high charge state ion production. One of the key loss channels for heated electrons is thought to be induced by the injected microwaves. While this loss mechanism, referred to as rf-induced pitch angle scattering, has been studied theoretically and with computational tools, direct experimental evidence of its significance in minimum-B ECRIS plasmas remains limited. In this work, experimental evidence of microwave induced electron losses in the axial…

Ion source and low energy beam transport prototyping for a single-ended heavy ion ToF-ERDA facility

We present the status of the ion source and low energy beam transport prototyping activities for a heavy ion time-of-flight elastic recoil detection analysis (ToF-ERDA) equipment, designed to accelerate a flux of 1–10 particle nano-Ampere of 40Ar6-12+ ions to 3–6 MeV energy for depth profiling of light elements. The prototype injector consists of a novel permanent magnet electron cyclotron resonance ion source CUBE-ECRIS with a minimum-B quadrupole field topology, and a 90° permanent magnet dipole with adjustable field strength for charge state selection. We report experimentally measured argon beam currents as a function of the applied microwave power and ion source potential to demonstrat…

Periodic Beam Current Oscillations Driven by Electron Cyclotron Instabilities in ECRIS Plasmas

Experimental observation of cyclotron instabilities in electron cyclotron resonance ion source plasma operated in cwmode is reported. The instabilities are associated with strong microwave emission and a burst of energetic electrons escaping the plasma, and explain the periodic oscillations of the extracted beam currents. The instabilities are shown to restrict the parameter space available for the optimization of high charge state ion currents. nonPeerReviewed

Double einzel lens extraction for the JYFL 14 GHz ECR ion source designed with IBSimu

In order to improve the performance of the JYFL 14 GHz electron cyclotron resonance ion source (ECRIS) and initiate low energy beam transport (LEBT) upgrade at the University of Jyvaskyla, Department of Physics (JYFL) accelerator laboratory, a new ion beam extraction system has been designed and installed. The development of the new extraction was performed with the ion optical code IBSimu, making it the first ECRIS extraction designed with the code. The measured performance of the new extraction is in good agreement with the simulations. Compared to the old extraction the new system provides improved beam quality, i.e. lower transverse emittance values and improved structure of beam profil…

Studying the double-frequency heating mode in ECRIS plasma using Kα diagnostics

Despite the success of double-heating frequency in enhancing high charge state production, the underlying physics remains poorly understood. By combining three different diagnostic techniques i.e. Kα emission, optical emission and the extracted charge state distribution, it is now possible to assess the proposed explanations for the effectiveness of double-frequency heating against the experimental results. These results seem to indicate that the increase of plasma density accounts largely for the favorable behavior of this operation mode compared to single-frequency mode. peerReviewed

Estimating ion confinement times from beam current transients in conventional and charge breeder ECRIS

International audience; Cumulative ion confinement times are probed by measuring decaying ion current transients in pulsed material injection mode. The method is applied in a charge breeder and conventional ECRIS yielding mutually corroborative results. The cumulative confinement time estimates vary from approximately 2 ms–60 ms with a clear dependence on the ion charge-to-mass ratio—higher charges having longer residence times. The long cumulative confinement times are proposed as a partial explanation to recently observed unexpectedly high ion temperatures. The results are relevant for rare ion beam (RIB) production as the confinement time and the lifetime of stable isotopes can be used f…

Photo-enhanced O−, H− and Br− ion production in caesium sputter negative ion source : no evidence for resonant ion pair production

It has been proposed that the negative ion yield of a caesium sputter ion source could be enhanced by promoting neutral caesium atoms to electronically excited 7p states supporting resonant ion pair production. We have tested this hypothesis by illuminating the cathode of a caesium sputter ion source with an adjustable wavelength laser and measuring its effect on the extracted beam currents of O−, H− and Br− anions. The laser exposure causes the beam currents to increase but the effect is independent of the wavelength in the range of 440-460 nm, which leads us to conclude that there is no evidence for resonant ion pair production. The photon-induced beam current enhancement scales with the …

Plasma instability in the afterglow of electron cyclotron resonance discharge sustained in a mirror trap

The work presented in this article is devoted to time-resolved diagnostics of non-linear effects observed during the afterglow plasma decay of a 14 GHz electron cyclotron resonance ion source operated in pulsed mode. Plasma instabilities that cause perturbations of the extracted ion current during the decay were observed and studied. It is shown that these perturbations are associated with precipitation of high energy electrons along the magnetic field lines and strong bursts of bremsstrahlung emission. The effect of ion source settings on the onset of the observed instabilities was investigated. Based on the experimental data and estimated plasma properties, it is assumed that the instabil…

The effect of gas mixing and biased disc voltage on the preglow transient of electron cyclotron resonance ion source

The effect of gas mixing and biased disc voltage on the preglow of electron cyclotron resonance ion source plasma has been studied with the AECR-U type 14 GHz ion source. It was found that gas mixing has a significant effect on the preglow. The extracted transient beam currents and efficiency of the heavier species increase, while the currents and efficiency of the lighter species decrease when gas mixing is applied. The effect of the biased disc was found to be pronounced in continuous operation mode in comparison to preglow. The data provide information on the time scales of the plasma processes explaining the effects of gas mixing and biased disc. The results also have implications on pr…

Time Evolution of High-Energy Bremsstrahlung and Argon Ion Production in Electron Cyclotron Resonance Ion-Source Plasma

Bremsstrahlung radiation measurement is one of the most commonly used plasma-diagnostics methods. Most of the bremsstrahlung measurements with electron cyclotron resonance ion sources (ECRISs) have been performed in continuous-operation mode yielding information only on the steady-state bremsstrahlung emission. This paper describes results of bremsstrahlung and argon ion-current measurements with the JYFL 14-GHz ECRIS operated in a pulsed mode. The bremsstrahlung radiation was studied as a function of neutral-gas pressure and radio frequency power. The timescale of ECRIS bremsstrahlung production is compared to ion-production timescale for different charge states of argon for the first time…

Ion beam intensity and phase space measurement techniques for ion sources.

Ion sources produce beams used in accelerators and other applications. Both development and use of ion sources need beam diagnostics to probe the plasma processes and beam formation for optimization purposes and to produce beam parameters needed for transport tuning. These diagnostics include beam intensity measurements usually carried out with Faraday cups or inductive pickups, magnetic separation, profile measurements with scintillation screens and wires, and phase space measurements with different types of emittance scanners. peerReviewed

Gyrotron-driven high current ECR ion source for boron-neutron capture therapy neutron generator

Abstract Boron-neutron capture therapy (BNCT) is a perspective treatment method for radiation resistant tumors. Unfortunately its development is strongly held back by a several physical and medical problems. Neutron sources for BNCT currently are limited to nuclear reactors and accelerators. For wide spread of BNCT investigations more compact and cheap neutron source would be much more preferable. In present paper an approach for compact D–D neutron generator creation based on a high current ECR ion source is suggested. Results on dense proton beams production are presented. A possibility of ion beams formation with current density up to 600 mA/cm 2 is demonstrated. Estimations based on obt…

Photoelectron Emission from Metal Surfaces Induced by VUV-emission of Filament Driven Hydrogen Arc Discharge Plasma

Photoelectron emission measurements have been performed using a filament-driven multi-cusp arc discharge volume production H^- ion source (LIISA). It has been found that photoelectron currents obtained with Al, Cu, Mo, Ta and stainless steel (SAE 304) are on the same order of magnitude. The photoelectron currents depend linearly on the discharge power. It is shown experimentally that photoelectron emission is significant only in the short wavelength range of hydrogen spectrum due to the energy dependence of the quantum efficiency. It is estimated from the measured data that the maximum photoelectron flux from plasma chamber walls is on the order of 1 A per kW of discharge power.

High current proton beams production at Simple Mirror Ion Source 37

This paper presents the latest results of high current proton beam production at Simple Mirror Ion Source (SMIS) 37 facility at the Institute of Applied Physics (IAP RAS). In this experimental setup, the plasma is created and the electrons are heated by 37.5 GHz gyrotron radiation with power up to 100 kW in a simple mirror trap fulfilling the ECR condition. Latest experiments at SMIS 37 were performed using a single-aperture two-electrode extraction system. Proton beams with currents up to 450 mA at high voltages below 45 kV were obtained. The maximum beam current density was measured to be 600 mA/cm2. A possibility of further improvement through the development of an advanced extraction sy…

An Experimental Study of Waveguide Coupled Microwave Heating with Conventional Multicusp Negative Ion Source

Negative ion production with conventional multicusp plasma chambers utilizing 2.45 GHz microwave heating is demonstrated. The experimental results were obtained with the multicusp plasma chambers and extraction systems of the RFdriven RADIS ion source and the filament driven arc discharge ion source LIISA. A waveguide microwave coupling system, which is almost similar to the one used with the SILHI ion source, was used. The results demonstrate that at least one third of negative ion beam obtained with inductive RF-coupling (RADIS) or arc discharge (LIISA) can be achieved with 1 kW of 2.45 GHz microwave power in CW mode without any modification of the plasma chamber. The co-extracted electro…

The effect of magnetic field strength on the time evolution of high energy bremsstrahlung radiation created by an electron cyclotron resonance ion source

Abstract An electron cyclotron resonance (ECR) ion source is one of the most used ion source types for high charge state heavy ion production. In ECR plasma the electrons are heated by radio frequency microwaves in order to provide ionization of neutral gases. As a consequence, ECR heating also generates very high electron energies (up to MeV region) which can produce a vast amount of bremsstrahlung radiation causing problems with radiation shielding and heating superconducting cryostat of an ECR ion source. To gain information about the time evolution of the electron energies in ECR plasma radial bremsstrahlung measurements were performed. JYFL 14 GHz ECR ion source was operated in pulsed …

Status of new 18 GHz ECRIS HIISI

A new 18 GHz ECR ion source HIISI is under commissioning at the Accelerator Laboratory at the University of Jyvaskyla (JYFL). The main purpose of HIISI is to produce high-energy beam cocktails, e.g. Xe44+, for radiation effects testing of electronics with the K130 cyclotron. The initial commissioning results in 18+14 GHz operation with oxygen, argon and xenon are reported. The beam currents are compared to those produced by reference ion sources (JYFL 14 GHz ECRIS, GTS and SuSI). At the moment (October 2017) 560 µA of O6+ and 310 µA of Ar13+, for example, have been reached with HIISI at 2.3 kW total power.A new 18 GHz ECR ion source HIISI is under commissioning at the Accelerator Laboratory…

Hydrogen plasma induced photoelectron emission from low work function cesium covered metal surfaces

Experimental results of hydrogen plasma induced photoelectron emission from cesium covered metal surfaces under ion source relevant conditions are reported. The transient photoelectron current during the Cs deposition process is measured from Mo, Al, Cu, Ta, Y, Ni, and stainless steel (SAE 304) surfaces. The photoelectron emission is 2–3.5 times higher at optimal Cs layer thickness in comparison to the clean substrate material. Emission from the thick layer of Cs is found to be 60%–80% lower than the emission from clean substrates. peerReviewed

Hybrid simulation of electron cyclotron resonance heating

Electron Cyclotron Resonance (ECR) heating is a fundamentally important aspect in understanding the physics of Electron Cyclotron Resonance Ion Sources (ECRIS). Absorption of the radio frequency (RF) microwave power by electron heating in the resonance zone depends on many parameters including frequency and electric field strength of the microwave, magnetic field structure and electron and ion density profiles. ECR absorption has been studied in the past by e.g. modelling electric field behaviour in the resonance zone and its near proximity. This paper introduces a new ECR heating code that implements damping of the microwave power in the vicinity of the resonance zone, utilizes electron de…

Correlations between density distributions, optical spectra, and ion species in a hydrogen plasma (invited)

An experimental study of plasma distributions in a 2.45 GHz hydrogen discharge operated at 100 Hz repetition rate is presented. Ultrafast photography, time integrated visible light emission spectra, time resolved Balmer-alpha emission, time resolved Fulcher Band emission, ion species mass spectra, and time resolved ion species fraction measurements have been implemented as diagnostic tools in a broad range of plasma conditions. Results of plasma distributions and optical emissions correlated with H + , H + 2 , and H + 3 ion currents by using a Wien filter system with optical observation capability are reported. The magnetic field distribution and strength is found as the most critical facto…

The effect of microwave power on the Ar9+ and Ar13+ optical emission intensities and ion beam currents in ECRIS

The production of Ar9+ and Ar13+ ions in an ECRIS plasma and the efficiency of the ion beam extraction and transport of the resulting Ar9+ and Ar13+ ion beams have been studied with the JYFL 14 GHz ECRIS by using optical emission spectroscopy and measurement of the m/q analyzed beam currents. The relative changes in both the optical emission and the ion beam current in CW mode as function of microwave power and in amplitude modulation (AM) operation mode are reported. The results indicate a discrepancy between the parametric dependence of high charge state ion densities in the core plasma and their extracted beam currents. The observation implies that in CW mode the ion currents could be li…

Photoelectron emission induced by low temperature hydrogen plasmas

Experimental results of low temperature hydrogen plasma induced photoelectron emission measurements comparing two different plasma heating methods are summarized. By exposing the samples to the vacuum ultraviolet radiation of a filament-driven multi-cusp arc discharge ion source and a 2.45 GHz microwave-driven ion source, it has been measured that the total photoelectron emission from various metal surfaces is on the order of 1 A per kW of plasma heating power, which can be increased by a factor of 2–3.5 with a thin layer of alkali metal. The possible effects of the photoelectrons on the plasma sheath structure are studied with a 1D collisionless model extended to include the contribution o…

Nano-graphite cold cathodes for electric solar wind sail

The nanographite (NG) films consisting of tiny graphite crystallites (nanowalls) are produced by carbon condensation from methane–hydrogen gas mixture activated by a direct current discharge. High aspect ratio and structural features of the NG crystallites provides efficient field electron emission (FE). Applicability and performance of the NG films in an electron gun (E-gun) of a solar wind thruster system with an electric sail (E-sail) is tested. The long-term tests are demonstrated suitability of E-gun assembly with the NG cathodes for the real space missions. The results of the tests are analyzed and physical mechanisms of the cathode aging and practical methods for improvement performa…

Cyclotron instability in the afterglow mode of minimum-B ECRIS.

It was shown recently that cyclotron instability in non-equilibrium plasma of a minimum-B electron cyclotron resonance ion source (ECRIS) causes perturbation of the extracted ion current and generation of strong bursts of bremsstrahlung emission, which limit the performance of the ion source. The present work is devoted to the dynamic regimes of plasma instability in ECRIS operated in pulsed mode. Instability develops in decaying plasma shortly after heating microwaves are switched off and manifests itself in the form of powerful pulses of electromagnetic emission associated with precipitation of high energy electrons. Time-resolved measurements of microwave emission bursts are presented. I…

The effect of cavity tuning on oxygen beam currents of an A-ECR type 14 GHz electron cyclotron resonance ion source.

The efficiency of the microwave-plasma coupling plays a significant role in the production of highly charged ion beams with electron cyclotron resonance ion sources (ECRISs). The coupling properties are affected by the mechanical design of the ion source plasma chamber and microwave launching system, as well as damping of the microwave electric field by the plasma. Several experiments attempting to optimize the microwave-plasma coupling characteristics by fine-tuning the frequency of the injected microwaves have been conducted with varying degrees of success. The inherent difficulty in interpretation of the frequency tuning results is that the effects of microwave coupling system and the ca…

The biased disc of an electron cyclotron resonance ion source as a probe of instability-induced electron and ion losses

International audience; Electron Cyclotron Resonance Ion Source (ECRIS) plasmas are prone to kinetic instabilities resulting in loss of electron and ion confinement. It is demonstrated that the biased disk of an ECRIS can be used as a probe to quantify such instability-induced electron and ion losses occurring in less than 10 µs. The qualitative interpretation of the data is supported by the measurement of the energy spread of the extracted ion beams implying a transient plasma potential >1.5 kV during the instability. A parametric study of the electron losses combined with electron tracking simulations allows for estimating the fraction of electrons expelled in each instability event to be…

Plasma response to amplitude modulation of the microwave power on a 14 GHz electron cyclotron resonance ion source

This paper reports the effects of sinusoidal microwave power Amplitude Modulation (AM) on the performance of Electron Cyclotron Resonance (ECR) ion sources. The study was conducted on the 14 GHz ECR ion source ECR2 at the University of Jyväskylä. The klystron output was intentionally altered by a variable frequency sinusoidal amplitude modulation. The average microwave power 350 W was modulated between 530 W and 180 W from 0.011-25 kHz. The integrated x-ray energy, the mass analyzed beam current and the forward and reflected microwave power were measured. The energy integrated x-ray signal responded strongly with low frequency modulation and was no longer observable at approximately 2.2 kHz…

H− extraction systems for CERN’s Linac4 H− ion source

Abstract Linac4 is a 160 MeV linear H −  accelerator at CERN. It is an essential part of the beam luminosity upgrade of the Large Hadron Collider (LHC) and will be the primary injector into the chain of circular accelerators. It aims at increasing the beam brightness by a factor of 2, when compared to the currently used 50 MeV linear proton accelerator, Linac2. Linac4’s ion source is a cesiated RF-plasma H −  ion source. Several beam extraction systems were designed for H −  beams of 45 keV energy, 50 mA intensity and an electron to H −  ratio smaller than 5. The goal was to extract a beam with an rms-emittance of 0 . 25 π  mm mrad. One of the main challenges in designing an H −  extraction…

A study of VUV emission and the extracted electron-ion ratio in hydrogen and deuterium plasmas of a filament-driven H−/D− ion source

Vacuum ultraviolet (VUV) emission diagnostics for studying differences of electron impact processes in hydrogen and deuterium plasmas are presented. The method is applied to study a filament driven multicusp arc discharge negative ion source by comparing the VUV-emission intensities of different emission bands and extracted currents of H−/D− ions and electrons. It was found that the ratio of coextracted electrons to extracted ions is four times higher for deuterium than for hydrogen. No significant differences of the VUV-spectra or volumetric rates of ionization, excitation, production of high vibrational states, and dissociation were found between the plasmas of the two isotopes. The volum…

Dynamic regimes of cyclotron instability in the afterglow mode of minimum-B electron cyclotron resonance ion source plasma

The paper is concerned with the dynamic regimes of cyclotron instabilities in non-equilibrium plasma of a minimum-B electron cyclotron resonance ion source operated in pulsed mode. The instability appears in decaying ion source plasma shortly (1–10 ms) after switching off the microwave radiation of the klystron, and manifests itself in the form of powerful pulses of electromagnetic emission associated with precipitation of high-energy electrons along the magnetic field lines. Recently it was shown that this plasma instability causes perturbations of the extracted ion current, which limits the performance of the ion source and generates strong bursts of bremsstrahlung emission. In this artic…

Deviation of H− beam extraction simulation model

Negative hydrogen ion source extraction system development is dependent on accurate and fast simulation methods for modelling the behaviour of ion and electron beams. Traditionally this type of work has been done using ray-tracing extraction codes, such as IBSimu. The plasma extraction model in IBSimu has been observed to under-estimate the charge density near the plasma sheath, leading to incorrect prediction of the current at which the system produces the optimum emittance. It is suspected that this deviation results from the approximations made by the model, neglecting the magnetic field and collisional effects near the sheath region. Results and comparisons to simulations are presented …

Beam current oscillations driven by cyclotron instabilities in a minimum-Belectron cyclotron resonance ion source plasma

Experimental observation of cyclotron instabilities in a minimum-B confined electron cyclotron resonance ion source plasma is reported. The instabilities are associated with strong microwave emission and a burst of energetic electrons escaping the plasma, and explain the periodic ms-scale oscillation of the extracted beam currents. Such non-linear effects are detrimental for the confinement of highly charged ions due to plasma perturbations at shorter periodic intervals in comparison with their production time. It is shown that the repetition rate of the periodic instabilities in oxygen plasmas increases with increasing magnetic field strength and microwave power and decreases with increasi…

Measurement of the energy distribution of electrons escaping minimum-B ECR plasmas

The measurement of the electron energy distribution (EED) of electrons escaping axially from a minimum-B electron cyclotron resonance ion source (ECRIS) is reported. The experimental data were recorded with a room-temperature 14 GHz ECRIS at the JYFL accelerator laboratory. The electrons escaping through the extraction mirror of the ion source were detected with a secondary electron amplifier placed downstream from a dipole magnet serving as an electron spectrometer with 500 eV resolution. It was discovered that the EED in the range of 5–250 keV is strongly non-Maxwellian and exhibits several local maxima below 20 keV energy. It was observed that the most influential ion source operating pa…

Limitation of the ECRIS performance by kinetic plasma instabilities (invited).

Electron cyclotron resonance ion source (ECRIS) plasmas are prone to kinetic instabilities due to anisotropic electron velocity distribution. The instabilities are associated with strong microwave emission and periodic bursts of energetic electrons escaping the magnetic confinement. The instabilities explain the periodic ms-scale oscillation of the extracted beam current observed with several high performance ECRISs and restrict the parameter space available for the optimization of extracted beam currents of highly charged ions. Experiments with the JYFL 14 GHz ECRIS have demonstrated that due to the instabilities the optimum Bmin-field is less than 0.8BECR, which is the value suggested by …

Spectroscopic study of ion temperature in minimum-B ECRIS plasma

Experimentally determined ion temperatures of different charge states and elements in minimum-B confined electron cyclotron resonance ion source (ECRIS) plasma are reported. It is demonstrated with optical emission spectroscopy, complemented by the energy spread measurements of the extracted ion beams, that the ion temperature in the JYFL 14 GHz ECRIS is 5–28 eV depending on the plasma species and charge state. The reported ion temperatures are an order of magnitude higher than previously deduced from indirect diagnostics and used in simulations, but agree with those reported for a quadrupole mirror fusion experiment. The diagnostics setup and data interpretation are discussed in detail to …

Microwave emission related to cyclotron instabilities in a minimum-Belectron cyclotron resonance ion source plasma

Electron cyclotron resonance ion sources (ECRIS) have been essential in the research and applications of nuclear physics over the past 40 years. They are extensively used in a wide range of large-scale accelerator facilities for the production of highly charged heavy ion beams of stable and radioactive elements. ECRISs are susceptible to kinetic instabilities due to resonance heating mechanism leading to anisotropic electron velocity distribution function. Instabilities of cyclotron type are a proven cause of frequently observed periodic bursts of 'hot' electrons and bremsstrahlung, accompanied with emission of microwave radiation and followed by considerable drop of multiply charged ions c…

Effect of space charge on the negative oxygen flux during reactive sputtering

Negative ions often play a distinctive role in the phase formation during reactive sputter deposition. The path of these high energetic ions is often assumed to be straight. In this paper, it is shown that in the context of reactive magnetron sputtering space charge effects are decisive for the energetic negative ion trajectories. To investigate the effect of space charge spreading, reactive magnetron sputter experiments were performed in compound mode with target materials that are expected to have a high secondary ion emission yield (MgO and CeO2). By the combination of energy flux measurements, and simulations, a quantitative value for the negative oxygen ion yield can be derived.

Status of new 18 GHz ECRIS HIISI

A new 18 GHz ECR ion source HIISI is under commissioning at the Accelerator Laboratory at the University of Jyväskylä (JYFL). The main purpose of HIISI is to produce high-energy beam cocktails, e.g. Xe44+, for radiation effects testing of electronics with the K130 cyclotron. The initial commissioning results in 18+14 GHz operation with oxygen, argon and xenon are reported. The beam currents are compared to those produced by reference ion sources (JYFL 14 GHz ECRIS, GTS and SuSI). At the moment (October 2017) 560 µA of O6+ and 310 µA of Ar13+, for example, have been reached with HIISI at 2.3 kW total power. peerReviewed

Design of a 10 GHz minimum-B quadrupole permanent magnet electron cyclotron resonance ion source

This paper presents a simulation study of a permanent magnet electron cyclotron resonance ion source (ECRIS) with a minimum-B quadrupole magnetic field topology. The magnetic field is made to conform to conventional ECRIS with $B_\textrm{min}/B_\textrm{ECR}$ of 0.67 and a last closed magnetic isosurface of 1.86$B_\textrm{ECR}$ at 10 GHz. The distribution of magnetic field gradients parallel to the field, affecting the electron heating efficiency, cover a range from 0 to 13 T/m, being similar to conventional ECRIS. Therefore it is expected that the novel ion source produces warm electrons and high charge state ions in significant number. Single electron tracking simulations are used to estim…

Application and development of ion-source technology for radiation-effects testing of electronics

Abstract Studies of heavy-ion induced single event effect (SEE) on space electronics are necessary to verify the operation of the components in the harsh radiation environment. These studies are conducted by using high-energy heavy-ion beams to simulate the radiation effects in space. The ion beams are accelerated as so-called ion cocktails, containing several ion beam species with similar mass-to-charge ratio, covering a wide range of linear energy transfer (LET) values also present in space. The use of cocktails enables fast switching between beam species during testing. Production of these high-energy ion cocktails poses challenging requirements to the ion sources because in most laborat…

Kinetic instabilities in pulsed operation mode of a 14 GHz electron cyclotron resonance ion source

The occurrence of kinetic plasma instabilities is studied in pulsed operation mode of a 14 GHz Aelectron cyclotron resonance type electron cyclotron resonance ion source. It is shown that the temporal delay between the plasma breakdown and the appearance of the instabilities is on the order of 10- 100 ms. The most important parameters affecting the delay are magnetic field strength and neutral gas pressure. It is demonstrated that kinetic instabilities limit the high charge state ion beam production in the unstable operating regime. peerReviewed

Power efficiency improvements with the radio frequency H− ion source

CW 13.56 MHz radio frequency-driven H− ion source is under development at the University of Jyväskylä for replacing an existing filament-driven ion source at the MCC30/15 cyclotron. Previously, production of 1 mA H− beam, which is the target intensity of the ion source, has been reported at 3 kW of RF power. The original ion source front plate with an adjustable electromagnet based filter field has been replaced with a new front plate with permanent magnet filter field. The new structure is more open and enables a higher flux of ro-vibrationally excited molecules towards the plasma electrode and provides a better control of the potential near the extraction due to a stronger separation of t…

Ion source research and development at University of Jyväskylä: Studies of different plasma processes and towards the higher beam intensities

MonPS16; International audience; The long-term operation of high charge state electron cyclotron resonance ion sources fed withhigh microwave power has caused damage to the plasma chamber wall in several laboratories.Porosity, or a small hole, can be progressively created in the wall on a year time scale, which cancause a water leak from the cooling system into the plasma chamber vacuum. A burnout of theVENUS chamber is investigated. Information on the hole formation and on the necessary localhot electron power density is presented. Next, the hot electron flux to the wall is studied bymeans of simulations. First, the results of a simple model assuming that electrons are fullymagnetized and …

Power efficiency improvements with the radio frequency H− ion source

CW 13.56 MHz radio frequency-driven H(-) ion source is under development at the University of Jyväskylä for replacing an existing filament-driven ion source at the MCC30/15 cyclotron. Previously, production of 1 mA H(-) beam, which is the target intensity of the ion source, has been reported at 3 kW of RF power. The original ion source front plate with an adjustable electromagnet based filter field has been replaced with a new front plate with permanent magnet filter field. The new structure is more open and enables a higher flux of ro-vibrationally excited molecules towards the plasma electrode and provides a better control of the potential near the extraction due to a stronger separation …

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Neutron generator for BNCT based on high current ECR ion source with gyrotron plasma heating

BNCT development nowadays is constrained by a progress in neutron sources design. Creation of a cheap and compact intense neutron source would significantly simplify trial treatments avoiding use of expensive and complicated nuclear reactors and accelerators. D-D or D-T neutron generator is one of alternative types of such sources for. A so-called high current quasi-gasdynamic ECR ion source with plasma heating by millimeter wave gyrotron radiation is suggested to be used in a scheme of D-D neutron generator in the present work. Ion source of that type was developed in the Institute of Applied Physics of Russian Academy of Sciences (Nizhny Novgorod, Russia). It can produce deuteron ion beam…

High Current Proton and Deuteron Beams for Accelerators and Neutron Generators

This paper presents the latest results of high current proton and deuteron beam production at SMIS 37 facility at the Institute of Applied Physics (IAP RAS). In this experimental setup the plasma is created and the electrons are heated by 37.5 GHz gyrotron radiation with power up to 100 kW in a simple mirror trap fulfilling the ECR condition. High microwave power and frequency allow sustaining higher density hydrogen plasma (ne up to 2·1013 cm-3) in comparison to conventional ECRIS’s or microwave sources. The low ion temperature, on the order of a few eV, is beneficial to produce proton beams with low emittance. Latest experiments at SMIS 37 were performed using a single-aperture two-electr…

New progress of high current gasdynamic ion source (invited).

The experimental and theoretical research carried out at the Institute of Applied Physics resulted in development of a new type of electron cyclotron resonance ion sources (ECRISs)—the gasdynamic ECRIS. The gasdynamic ECRIS features a confinement mechanism in a magnetic trap that is different from Geller’s ECRIS confinement, i.e., the quasi-gasdynamic one similar to that in fusion mirror traps. Experimental studies of gasdynamic ECRIS were performed at Simple Mirror Ion Source (SMIS) 37 facility. The plasma was created by 37.5 and 75 GHz gyrotron radiation with power up to 100 kW. High frequency microwaves allowed to create and sustain plasma with significant density (up to 8 × 1013 cm−3 ) …

H$^{-}$ extraction systems for CERN’s Linac4 H$^{-}$ ion source

Linac4 is a 160 MeV linear H accelerator at CERN. It is an essential part of the beam luminosity upgrade of the Large Hadron Collider (LHC) and will be the primary injector into the chain of circular accelerators. It aims at increasing the beam brightness by a factor of 2, when compared to the currently used 50 MeV linear proton accelerator, Linac2. Linac4’s ion source is a cesiated RF-plasma H ion source. Several beam extraction systems were designed for H beams of 45 keV energy, 50 mA intensity and an electron to H ratio smaller than 5. The goal was to extract a beam with an rms-emittance of mm mrad. One of the main challenges in designing an H extraction system is dumping of the co-extra…

Method for estimating charge breeder ECR ion source plasma parameters with short pulse 1+ injection of metal ions

Abstract A new method for determining plasma parameters from beam current transients resulting from short pulse 1+ injection of metal ions into a charge breeder electron cyclotron resonance ion source has been developed. The proposed method relies on few assumptions, and yields local values for the ionisation times 1 / n e σ v q → q + 1 inz , charge exchange times 1 / n 0 σ v q → q − 1 cx , the ion confinement times τ q , as well as estimates for the minimum plasma energy contents n e E e and the plasma triple products n e E e τ q . The method is based on fitting the current balance equation on the extracted beam currents of high charge state ions, and using the fitting coefficients to dete…

Dynamic regimes of cyclotron instability in the afterglow mode of minimum-Belectron cyclotron resonance ion source plasma

The paper is concerned with the dynamic regimes of cyclotron instabilities in non-equilibrium plasma of a minimum-B electron cyclotron resonance ion source operated in pulsed mode. The instability appears in decaying ion source plasma shortly (1–10 ms) after switching off the microwave radiation of the klystron, and manifests itself in the form of powerful pulses of electromagnetic emission associated with precipitation of high-energy electrons along the magnetic field lines. Recently it was shown that this plasma instability causes perturbations of the extracted ion current, which limits the performance of the ion source and generates strong bursts of bremsstrahlung emission. In this artic…

High yield neutron generator based on a high-current gasdynamic electron cyclotron resonance ion source

In present paper, an approach for high yield compact D-D neutron generator based on a high current gasdynamic electron cyclotron resonance ion source is suggested. Results on dense pulsed deuteron beam production with current up to 500 mA and current density up to 750 mA/cm2 are demonstrated. Neutron yield from D2O and TiD2 targets was measured in case of its bombardment by pulsed 300 mA Dþ beam with 45 keV energy. Neutron yield density at target surface of 109 s 1 cm2 was detected with a system of two 3 He proportional counters. Estimations based on obtained experimental results show that neutron yield from a high quality TiD2 target bombarded by Dþ beam demonstrated in present work accele…

Electron cyclotron resonance ion sources – physics, technology and future challenges

This article has no abstract. peerReviewed

E-sail test payload of the ESTCube-1 nanosatellite

The scientific mission of ESTCube-1, launched in May 2013, is to measure the electric solar wind sail (E-sail) force in orbit. The experiment is planned to push forward the development of the E-sail, a propulsion method recently invented at the Finnish Meteorological Institute. The E-sail is based on extracting momentum from the solar wind plasma flow by using long thin electrically charged tethers. ESTCube-1 is equipped with one such tether, together with hardware capable of deploying and charging it. At the orbital altitude of ESTCube-1 (660–680 km) there is no solar wind present. Instead, ESTCube-1 shall observe the interaction between the charged tether and the ionospheric plasma. The E…

Plasma instabilities of a charge breeder ECRIS

International audience; Experimental observation of plasma instabilities in a charge breeder electron cyclotron resonance ion source (CB-ECRIS) is reported. It is demonstrated that the injection of 133Cs+ or 85Rb+ ion beam into the oxygen discharge of the CB-ECRIS can trigger electron cyclotron instabilities, which restricts the parameter space available for the optimization of the charge breeding efficiency. It is concluded that the transition from a stable to unstable plasma regime is caused by gradual accumulation and ionization of Cs/Rb and simultaneous change of the discharge parameters in 10–100 ms time scale, not by a prompt interaction between the incident ion beam and the ECRIS pla…

CERN’s Linac4 cesiated surface H− source

Linac4 cesiated surface H− sources are routinely operated for the commissioning of the CERN’s Linac4 and on an ion source test stand. Stable current of 40-50 mA are achieved but the transmission through the LEBT of 80% was below expectations and triggered additional beam simulation and characterization. The H− beam profile is not Gaussian and emittance measurements are larger than simulation. The status of ongoing development work is described; 36 mA H− and 20 mA D− beams were produced with a 5.5 mm aperture cesiated surface ion source. The emittances measured at the test stand are presented. During a preliminary test, the Linac4 proton source delivered a total beam intensity of 70 mA (p, H…

High current proton source based on ECR discharge sustained by 37.5 GHz gyrotron radiation

Formation of hydrogen ion beams with high intensity and low transverse emittance is one of the key challenges in accelerator technology. Present work is devoted to experimental investigation of proton beam production from dense plasma (Ne > 1013 cm−3) of an ECR discharge sustained by 37.5 GHz, 100 kW gyrotron radiation at SMIS 37 facility at IAP RAS. The anticipated advantages of the SMIS 37 gasdynamic ion source over the current state-of-the-art proton source technology based on 2.45 GHz hydrogen discharges are described. Experimental result obtained with different extraction configurations i.e. single- and multi-aperture systems are presented. It was demonstrated that ultra bright proton …

IBSIMU: a three-dimensional simulation software for charged particle optics.

A general-purpose three-dimensional (3D) simulation code IBSIMU for charged particle optics with space charge is under development at JYFL. The code was originally developed for designing a slit-beam plasma extraction and nanosecond scale chopping for pulsed neutron generator, but has been developed further and has been used for many applications. The code features a nonlinear FDM Poisson's equation solver based on fast stabilized biconjugate gradient method with ILU0 preconditioner for solving electrostatic fields. A generally accepted nonlinear plasma model is used for plasma extraction. Magnetic fields can be imported to the simulations from other programs. The particle trajectories are …

Broadband microwave emission spectrum associated with kinetic instabilities in minimum-B ECR plasmas

Plasmas of electron cyclotron resonance ion sources (ECRISs) are prone to kinetic instabilities due to the resonant heating mechanism resulting in anisotropic electron velocity distribution. Frequently observed periodic oscillations of extracted ion beam current in the case of high plasma heating power and/or strong magnetic field have been proven to be caused by cyclotrontype instabilities leading to a notable reduction and temporal variation of highly charged ion production. Thus, investigations of such instabilities and techniques for their suppression have become important topics in ECRIS research. The microwave emission caused by the instabilities contains information on the electron e…

A new 18 GHz room temperature electron cyclotron resonance ion source for highly charged ion beams

An innovative 18 GHz HIISI (Heavy Ion Ion Source Injector) room temperature Electron Cyclotron Resonance (ECR) ion source (ECRIS) has been designed and constructed at the Department of Physics, University of Jyväskylä (JYFL), for the nuclear physics program of the JYFL Accelerator Laboratory. The primary objective of HIISI is to increase the intensities of medium charge states (M/Q ≅ 5) by a factor of 10 in comparison with the JYFL 14 GHz ECRIS and to increase the maximum usable xenon charge state from 35+ to 44+ to serve the space electronics irradiation testing program. HIISI is equipped with a refrigerated permanent magnet hexapole and a noncylindrical plasma chamber to achieve very stro…

Investigation of ISIS and Brookhaven National Laboratory ion source electrodes after extended operation.

Linac4 accelerator of Centre Européen de Recherches Nucléaires is under construction and a RFdriven H− ion source is being developed. The beam current requirement for Linac4 is very challenging: 80 mA must be provided. Cesiated plasma discharge ion sources such as Penning or magnetron sources are also potential candidates. Accelerator ion sources must achieve typical reliability figures of 95% and above. Investigating and understanding the underlying mechanisms involved with source failure or ageing is critical when selecting the ion source technology. Plasma discharge driven surface ion sources rely on molybdenum cathodes. Deformation of the cathode surfaces is visible after extended opera…

VUV irradiance measurement of a 2.45 GHz microwave-driven hydrogen discharge

Absolute values of VUV-emission of a 2.45 GHz microwave-driven hydrogen discharge are reported. The measurements were performed with a robust and straightforward method based on a photodiode and optical filters. It was found that the volumetric photon emission rate in the VUV-range (80-250 nm) is $10^{16}$-$10^{17}$ 1/cm$^3$s, which corresponds to approximately 8% dissipation of injected microwave power by VUV photon emission. The volumetric emission of characteristic emission bands was utilized to diagnostics of molecular plasma processes including volumetric rates of ionization, dissociation and excitation to high vibrational levels and metastable states. The estimated reaction rates impl…

Simulation of H- ion source extraction systems for the Spallation Neutron Source with Ion Beam Simulator.

A three-dimensional ion optical code IBSimu, which is being developed at the University of Jyväskylä, features positive and negative ion plasma extraction models and self-consistent space charge calculation. The code has been utilized for modeling the existing extraction system of the H(-) ion source of the Spallation Neutron Source. Simulation results are in good agreement with experimental data. A high-current extraction system with downstream electron dumping at intermediate energy has been designed. According to the simulations it provides lower emittance compared to the baseline system at H(-) currents exceeding 40 mA. A magnetic low energy beam transport section consisting of two sole…

Status and development of the MARA low-energy branch

The MARA Low-Energy Branch is under development at the Accelerator Laboratory of the University of Jyvaskylä. The facility will be employed for laser ionisation and spectroscopy studies and for mass measurements of nuclei close to the proton drip line. This article presents an updated status of the ongoing development of the different parts of this facility, including the buffer gas cell, the ion transport system, the laser system and the detector stations. peerReviewed

The role of seed electrons on the plasma breakdown and preglow of electron cyclotron resonance ion source

The 14 GHz Electron Cyclotron Resonance Ion Source at University of Jyväskylä, Department of Physics (JYFL) has been operated in pulsed mode in order to study the plasma breakdown and preglow effect. It was observed that the plasma breakdown time and preglow characteristics are affected by seed electrons provided by a continuous low power microwave signal at secondary frequency. Sustaining low density plasma during the off-period of high power microwave pulses at the primary frequency shifts the charge state distribution of the preglow transient toward higher charge states. This could be exploited for applications requiring fast and efficient ionization of radioactive elements as proposed f…

Photoelectron emission experiments with ECR-driven multi-dipolar negative ion plasma source

Photoelectron emission measurements have been performed using a 2.45 GHz ECR-driven multi-dipolar plasma source in a low pressure hydrogen discharge. Photoelectron currents induced by light emitted from ECR zone and H− production region are measured from Al, Cu, Mo, Ta, and stainless steel (SAE 304) surfaces as a function of microwave power and neutral hydrogen pressure. The total photoelectron current from the plasma chamber wall is estimated to reach values up to 1 A for 900 W of injected microwave power. It is concluded that the volumetric photon emission rate in wavelength range relevant for photoelectron emission is a few times higher in arc discharge. peerReviewed

Prospects for advanced electron cyclotron resonance and electron beam ion source charge breeding methods for EURISOL

International audience; As the most ambitious concept of isotope separation on line (ISOL) facility, EURISOL aims at producing unprecedented intensities of post-accelerated radioactive isotopes. Charge breeding, which transforms the charge state of radioactive beams from 1+ to an n+ charge state prior to postacceleration, is a key technology which has to overcome the following challenges: high charge states for high energies, efficiency, rapidity and purity. On the roadmap to EURISOL, a dedicated R&D is being undertaken to push forward the frontiers of the present state-of-the-art techniques which use either electron cyclotron resonance or electron beam ion sources. We describe here the gui…

The effect of rf pulse pattern on bremsstrahlung and ion current time evolution of an ECRIS.

Time-resolved helium ion production and bremsstrahlung emission from JYFL 14 GHz ECRIS is presented with different radio frequency pulse lengths. rf on times are varied from 5 to 50 ms and rf off times from 10 to 1000 ms between different measurement sets. It is observed that the plasma breakdown occurs a few milliseconds after launching the rf power into the plasma chamber, and in the beginning of the rf pulses a preglow transient is seen. During this transient the ion beam currents are increased by several factors compared to a steady state situation. By adjusting the rf pulse separation the maximum ion beam currents can be maintained during the so-called preglow regime while the amount o…

Spectroscopic method to study low charge state ion and cold electron population in ECRIS plasma

The results of optical emission spectroscopy experiments probing the cold electron population of a 14 GHz Electron Cyclotron Resonance Ion Source (ECRIS) are reported. The study has been conducted with a high resolution spectrometer and data acquisition setup developed specifically for the diagnostics of weak emission line characteristic to ECRIS plasmas. The optical emission lines of low charge state ions and neutral atoms of neon have been measured and analyzed with the line-ratio method. The aforementioned electron population temperature of the cold electron population (Te < 100 eV) is determined for Maxwell-Boltzmann and Druyvesteyn energy distributions to demonstrate the applicability …

ECRIS plasma spectroscopy with a high resolution spectrometer

Electron Cyclotron Resonance Ion Source (ECRIS) plasmas contain high-energy electrons and highly charged ions implying that only noninvasive methods such as optical emission spectroscopy are reliable in their characterization. A high-resolution spectrometer (10 pm FWHM at 632 nm) enabling the detection of weak emission lines has been developed at University of Jyväskylä, Department of Physics (JYFL) for this purpose. Diagnostics results probing the densities of ions, neutral atoms, and the temperature of the cold electron population in the JYFL 14 GHz ECRIS are described. For example, it has been observed that the cold electron temperature drops from 40 eV to 20 eV when the extraction volta…

Studies of plasma breakdown and electron heating on a 14 GHz ECR ion source through measurement of plasma bremsstrahlung

Temporal evolution of plasma bremsstrahlung emitted by a 14?GHz electron cyclotron resonance ion source (ECRIS) operated in pulsed mode is presented in the energy range 1.5?400?keV with 100??s resolution. Such a high temporal resolution together with this energy range has never been measured before with an ECRIS. Data are presented as a function of microwave power, neutral gas pressure, magnetic field configuration and seed electron density. The saturation time of the bremsstrahlung count rate is almost independent of the photon energy up to 100?keV and exhibits similar characteristics with the neutral gas balance. The average photon energy during the plasma breakdown is significantly highe…