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…

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

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.

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…

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

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…

Hydrogen plasma induced photoelectron emission from metal surfaces

Low temperature hydrogen plasmas are strong sources of vacuum ultraviolet radiation. The properties of laboratory plasmas can be influenced by surface processes induced by photons with their energies exceeding the surface work function of the wall material. In this work, the plasma induced photoelectron emission has been studied with different ion sources. The emission depends on the mechanical design of the plasma device, plasma heating method and the discharge power (density). Parametric studies include the quantifying of the emission from different metal surfaces, commonly used as plasma facing materials in ion sources, as well as alkali metal covered surfaces. Experimental studies sugges…

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…

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…

Microwave emission from ECR plasmas under conditions of two-frequency heating induced by kinetic instabilities

Multiple frequency heating is one of the most effective techniques to improve the performances of ECR ion sources. It has been demonstrated that the appearance of the periodic ion beam current oscillations in ECRIS at high heating power and low magnetic field gradient is associated with kinetic plasma instabilities. Recently it was proven that one of the main features of multiple frequency heating is connected with stabilizing effect, namely the suppression of electron cyclotron instability in ECRIS plasmas. Due to this kind of stabilization it is possible to run the ion source in stable mode using higher total microwave power and thus to obtain better ion beam parameters. Unfortunately, ev…

Mikroaalloilla lämmitetyn vetyplasman emittoiman valon aiheuttama fotoelektroniemissio metallipinnoilla

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…

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…

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

VUV emission spectroscopy combined with H- density measurements in the ion source Prometheus I

“Prometheus I” is a volume H− negative ion source, driven by a network of dipolar electron cyclotron resonance (ECR; 2.45 GHz) modules. The vacuum-ultraviolet (VUV) emission spectrum of low-temperature hydrogen plasmas may be related to molecular and atomic processes involved directly or indirectly in the production of negative ions. In this work, VUV spectroscopy has been performed in the above source, Prometheus I, both in the ECR zones and the bulk (far from ECR zones and surfaces) plasma. The acquired VUV spectra are correlated with the negative ion densities, as measured by means of laser photodetachment, and the possible mechanisms of negative ion production are considered. The well-e…

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 …

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…

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…

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