Kinetic whistler instability in a mirror-confined plasma of a continuous ECR ion source

Kinetic instabilities in a dense plasma of a continuous ECR discharge in a mirror magnetic trap at the GISMO setup are studied. We experimentally define unstable regimes and corresponding plasma parameters, where the excitation of electromagnetic emission is observed, accompanied by the precipitation of energetic electrons from the magnetic trap. Comprehensive experimental study of the precipitating electron energy distribution and plasma electromagnetic emission spectra, together with theoretical estimates of the cyclotron instability increment proves that under the experimental conditions the observed instability is related to the excitation of whistler-mode waves, which are a driver of losses of energetic electrons from the magnetic trap. The results of this study are important for the further development of the GISMO ECRIS facility and for the improvement of its parameters as an ion source. Also, this research of plasma kinetic instabilities is of fundamental interest and provides experimental tools to simultaneously study plasma electromagnetic activity and corresponding changes in a resonant electron energy distribution.Comment: 9 pages, 9 figures, submitted to Physics of Plasma

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 parameter on the EED is the magnetic field strength, which affected the EED predominantly at energies less than 100 keV. The effects of the microwave power and frequency, ranging from 100 to 600 W and 11 to 14 GHz respectively, on the EED were found to be less significant. The presented technique and experiments enable the comparison between direct measurement of the EED and results derived from bremsstrahlung diagnostics, the latter being severely complicated by the non-Maxwellian nature of the EED reported here. The role of RF pitch angle scattering on electron losses and the relation between the EED of the axially escaping electrons and the EED of the confined electrons are discussed

Status of the 60 GHz ECR Ion Source Research

International audienceSEISM is a compact ECR ion source operating at 60 GHz developed up to 2014. The prototype uses a magnetic cusp to confine the plasma. This simple magnetic geometry was chosen to allow the use of polyhelix coils (developed at the LNCMI, Grenoble) to generate a strong magnetic confinement featuring a closed ECR surface at 2.1 T. The plasma is sustained by a 300 kW microwave pulse of 1 ms duration and with a 2 Hz repetition rate. Previous experiments at LNCMI have successfully demonstrated the establishment of the nominal magnetic field and the extraction of ion beams with a current density up to ~ 1A/cm². The presence of "afterglow" peaks was also observed, proving the existence of ion confinement in a cusp ECR ion source. The last run was prematurely stopped but the project restarted in 2018 and new experiments are planned in 2021. A new transport beam line has been designed to improve ion beam transport towards the beam detectors. Short- and long-term research plans are presented, including numerical simulations of the beam transport line and future upgrades of the ion source with the main goal to transform the high current density measured into a real high intensity ion beam

Plasma instabilities of a charge breeder ECRIS

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 plasma. The instabilities lead to loss of ion confinement, which results in the sputtering of the surfaces in contact with the plasma, followed by up to an order of magnitude increase of impurity currents in the extracted $n+$ ion beam.peerReviewe

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, even with using of such technique at some threshold level the plasma becomes unstable. This work is devoted to experimental investigations of the peculiarities of cyclotron instability in the case of two-frequency heating. It was found out that the plasma microwave emission spectrum related to instabilities is affected by the division of injected power shared between the heating frequencies, though the main emission lines in the spectrum are proven to be independent on heating frequencies.peerReviewe

Cavity Tuning Experiments with the JYFL 14 GHz ECRIS

Experimental results showing the effect of cavity tuning on oxygen beam currents extracted from the AECR-type JYFL 14 GHz ECRIS are reported. The microwave-plasma coupling properties of the ion source were adjusted by inserting a conducting tuner stub through the injection plug, thus changing the dimensions of the plasma chamber and affecting the cavity properties of the system. The beam currents of high charge state ions were observed to vary up to some tens of percent depending on the tuner position and the microwave frequency.Experimental results showing the effect of cavity tuning on oxygen beam currents extracted from the AECR-type JYFL 14 GHz ECRIS are reported. The microwave-plasma coupling properties of the ion source were adjusted by in- serting a conducting tuner stub through the injection plug, thus changing the dimensions of the plasma chamber and affecting the cavity properties of the system. The beam cur- rents of high charge state ions were observed to vary up to some tens of percent depending on the tuner position and the microwave frequency. In this paper we focus on reporting results obtained in the frequency range of 10.75–12.75 GHz

The effect of plasma instabilities on the background impurities in charge breeder ECRIS

International audienceExperimental observations of plasma instabilities in the 14.5 GHz PHOENIX charge breeder ECRIS are summarized. It has been found that the injection of 133Cs+ or 85Rb+ into oxygen discharge of the CB-ECRIS can trigger electron cyclotron instabilities, which results to sputtering of the surfaces exposed to the plasma, followed by up to an order of magnitude increase of impurity currents in the extracted n+ charge state distribution. The transition from stable to unstable plasma regime is caused by gradual accumulation and ionization of Cs/Rb altering the discharge parameters in 10 - 100 ms time scale, not by a prompt interaction between the incident ion beam and the ECRIS plasma. This time scale is similar to the reported breeding times of the high charge state Cs and Rb ions. Since the commonly applied method of measuring the breeding time, i.e. pulsing the 1+ injection, clearly affects the buffer gas discharge, it is argued that the actual breeding times in continuous operation can differ from those obtained by studying the injection transient

Status and prospects of the 60 GHz SEISM ion source

International audienceSEISM is a unique ECR ion source operating at a frequency of 60 GHz. The prototype is based on a simple magnetic geometry, the cusp, allowing the use of polyhelix coils (developed with LNCMI, Grenoble) to generate the closed ECR surface at 2.14T. The plasma is sustained by a high intensity HF pulse (up to 300kW). Previous experiments at LNCMI have successfully demonstrated the establishment of the nominal magnetic field and the extraction of ion beams with a current density up to 1 A cm$^{-2}$. The presence of afterglow peaks was also observed, proving the existence of ion confinement in a CUSP ECR source. An experimental campaign is carried out in 2021 using a new transport line designed to improve the transmission of the beam to the new detectors. Short and long-term research plans are presented to transform this high current density into a high intensity ion beam that can be used for accelerators of the future