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

Abstract

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 increasing neutral gas pressure, the magnetic field strength being the most critical parameter. The occurrence of plasma turbulence is demonstrated to restrict the parameter space available for the optimization of extracted currents of highly charged ions.

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

“…A thorough description of the experimental setup is presented elsewhere. 5,13 • Microwave detector diode (0.01-50 GHz, 10 ns resolution) connected to a WR75 waveguide port at the injection of the ion source.…”

“…10 Kinetic plasma instabilities driven by the anisotropy of the EED have been detected and studied in ECR-heated mirror plasmas 11,12 and recently in a minimum-B ECRIS. 5,13 Each onset of the instability is associated with strong microwave emission from the plasma followed by a burst of electrons ejected into the loss cone and thus escaping the magnetic trap. In a magnetic mirror machine, the instabilities are responsible for plasma (energy) losses up to 25%, 9 which in an ECRIS would be detrimental for high charge state production.…”

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

“…The most critical parameter affecting the occurrence of the instabilities has been shown to be the (solenoid) magnetic field strength, which defines the threshold between stable and unstable operating regimes, while the microwave power and neutral gas pressure affect the repetition rate of the periodic instabilities in continuous (cw) operating mode. 1 It has been shown recently that the instabilities restrict the parameter space available for optimizing the extracted currents of high charge state ions in cw operation of minimum-B ECR ion sources. 1,2 This is further illustrated in Fig.…”

“…1 It has been shown recently that the instabilities restrict the parameter space available for optimizing the extracted currents of high charge state ions in cw operation of minimum-B ECR ion sources. 1,2 This is further illustrated in Fig. 1 showing examples of temporally averaged (normalized) beam currents of He 2+ , O 7+ , and Ar 14+ extracted from the JYFL 14 GHz ECRIS 3 when sweeping the magnetic field strength, indicated by the B min /B ECR -ratio, across the transition from stable to unstable operating regime.…”

Kinetic instabilities in non-equilibrium plasma: a review of observations

“…Significant losses of charged particles from the trap as a result of instability leads to the generation of bremsstrahlung X-ray radiation, which is absorbed by the coolant of the magnetic systems (cryomagnet) and increases the thermal load. For example, the ECR ion sources powerful emission of highenergy electrons from the traps can lead to disturbance of the ambipolar potential and undesirable oscillations of the currents of multiply charged ions [8]. In this sense, the cyclotron instability, which is inherent with ECR heating systems, in natural way (and sometimes very significantly!)…”