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

Abstract

Izotov, I.; Kalvas, Taneli; Koivisto, Hannu; Kronholm, Risto; Mansfeld, D.; Skalyga, V.; Tarvainen, Olli Izotov, I., Kalvas, T., Koivisto, H., Kronholm, R., Mansfeld, D., Skalyga, V., & Tarvainen, O. (2017). Broadband microwave emission spectrum associated with kinetic instabilities in minimum-B ECR plasmas. Physics of Plasmas, 24 (4) 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 energy distribution and growth mechanism of the instability. The emission has been studied earlier in the frequency range of 8-14 GHz allowing us to deduce the probable excited mode. A more detailed study of the microwave emission spectrum of a minimum-B ECR plasma, sustained by 14 GHz microwave radiation, is presented in this work. It was found that the frequencies of the microwaves emitted by the plasma consist of several harmonics of the main band and extend from 6 GHz up to 25 GHz, being independent of the plasma parameters. Published by AIP Publishing.[http://dx

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

“…However, the emission power distribution over the spectrum is affected by the heating power distribution. Moreover, it can be noted that the main emission lines are the same as in the case of single frequency heating reported before [5]. In the case of 14 GHz wave power within 300-550 W and 12.45 GHz wave at 200 W no emission was found, which is consistent with the observed parametric space of two-frequency stabilization [10].…”

“…The electron energy distribution resulting from the resonant heating and long confinement time of energetic electrons is strongly anisotropic and is considered to consist of (at least) two main populations: cold (and warm) electrons with an average energy of Ee,cold = 10 eV-10 keV and hot electrons with Ee,hot > 10 keV up to 1 MeV. Such non-equilibrium plasmas are prone to kinetic instabilities driven by warm and hot electrons whose transverse (with respect to the external magnetic field) velocity dominates over the longitudinal velocity [5]. It has been proven that the kinetic instabilities of cyclotron type are the reason for the periodic oscillation of the extracted beam current with a maximum amplitude of several tens of percent [6,7] and simultaneous powerful bursts of microwave and x-ray radiation.…”

“…An inevitable feature of the cyclotron instability is the emission of electromagnetic (EM) radiation due to resonant amplification of plasma waves as a result of nonlinear interaction with "hot" electrons. The microwave emission during the onset of the cyclotron instability has been thoroughly studied in various experiments of ECR-heated plasmas in a simple mirror and minimum-B traps [5]. The present paper is devoted to investigations of microwave emission pattern induced by kinetic instabilities under two-frequency heating of ECRIS plasmas.…”

Measurements of the energy distribution of electrons lost from the minimum B-field—The effect of instabilities and two-frequency heating

“…The hump at higher energy is presumably linked to the B min value as discussed in the previous section. The hump at lower energy could be of similar origin, as a rich electromagnetic spectrum interacting with the electrons is generated during the instability event [6,15,16].…”

Observation of Poincaré-Andronov-Hopf Bifurcation in Cyclotron Maser Emission from a Magnetic Plasma Trap

“…Plasma microwave emission was detected with a highbandwidth digital oscilloscope allowing direct recording of the waveforms of electromagnetic field emitted by the plasma with temporal resolution of 12.5 ps. More details on the diagnostic technique may be found in [41,42]. An example of the measured signal and its dynamic spectrogram in the frequency band of 8 − 12 GHz is presented in Fig.…”