Observation of parametric decay spectrum in a cylindrical electron cyclotron resonance plasma system

Abstract

Nonlinear wave coupling phenomena in an electron cyclotron resonance (ECR) produced plasma are observed. Experiments are carried out in a small linear cylindrical system with hydrogen plasma. Microwave power in the system is delivered by a magnetron at 2.45 GHz in TE10 mode and is launched radially to have extraordinary (X) wave in plasma. The axial magnetic field configuration, required for ECR to occur in the system, is such that the first ECR surface (B=875 G) resides at the center of the system. Parametric decay spectrum of the launched electromagnetic wave is observed at the upper hybrid resonance (UHR) layer with the appearance of sidebands on the higher and lower side of the launched frequency. Experiments show that the appearance of parametric decay spectrum has a threshold on input microwave power. The decay spectrum is observed only at the UHR surface indicating strong spatial dependence. The parametric decay is also verified in argon and helium plasmas.

Schemes of microwave heating of overdense plasmas in the stellarator TJ-K

“…A maximum of the wave electric field was measured at the UHR [9]. These high values of the wave electric field can give rise to nonlinear phenomena, like parametric decay instabilities [12,13]. The simplest case of such an instability is the three wave coupling in which a pump wave with the frequency f 0 excites two waves with the frequency f 1 and f 2 .…”

Electron Bernstein wave generation in a linear plasma system

“…The unknown frequency signal picked by the radial capacitive probe, placed at the UHR layer ͑R =2 cm͒, is fed into the rf port and the known reference signal from the forward port of the directional coupler, which is 40 dB down, is fed into the LO port of the DBM. The IF port output is LH frequency 30 and is shown as the oscillogram in Fig. 10.…”

“…This result is consistent with the earlier experiment of observation of parametric decay spectrum. 30 The EBW frequency is determined using the DBM. The unknown frequency signal picked by the radial capacitive probe, placed at the UHR layer ͑R =2 cm͒, is fed into the rf port and the known reference signal from the forward port of the directional coupler, which is 40 dB down, is fed into the LO port of the DBM.…”

“…The LH frequency measured here directly with the capacitive probe matches fairly well with the residual frequency in the parametric decay experiment. 30 The EBW phase velocity is determined by the time-offlight measurement technique. The reference signal from the directional coupler is divided into two equal parts and fed into the LO port of the two DBMs.…”

INVESTIGATION OF EBW THERMAL EMISSION AND MODE CONVERSION PHYSICS IN H-MODE PLASMAS ON NSTX

“…Numerical modeling has shown that by adding 1 MA of off-axis EBWCD to an ST-CTF plasma, the stability is significantly increased by increasing the β n and β t stability limits [3]. The EBW emission (EBE) physics of B-X-O MC is essentially the reverse process of the O-X-B MC process used for heating [4], except that the introduction of RF power may excite lossy nonlinear phenomena like parametric decay [5,6]. The present work focuses on investigating emission via the B-X-O mode conversion process in H-mode plasmas in order to economically assess the feasibility of O-X-B coupling.…”