Non-inductive plasma current ramp-up through oblique injection of harmonic electron cyclotron waves on the QUEST spherical tokamak

Abstract

The plasma current is ramped up primarily by a 28 GHz electron cyclotron wave (ECW) in the Q-shu University experiment Steady-State Spherical Tokamak (QUEST), with multiple harmonic resonance layers from the second to the fourth stay in the plasma core. A steering antenna comprising two quasi-optical mirrors enhances the power density of ECWs. The ECW beam is injected obliquely from the low-field side where the parallel refractive index is N∥ = 0.75 at the second-harmonic resonance layer. Analysis of the resonance condition has found that energetic electrons moving forward along the magnetic field resonate more effectively than those moving backward. Such symmetry breaking is consistent with the results of the current ramp-up experiment. The peak plasma current reaches Ip>70 kA, constantly injecting a beam of radio frequency power of 100 kW. Ray-tracing by the TASK/WR code demonstrates that the power of the 28 GHz extraordinary mode is absorbed by energetic electrons via single-pass cyclotron absorption.

Investigation of the effectiveness of non-inductive `multi-harmonic' electron cyclotron current drive through modeling multi-pass absorptions in the EXL-50 spherical tokamak

“…Recent simulation studies on the QUEST ST [15,34] -an EC wave driven noninductive plasma operation similar to EXL-50 -has also investigated the dominancy of energetic electron fraction over the bulk thermal electron population in absorbing EC-wave power, and predicted the usefulness of multi-harmonic on current drive by examining the relativistic ECR condition in high energy domain. In another modeling of non-inductive plasma current [35], it was reported, in their terminology, minority hot electrons by the 2nd harmonic ECR power absorption could drive most of the current observed on QUEST.…”

“…Thus created energetic electrons eventually resonate with higher harmonics (n > 2) by means of a two-step mechanism; first, through the relativistic Doppler shifted resonance and second, via a nonlinear interaction with different harmonics. The first step is widely interpreted as being effective for power absorption in fully EC wave powered operation on QUEST [15]. This article explores In CQL3D, Z ef f appears in the Coulomb collision calculation between electrons and different impurity ion species.…”

BORAY: An Axisymmetric Ray Tracing Code Supports Both Closed and Open Field Lines Plasmas

“…In some STs, such as QUEST [19] and EXL-50 [20], the high energy electrons (>10keV) are an important component. Ref.…”

Solenoid-free current drive via ECRH in EXL-50 spherical torus plasmas

“…One of the key EXL-50 experimental goals is to test the efficacy of electron cyclotron resonance heating (ECRH) and current drive in the absence of an CS magnet. CS-free ECRH and current drive has been tested in several earlier ST devices (CDX-U [10], LATE [11][12][13][14][15], TST-2 [16][17], MAST [18][19], and QUEST [20][21][22][23][24][25][26]). A toroidal current of 1.05 kA was generated using about 8 kW of ECRH power on CDX-U [9], proving the possibility of current start-up by ECRH alone.…”

“…including those on the center column, assist in ensuring multiple reflected paths of the injected ECW back to the plasma. Wall-reflection further helps by converting O-mode wave to the Xmode and vice versa, thus taking advantage of the higher efficiency of X-modes by energetic electrons [26]. Another notable feature of EXL-50's plasma is that the cross section of the plasma current carried by the energetic electrons is much bigger than that of CFS during the flattop phase of plasma current.…”