Measurements of the fast electron bremsstrahlung during lower hybrid current drive in the HL-2A tokamak

Y P Zhang,J Zhou,M Xu,L Delpech,A Ekedahl,Y Peysson,M Isobe,J Zhang,W Chen,D Mazon,Y Liu,P F Zhang,P Malard,X Li,X L Zou,Y G Li,Yi Liu,T Hoang,Guoliang Yuan ,Xue Bai ,W.l Zhong ,Huang Xu ,B Lü ,X.t Ding ,Z.b Shi ,Jianhua Yang ,Qingwei Yang ,Xiao Yan Song ,X.r Duan

doi:10.1063/1.5110233

Abstract

Physics related to fast electrons in lower hybrid (LH) current drive (LHCD) plasma is a very important issue, since these particles will play an important role in runaway electron (RE) generation and lower hybrid wave (LHW)-related physics. Utilizing a new hard X-ray (HXR) pinhole camera, recent HL-2A tokamak experiments have devoted to enhancing the understanding of the physics on fast electrons and LHW. The fast electron bremsstrahlung (FEB) emission in the HXR energy range between 20 and 200 keV was measured by the HXR camera. To study the conversion of LHW-produced fast electrons into REs, a very short pulse of LHW, so-called “blip”, with duration of 5 ms was injected into the plasma during the current flattop phase. A strong enhancement of REs was induced by the blip injection. Measurements from the HXR camera show that the fast electrons generated by LHWs is mainly concentrated in 40-60 keV, which is well consistent with the calculated value based on Landau damping theory. The energy of these seed electrons is higher than the critical runaway energy. This phenomenon may be come from the synergetic effects of Dreicer and avalanche RE generation. Moreover, the measurements indicate that the spatial distribution of the fast electrons during LHCD has a peaked profile, implying that the fast electrons are mainly produced in the plasma core. It also suggests that the energy of the LHW mainly deposited in the plasma core region.

Highlights

Sustainment of the plasma steady-state operation and control of the current density profile are crucial issues in an advanced operation of a future tokamak fusion reactor,1–4 such as international thermonuclear experimental reactor (ITER)
It suggests that the energy of the lower hybrid wave (LHW) mainly deposited in the plasma core region
A hard X-ray (HXR) pinhole camera system has been recently developed in the HL-2A tokamak to measure the spatial and temporal evolution of the fast electron bremsstrahlung (FEB) emission in the HXR energy range between 20 and 200 keV, enhancing the understanding of the physics on runaway electron (RE) and LHCD

Summary

INTRODUCTION

Sustainment of the plasma steady-state operation and control of the current density profile are crucial issues in an advanced operation of a future tokamak fusion reactor, such as international thermonuclear experimental reactor (ITER). The measurements of the FEB emission and REs presented in this paper were performed using the following diagnostic systems: (a) a HXR pinhole camera; (a) a high-energy HXR detection system; (c) a neutron flux monitoring system. (a) The FEB emission in the energy range of 20-200 keV is measured by means of a HXR pinhole camera which has recently been developed and operated in the HL-2A tokamak. The camera plays an important role in the experiments about runaway electrons and LHCD, detecting the fast electron population together with the energy and spatial-temporal distribution. 21 ≤ 0.7a 20-200 keV CdTe 20 keV 2-16 ms 2 cm the neutron flux monitor is an accurate tool for RE energy characterization

ENHANCEMENT OF RE GENERATION INDUCED BY A LHCD BLIP

ENERGY AND SPATIAL DISTRIBUTION OF FAST ELECTRONS DURING LHCD

CONCLUSIONS