Non-inductive plasma start-up experiments on the TST-2 spherical tokamak using waves in the lower-hybrid frequency range

Abstract

Non-inductive plasma current start-up and sustainment by waves in the lower-hybrid frequency range (200 MHz) have been studied on the TST-2 spherical tokamak (R 0 0.38 m, a 0.25 m, B t0 0.3 T, I p 0.14 MA) using three types of antenna: the 11-element inductively-coupled combline antenna, the dielectric loaded 4-waveguide array antenna, and the 13-element capacitively-coupled combline (CCC) antenna. The maximum plasma currents of 15 kA, 10 kA and 16 kA were achieved, respectively. The highest current drive figure of merit η CD ≡ n e I p R/P RF was achieved by the CCC antenna. The efficiency of current drive should improve by reducing prompt orbit losses of high energy electrons by operating at higher plasma current (to improve orbit confinement) and higher toroidal magnetic field (to improve wave accessibility to the plasma core), while keeping the density high enough (to avoid excessive acceleration of electrons), but under the 'density limit'.

First Measurement of Electron Temperature and Density Profiles for Spherical Tokamak Plasmas Sustained by Lower Hybrid Waves

“…Plasma current start-up experiments using electron cyclotron wave (ECW) have been performed in several ST devices [1][2][3][4]. On the other hand, the lower hybrid wave (LHW) is used in the TST-2 [5]. A successful lower hybrid current drive (LHCD) was achieved with a high efficiency in many conventional tokamaks, but the efficiency is low for ST devices [6,7].…”

Expansion of the Operation Region of Tokamak Plasmas Using the Stationary Direct Current of a Central Solenoid

“…On the TST-2 spherical tokamak device [1,2], tokamak plasma formation by non-inductive heating has been studied using RF heating at 2450, 200, and 21 MHz [3][4][5][6]. In the tokamak plasma formation experiments by the 2450 MHz heating, the dependences of tokamak plasma formation on the n-index and injection with O-and Xmodes were studied [3].…”

“…In the tokamak plasma formation experiments by the 2450 MHz heating, the dependences of tokamak plasma formation on the n-index and injection with O-and Xmodes were studied [3]. For the non-inductive heating experiments using the 200 MHz, several new antennas were developed [4,5] to improve the toroidal plasma current drive efficiency. By heating at 21 MHz, the minimum heating energies necessary for tokamak plasma sustainment were compared between hydrogen and deuterium plasmas [7].…”

Current Drive Experiment Using Top/Outboard Side Lower Hybrid Wave Injection on TST-2 Spherical Tokamak

“…TST-2 is a spherical tokamak at the University of Tokyo, with the plasma major radius 0.36 m and minor radius 0.23 m (aspect ratio 1.6) [1,2]. In TST-2, CCCAs are used for LHW excitation [3,4], characterized by a sharp wavenumber spectrum with peak parallel refractive index N ∼ −5. At present, there are four RF amplification systems capable of 100 kW output each.…”