Startup and Quasistationary Drive of Plasma Current by Lower Hybrid Waves in a Tokamak

Abstract

A plasma current is initiated and raised to a quasistationary level of about 20 kA by injection of the lower hybrid wave into a cold and low-density plasma produced by electron cyclotron resonance. The plasma current rises more slowly than the experimentally obtained L p /Rp magnetic diffusion time of the bulk plasma. The current rise time is inversely proportional to the bulk electron density, and agrees well with the collision time of the currentcarrying high-energy electrons with the bulk plasma.PACS numbers: 52.40.Db, 52.55.Gb A plasma current generated by an Ohmic heating electric field is maintained by injecting lower hybrid waves (LHW) in many tokamaks. 1 "" 6 These experimental results demonstrate the possibility of a steady-state operation in a future large tokamak.The startup of plasma current by a noninductive method is attractive for the further saving of the volt-seconds of the Ohmic heating transformer.Recently, experiments on current-startup by LHW have been tried in a target plasma produced by LHW alone 7 or by electron cyclotron resonance (ECR). 8 In the WT-2 tokamak, the plasma current increases linearly in time. However, the quasisteady state is not attained since the pulse duration of LHW is relatively short. The realization of the quasisteady state in the plasma current initiated by rf is necessary to clarify the mechanisms of the startup and quasistationary drive and to establish the startup scenario in a future large tokamak.In this Letter, we report on the startup and quasistationary drive of the plasma current by LHW in which the pulse width (=170 ms) is much longer than the experimentally obtained L p /R p time, where L p and R p are the total inductance and resistance of the bulk plasma. The experiments are carried out on the Japanese Institute of Plasma Physics T-IIU tokamak (major radius R 0 = 0.93 m and minor radius a L = 0.25 m). First, a cold and lowdensity target plasma is produced by the electron cyclotron wave of an ordinary mode (/=35.5 GHz) which is injected from the low-field side. 9 The electron cyclotron-resonance layer (ECR layer) is located at R =0.91 m where the toroidal field B t is L27 T. The initial filling-gas pressure P is 5x 10" 5 Torr for hydrogen. The electron temperature and density of the ECR plasma are about 20 eV and 2x 10 12 cm" 3 , which are measured by a movable floating double probe. Next, the LHW is injected into the ECR plasma via the launcher of a pair of C-shaped waveguides. 4 The calculated spectrum of the power emitted from the waveguides has a wide spread of parallel refractive index (n\\) from 4 to 1.4, which corresponds to a critical value of the accessibility condition for n e = 2x 10 12 cm -3 . In the current startup by rf alone, it is particularly essential to control the vertical field carefully. A quasistationary vertical field of about 10 G is always applied at the beginning of the LHW pulse to bring the Larmor radius of a high-energy electron beam inside the vacuum vessel, 10 where the stray field is estimated at below 2 G under this exper...

Physics and Technological Issues for Steady-State Tokamak Operation on TRIAM-1M

“…Although so far many current ramp-up experiments by non-inductive methods have been performed [43][44][45][46], relatively low density plasma (0.05 ~ 0.5 × 10 19 m -3 ) has been used, as shown in Fig. 14. A recent JT-60U result was obtained at ~ 0.3 × 10 19 m -3 [19].…”

Cross-field potential hill arisen eccentrically in toroidal electron cyclotron resonance plasmas in the Low Aspect ratio Torus Experiment device to regulate electron and ion flows from source to boundary

“…This is a standard method of pre-ionization and heating to assist the start-up of plasma current in tokamaks [3]. The ECR plasmas also served as a target plasma for solenoid-free start-up of tokamaks by lower hybrid waves [4,5] and their characteristics was investigated in connection with confinement and equilibrium in the open field configuration [6,7]. In addition to the usage for tokamak startup, ECR-heated plasmas in small toroidal devices, which are often produced by 2.45 GHz microwave power at a power level of 1 kW, have been frequently used as a bed of various basic studies in plasma physics.…”

Current Ramp-up Experiments Using ECRW on TRIAM-1M

“…On the many tokamak devices, including WT-3 [1], JIPP T-II [2], JT-60U [3], TRIAM-1M [4], and Tore Supra [5], many kinds of current drive experiments have been performed. Most notably on the superconducing high-field tokamak TRIAM-1M at Kyushu University, although the line averaged electron density is rather low (3 × 10 18 m -3 ), an ultra-long discharge for longer than 3 hours has been demonstrated using lower hybrid current drive (LHCD) [6].…”