Time-resolved spatial profiles of Zeff in JIPP T-II tokamak plasmas have been determined from the measurement of bremsstrahlung in the visible spectral region. The effective ion charge Zeff increases at the plasma centre and decreases at the plasma periphery in the high-electron-density regime. The emissions from low ionization states of light impurities increase notably in the region of lower electron temperatures in this regime. As a result, impurities accumulate in the plasma centre in the high-density regime. The results obtained with respect to the plasma periphery support the interpretation of the behaviour of the impurities, which is based on VUV spectroscopic studies.

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...

Current driven by injecting lower hybrid waves has been observed in low-density plasmas in the JIPP T-II. It is confirmed that RF-driven current is generated by momentum transfer from lower hybrid waves to suprathermal electrons with an energy of 8-25 keV. The driving efficiencies in tokamak and stellarator configurations are 1 kA'kW" 1 and 0.3 kA'kW" 1 , respectively, at a power level of 40 kW. Enhanced electron cyclotron emission due to pitch-angle scattering of RF-driven suprathermal electrons is observed, and spikes in loop voltage and X-ray bursts appear coincidentally. In a long RF-pulse, these rapid changes advance to relaxation oscillations. It is concluded that the pulsating changes originate in the instantaneous scattering of RF-driven suprathermal electrons by the unstable waves excited at an anomalous Doppler resonance.

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