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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...

The Tera Electronvolt Superconducting Linear Accelerator TESLA is the only linear electron-positron collider project based on superconductor technology for particle acceleration. In the first stage with 500 GeV center-of-mass energy an accelerating field of 23.4 MV/m is needed in the superconducting niobium cavities which are operated at a temperature of 2 K and a quality factor Q 0 of 10 10 . This performance has been reliably achieved in the cavities of the TESLA Test Facility (TTF) accelerator. The upgrade of TESLA to 800 GeV requires accelerating gradients of 35 MV/m. Using an improved cavity treatment by electrolytic polishing it has been possible to raise the gradient to 35 -43 MV/m in single cell resonators. Here we report on the successful transfer of the electropolishing technique to multi-cell cavities. Presently four nine-cell cavities have achieved 35 MV/m at Q 0 ≥ 5 × 10 9 , and a fifth cavity could be excited to 39 MV/m. In two high-power tests it could be verified that EP-cavities preserve their excellent performance after welding into the helium cryostat and assembly of the high-power coupler. One cavity has been operated for 1100 hours at the TESLA-800 gradient of 35 MV/m and 57 hours at 36 MV/m without loss in performance.

In the JIPP T-IIU tokamak an experiment to demonstrate the feasibility of fast wave current drive using five loop antennas has been successfully carried out with a relatively high density plasma (ωpe2/ωce2∼5). The RF frequency is 40 MHz and the toroidal field is 2 kG, which corresponds to ω = 13ωcH. The experiment is conducted in the density range n̄e ∼ 2 × 1018 m−3 where only the fast wave can propagate, eliminating the possibility of slow wave current drive. This density is two orders of magnitude higher than the density limit predicted for slow wave current drive. The dependence of the drive efficiency on the relative phase difference Δφ is clearly observed with a maximum of about Δφ = π/4. The plasma current was limited by MHD instability which begins to occur around qa = 10.

Current drive by fast magnetosonic waves is successfully performed in the JIPP T-IIU tokamak by means of a four-element dipole antenna array with a Faraday shield. A plasma current of about 50 kA is sustained by an rf power of about 80 kW in a low-density plasma (3 x 10 12 cm" 3 ) with an efficiency comparable to that of slow-wave current drive. A density limit for fast magnetosonic current drive is observed, contrary to theoretical expectations based on linear wave propagation.PACS numbers: 52.35.Hr, 52.40.Db, 52.55.Fa Though extensive studies of sustainment, rampup, and startup of plasma current by slow waves in the lower hybrid (LH) frequency range have been conducted successfully, 1 "" 9 it has been clearly indicated that there exists a density limit beyond which current cannot be driven by the slow wave. 10 The profile of current driven by the slow waves tends to be hollow when applied to large tokamaks confining a hightemperature plasma. 11 Recently, current drive by fast waves has started to attract attention because the fast wave may drive the plasma current even in hightemperature and high-density plasmas where the slow wave is not feasible. 12 The main reason lies in the different characteristics of the fast wave near the LH frequency: no mode conversion to electrostatic warm plasma wave and less absorption as a result of the weak Landau damping in the framework of linear theory. Nonlinear effects, which hinder the propagation of the fast wave to the high-density region, might be weakened because the electric field of fast waves is weak and no resonance cone exists for their propagation. 13 Because fast waves near the LH frequency with refractive indices N n~~0 (l) are mainly damped by electron Landau damping, tokamak plasmas with electron temperatures higher than several kiloelectronvolts, or with a fast electron tail, are imperative for greater efficiency in the current drive. Current drive by fast waves was studied experimentally in a low-temperature plasma by a few groups, and driven plasma currents of a few kiloamperes or less were observed. 14 " 17 In this Letter, we present the first successful experiment in a tokamak plasma of the "slide-away" regime 18,19 where a large current is sustained by fast waves near the LH frequency.The experiment was carried out in the JIPP T-IIU tokamak 20 with minor (major) radius a =0.25 m (#=0.93 m) at £, = 26.4 kG. A fast magnetosonic (FMS) wave at / 0 = 800 MHz (f 0 -20f cl )is launched into a hydrogen plasma in the slide-away regime from a four-element dipole antenna array, as shown in Fig. 1(a). Each dipole antenna, 2.0 cm in width and 19.6 cm in length, is separated from another by a distance of 3.6 cm. To prevent direct rf radiation in the toroidal direction the antenna system is housed in a FIG. 1. (a) Four-element dipole antenna array with Faraday shield; (b) rf voltage V r r picked up by a Hertz dipole located at the origin as a function of angle 0; (c) rf voltage K rf at the origin as a function of angle X; (d) rf field E y on the Faraday shield ...

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