A tokamak, which is the most successful device now on the road to controlled fusion, has the major disadvantage of pulsed operation because of a need to induce a toroidal current in the plasma. The application of rf to drive the current in steady-state tokamak reactors has been considered by a number of authors. 1 " 5 A method of producing continuous current carried by electrons in the tail of distribution function via quasilinear Landau damping of high-phase-velocity rf waves near the lower hybrid (LH) frequency has been proposed. 4,5 The linear and quasilinear Landau damping of slow electrostatic waves near LH frequency has been confirmed in a linear test device 6 and in the LH electron heating experiment on the tokamak (Doublet IL4). 7 These experiments provide a physical base for understanding the quasilinear Landau damping in the toroidal plasma with a relatively high electron temperature. Recently, the current generated by the unidirectional electron plasma waves has been observed in linear devices 8 * 9 and a toroidal device. 10 These experiments have been carried out in a plasma with a lower electron temperature, in which a transfer of momentum from LH waves to electrons via collisional absorption is significant.In order to make effective coupling between the LH waves and electrons, it is necessary to avoid the deposition of the rf energy into ions resulting ^. Mandelbrot, Fractals: Form, Chance, and Dimension (Freeman, San Francisco, 1977). e from the linear mode conversion and the excitation of parametric instabilities. The previous experiments on the rf ion heating indicated that i-for w Q /w lh {$) ^ l c 6 the ions did not interact with i the rf waves and the parametric decay instabilities almost disappeared, 11,12 where ou 0 is the frequency of the applied rf field and oo lh (0) is the LH QS frequency at the center of the plasma column. In this Letter, we report the experimental study on s the coupling between the rf waves and electrons under the conditions of oo 0 /uo lh (0) < 2 and the relatively high electron temperature in a tokamak. J-The experiment, with a 750-MHz rf source, e 6 was performed in the J FT-2 (JAERI Fusion Torus) i tokamak, which was a conventional tokamak with a major radius of JR 0 = 90 cm and a minor radius of a = 25 cm. The experimental setup and the discharges were reported in detail, 13 and hence will be described only briefly here. In the present l-experiment, the following discharge was used as a magnetohydrodynamically stable operation; toroidal magnetic field B t = 14 kG, plasma current I p = 3Q kA, mean line-of-sight electron density n ^3xl0 12 cm -3 , central electron temperature T^ -(250 eV)/k and effective ionic charge Z e ff of 2-5. The working gas was deuterium. The wave-3 guide array employed here consists of four indei pendently driven waveguides mounted 1.5 cm 5 away from the plasma edge, which is defined by It is observed that the waves launched from a phased array antenna of four waveguides couple effectively with electrons under the condition of oo 0 /oo lh (...
Electron cyclotron heating (ECH)-assisted startup experiments have been performed in JT-60U. The breakdown loop voltage was successfully reduced from 25 to 4 V (=0.26 V m−1) by 200 kW ECH. This is lower than the 0.3 V m−1, which corresponds to the maximum electric field in ITER. Parameter scans of ECH power, prefill pressure, resonance position and polarization were carried out. The sensitivity of the breakdown to polarization and resonance position was observed. A prefilling gas pressure scan showed that the initial breakdown density increases with prefill pressure when it is is lower than 8 × 10−5 Torr. Higher harmonic ECH was also attempted. The second harmonic ECH-assisted startup was possible with higher ECH power injection. However, the third harmonic ECH-assisted startup was not successful.
Tomidicity-induced Alfvdn eigen (TAE) modes are observed during minority-ion cyclotron resonance heating (ICRH) in the JT-60U. The toroidal mode numben of TAE modes are idenlified as 7, 8, 9, 10 and 1 I from the Doppler shift in the TAE modes with scanning toroidal rotation at a plasma current of 3 MA. The toroidal mode number of TAE modes tends to increase during a giant sawtooth by ICRH with a decreasing safety factor for the central region.The TAE mode number increases with pl3sma current, so that nine TAE modes are observed sequentially during B giant sawtooth al B plasma current of 4 MA. where the maximum toroidal made number is estimated lo be at least 13. There m no Alfvbn continuum gaps for TAE modes in the safety-fanor ranges of i -1/2n c y c i + 1/2n. ( I = 1.2,3, . . .), except for the gaps in ellipticity-induced Alfv6n eigen (EAE) modes, where n is the toroidal mode number of T4E modes. Therefore, control of the y profile pight provide a means of avoiding TAE modes.as long as the pressure gradient of the high-energy ions is Iodized.
An electron cyclotron range of frequency (ECRF) program has been initiated to study the local heating and current drive in JT-60U. A frequency of 110 GHz was adopted to couple the fundamental O-mode from the low-field side with an oblique toroidal injection angle for the current drive. Experiments were performed at an injection power of ~1.5 MW by using three gyrotrons, each of which has generated the output power up to ~0.8 MW for 3 seconds. A strongly peaked T e profile was observed and the central electron temperature increased up to ~15 keV when the O-mode was absorbed on the axis. The local electron heating clarified the significant difference in the heat pulse propagation between in the plasmas with internal transport barrier (ITB) and without. The driven current estimated by the Motional Stark Effect (MSE) diagnostic showed that the electron cyclotron (EC) waves drove the plasma current up to ~0.2 MA for an injected power of ~1.3 MW at the local electron temperature and density of T e ~6 keV, n e ~0.7×1019 m-3. The measured driven current near the axis was consistent with the theoretical prediction using a Fokker-Planck code. In the case of co-electron cyclotron current drive (ECCD), the sawtooth activity in neutral beam (NB) heated plasma was completely suppressed for 1.5 s with the deposition at the inversion radius, while the sawtooth was enhanced for counter-ECCD at the same deposition condition.
An output power of 1.5 MW for 1 s was achieved at 110 GHz in a recent gyrotron development using the JT-60U ECRF system. It is the world's highest power oscillation for a pulse duration of at least 1 s. The achievement was enabled by, in addition to the carefully designed cavity and collector, necessary because of thermal stress, an RF shield for the adjustment bellows and a low-dielectric-loss dc break. The way the power was modulated was improved upon by controlling the anode voltage, with high modulation frequency of 5 kHz being achieved in NTM stabilization experiments. Moreover, as a developmental step to realizing a reliable ECRF system for use in future fusion experiments, a long pulse demonstration of 0.4 MW and a 30 s injection into the plasma was achieved with real time control of the anode/cathode-heater. Confirmation was made that the temperature of the cooled components had been saturated with no evidence of any damage being discovered in the waveguides and antenna without forced cooling. An innovative antenna with a relatively wide range of beam steering capabilities utilizing a linearly moving-mirror concept was also designed for use as an active cooling antenna with longer pulses in the future, e.g. for JT-60SA. The beam profile and mechanical strength analyses proved the feasibility of the antenna.
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