6 research outputs found
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Enhanced lower hybrid penetration via intense multi-microsecond pulses
Applying lower-hybrid power in short, intense pulses can overcome Landau damping, allowing penetration into the core of reactor-grade plasmas. We present theoretical description of the absorption which accounts for transient collisional effects as well as nonlinear broadening of the resonant plateau. We show results from ray-tracing calculations which include the nonlinear absorption. We also derive the conditions required for pump depletion by parametric instabilities, and assess density depletion by ponderomotive effects, scattering by low-frequency background fluctuations, and filamentation. Consideration of all of the aforementioned effects as well as potential source availability and launcher requirements leads to the consideration of scenarios based on 5--10 GW 30--100 {mu}s pulses for the ITER Conceptual Design. Experimental tests of the concept can be done by launching waves with high enough parallel wavenumber that the resonant electrons are only moderately far out on the tail of the distribution function. The experiments could entail checking the predicted variation of the penetration with the duration and peak power of the pulses as well as the launcher area. We give sample experimental parameters for the Microwave Tokamak Experiment (MTX), Alcator C-Mod, Versator, and D3-D. 15 refs., 3 figs
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Fundamental and second harmonic hydrogen fast-wave heating on DIII-D
Ion cyclotron resonance heating (ICRH) with fast waves has been investigated on D3-D in both the fundamental hydrogen minority (32 MHz, 2.14 T) and second harmonic hydrogen majority (60 MHz, 1.97 T) regimes. The main purpose of these experiments was to characterize the fast wave coupling and propagation in preparation for upcoming fast wave current drive (FWCD) experiments. For the fundamental minority regime, the electron and ion heating, global confinement, and radiated power fraction are compared for ICRH and NBI discharges with P{sub aux} {approx} 1 MW. The ICRH experiments were conducted using a four strap antenna which was designed for current drive experiments. The antenna is fed by a single 2 MW 30--60 MHz transmitter. For ICRH experiments, either (0,0,0,0) or (0,{pi},0,{pi}) phasing was used. The launched parallel index of refraction for (0,{pi},0,{pi}) phasing is {vert bar}n{parallel}{vert bar} {approx} 21 at 32 MHz, and {vert bar}n{parallel}{vert bar} {approx} 11 at 60 MHz. 7 refs., 8 figs
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Fast wave current drive antenna performance on DIII-D
Fast wave current drive (FWCD) experiments at 60 MHz are being performed on the DIII-D tokamak for the first time in high electron temperature, high {beta} target plasmas. A four-element phased-array antenna is used to launch a directional wave spectrum with the peak n{sub {parallel}} value ({approx equal} 7) optimized for strong single-pass electron absorption due to electron Landau damping. For this experiment, high power FW injection (2 MW) must be accomplished without voltage breakdown in the transmission lines or antenna, and without significant impurity influx. In addition, there is the technological challenge of impedance matching a four-element antenna while maintaining equal currents and the correct phasing (90{degree}) in each of the straps for a directional spectrum. In this paper we describe the performance of the DIII-D FWCD antenna during initial FW electron heating and current drive experiments in terms of these requirements
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Fast wave current drive experiment on the DIII-D tokamak
One method of radio-frequency heating which shows theoretical promise for both heating and current drive in tokamak plasmas is the direct absorption by electrons of the fast Alfven wave (FW). Electrons can directly absorb fast waves via electron Landau damping and transit-time magnetic pumping when the resonance condition {omega} {minus} {kappa}{sub {parallel}e}{upsilon}{sup {parallel}e} = O is satisfied. Since the FW accelerates electrons traveling the same toroidal direction as the wave, plasma current can be generated non-inductively by launching FW which propagate in one toroidal direction. Fast wave current drive (FWCD) is considered an attractive means of sustaining the plasma current in reactor-grade tokamaks due to teh potentially high current drive efficiency achievable and excellent penetration of the wave power to the high temperature plasma core. Ongoing experiments on the DIII-D tokamak are aimed at a demonstration of FWCD in the ion cyclotron range of frequencies (ICRF). Using frequencies in the ICRF avoids the possibility of mode conversion between the fast and slow wave branches which characterized early tokamak FWCD experiments in the lower hybrid range of frequencies. Previously on DIII-D, efficient direct electron heating by FW was found using symmetric (non-current drive) antenna phasing. However, high FWCD efficiencies are not expected due to the relatively low electron temperatures (compared to a reactor) in DIII-D
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Direct electron absorption of fast waves on the D3-D tokamak
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Direct electron heating by 60 MHz fast waves on DIII-D
Efficient direct electron heating by fast waves has been observed on the DIII-D tokamak. A four strap antenna with (0,{pi},0,{pi}) phasing launched up to 1.6 MW of fast wave power with {vert bar}n{sub {parallel}}{vert bar} {approx} 11. This {vert bar}n{sub {parallel}}{vert bar} is suitable for strong electron interaction in ohmic target plasmas (T{sub e} {le} 2 keV). Ion cyclotron absorption was minimized by keeping the hydrogen fraction low ({lt}3%) in deuterium discharges and by operating at high ion cyclotron harmonics ({omega} = 4{Omega}{sub H} = 8{Omega}{sub D} at 1T). The fast wave electron heating was weak for central electron temperatures below 1 keV, but improved substantially with increasing T{sub e}. Although linear theory predicts a strong inverse magnetic field scaling of the first pass absorption, the measured fast-wave heating efficiency was independent of magnetic field. Multiple pass absorption of the fast waves appears to be occurring since at 2.1 T nearly 100% efficient plasma heating is observed while the calculated first pass absorption is 6% to 8%. The central electron temperature during fast wave heating also increased with magnetic field. The improved electron heating at higher magnetic fields may be due in part to a peaking of the ohmic plasma current and the ohmic electron temperature profiles. Centrally peaked deposition profiles were measured by modulating the fast wave power at 10 Hz and observing the local electron temperature response across the plasma. 11 refs., 10 figs