15 research outputs found
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LUMINANCE DISCRIMINATION OF BRIEF FLASHES UNDER VARIOUS CONDITIONS OF ADAPTATION.
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30-60 MHz FWCD system on DIII-D: Power division, phase control and tuning for a four-element antenna array
The 2 MW Fast Wave Current Drive system on DIII-D is intended to provide a near-term demonstration of up to 0.3 MA of current driven by the fast wave. The system used to drive the four element phased antenna array which produces the required directional spectrum is presented. This system must be able to cope with strong coupling between antenna elements and the time-varying plasma load seen by the antennas. Computer modelling shows that this system should be able to maintain a directional spectrum at full power under most anticipated load conditions. 5 refs., 1 fig
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Design and performance of fast wave current drive systems in the ICRF
Experiments have begun on D3-D using the fast wave current drive (FWCD) phased antenna array. The array consists of four elements with slotted septa between them to reduce mutual coupling. The passive phasing/matching circuit developed for the launcher incorporates only five tuning elements and is driven by a single rf power supply. The system has successfully operated in the presence of plasma at power levels up to 1.25 MW, with {pi}/2 relative phasing, and approximately equal currents and voltages on all elements. Tuning algorithms that allow proper setting of all five elements within 1--2 shots have been developed. In addition, substantial modeling has been undertaken in support of the D3-D FWCD program. Loading calculations that take into account currents induced in the septa as well as other effects related to antenna geometry have been performed, and the results agree well with the observed data. A circuit model has been developed that, in combination with the loading calculations, allows the simulation of shot-to-shot matching for various tuning algorithms. 6 refs., 8 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 system design for DIII-D
DIII-D has a major effort underway to develop the physics and technology of fast wave electron heating and current drive in conjunction with electron cyclotron heating. The present system consists of a four strap antenna driven by one 2 MW transmitter in the 32--60 MHz band. Experiments have been successful in demonstrating the physics of heating and current drive. In order to validate fast wave current drive for future machines a greater power capability is necessary to drive all of the plasma current. Advanced tokamak modeling for DIII-D has indicated that this goal can be met for plasma configurations of interest (i.e. high [beta] VH-mode discharges) with 8 MW of transmitter fast wave capability. It is proposed that four transmitters drive fast wave antennas at three locations in DIII-D to provide the power for current drive and current profile modification. As the next step in acquiring this capability, two modular four strap antennas are in design and the procurement of a high power transmitter in the 30--120 MHz range is in progress. Additionally, innovations in the technology are being investigated, such as the use of a coupled combine antenna to reduce the number of required feedthroughs and to provide for parallel phase velocity variation with a relatively small change in frequency, and the use of fast ferrite tuners to provide millisecond timescale impedance matching. A successful test of a low power fast ferrite prototype was conducted on DIII-D
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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 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
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Direct electron absorption of fast waves on the D3-D tokamak
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Confinement physics of H-mode discharges in DIII-D
The authors' data indicate that the L-mode to H-mode transition in the DIII-D tokamak is associated with the sudden reduction in anomalous, fluctuation-connected transport across the outer midplane of the plasma. In addition to the reduction in edge density and magnetic fluctuations observed at the transition, the edge radial electric field becomes more negative after the transition. They have determined the scaling of the H-mode power threshold with various plasma parameters; the roughly linear increase with plasma density and toroidal field are particularly significant. Control of the ELM frequency and duration by adjusting neutral beam input power has allowed us to produce H-mode plasmas with constant impurity levels and durations up to 5 s. Energy confinement time in ohmic H-mode plasmas and in deuterium H-mode plasmas with deuterium beam injection can exceed saturated ohmic confinement times by at least a factor of two. Energy confinement times above 0.3 s have been achieved in these beam-heated plasmas with plasma currents in the range of 2.0 to 2.5 MA. Local transport studies have shown that electron and ion thermal diffusivities and angular momentum diffusivity are comparable in magnitude and all decrease with increasing plasma current