51 research outputs found
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Physics of Target Plasmas in Minimum-B Geometries
Provides efficient targets for accumulation of a hot-ion plasma by energetic neutral injection
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Wall conditions in ORMAK
From surface effects in controlled thermonuclear fusion devices and reactors meeting; Argonne, Illnois, USA (10 Jan 1974). ORMAK is a diffuse toroidal pinch with typical plasma currents of 100 kA, electron temperatures of 800 eV, and ion temperatures of 300 eV. The walls of the plasma region are made of stainless steel coated with an intermediate layer of platinum 0.05 mu thick and an outer 1 to 2 mu layer of gold. Tests with an Ion Microprobe Mass Analyzer have shown that the platinum acts to decrease diffusion of impurities from the stalnless steel to the surface. Gold was chosen to inhibit the surface chemical adsorption of gases. Studies with a movable limiter indicate that electron energy is lost at the plasma edge mainly via line radiation and cooling on ions, while ions are lost from the plasma by charge exchange. Thus the walls are bombarded by energetic neutrals, line radiation and, in addition, bremsstrahlung x-rays. The flux of energetic neutrals is measured by a charge exchange analyzer. Wall bombardment by such neutrals should cause sputtering, and gold has been observed spectroscopically near the limiter, increasing with time during a shot, However, analysis of impurities coated on a window by the discharge indicated very little gold sputtering and re-deposition. To measure the sputterirg rate, a wall sample was coated with 105 A of radioactive gold and bombarded with neutrals from ORMAK during a day's run. No measurable sputtering was found within the counting statistics of the measurement, but surface carbon contamination of the sample prevented any final conclusions. (auth
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Varieties of sawtooth behavior in TFTR plasmas
The side-viewing soft-x-ray camera on the Tokamak Fusion Test Reactor (TFTR) has made possible the observation of several different forms of sawtooth oscillation, which can be categorized according to their position in the plasma, sequence of occurrence, and patterns of associated MHD oscillation. Some insight into the plasma conditions involved can be gained by examining the waveforms in detail, along with electron temperature profiles from electron cyclotron emission measurements
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ISX-A Graphite Limiter Experiment
Graphite limiters were installed and tested in the ISX-A tokamak as part of the ISX-A surface physics program and the TFTR materials research program. The puropse of the experiment was to compare plasma performance using graphite limiters as opposed to the standard ISX-A stainless steel limiters. Heaters were installed in the graphite limiters so that the effects of operation at elevated temperatures could be evaluated
Transport by intermittency in the boundary of the DIII-D tokamak
A271 TRANSPORT BY INTERMITTENCY IN THE BOUNDARY OF THE DIII-D TOKAMAK. Intermittent plasma objectives (IPOs) featuring higher pressure than the surrounding plasma, are responsible for {approx} 50% of the E x B{sub T} radial transport in the scrape off layer (SOL) of the DIII-D tokamak in L- and H-mode discharges. Conditional averaging reveals that the IPOs are positively charged and feature internal poloidal electric fields of up to 4000 V/m. The IPOs move radially with E x B{sub T}/B{sup 2} velocities of {approx} 2600 m/s near the last closed flux surface (LCFS), and {approx} 330 m/s near the wall. The IPOs slow down as they shrink in radial size from 4 cm at the LCFS to 0.5 cm near the wall. The skewness (i.e. asymmetry of fluctuations from the average) of probe and beam emission spectroscopy (BES) data indicate IPO formation at or near the LCFS and the existence of positive and negative IPOs which move in opposite directions. The particle content of the IPOs at the LCFS is linearly dependent on the local density and decays over {approx} 3 cm into the SOL while their temperature decays much faster ({approx} 1 cm)
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ELM Particle and Energy Transport in the SOL and Divertor of DIII-D
A271 ELM PARTICLE AND ENERGY TRANSPORT IN THE SOL AND DIVERTOR OF DIII-D. Results from a series of dedicated experiments measuring the effect of particle and energy pulses from Type-I Edge Localized Modes (ELMs) in the DIII-D scrape-off layer (SOL) and divertor are compared with a simple model of ELM propagation in the boundary plasma. The simple model asserts that the propagation of ELM particle and energy perturbations is dominated by ion parallel convection along SOL fields lines and the recovery from the ELM perturbation is determined by recycling physics. Time scales associated with the initial changes of boundary plasma parameters are expected to be on the order of the ion transit time from the outer midplane, where the ELM instability is initiated, to the divertor targets. To test the model, the ion convection velocity is changed in the experiment by varying the plasma density. At moderate to high density, n{sub e}/n{sub Gr} = 0.5-0.8, the delays in the response of the boundary plasma to the midplane ELM pulses, the density dependence of those delays and other observations are consistent with the model. However, at the lowest densities, n{sub e}/n{sub Gr} {approx} 0.35, small delays between the response sin the two divertors, and changes in the response of the pedestal thermal energy to ELM events, indicate that additional factors including electron conduction in the SOL, the pre-ELM condition of the divertor plasma, and the ratio of ELM instability duration to SOL transit time, may be playing a role. The results show that understanding the response of the SOL and divertor plasmas to ELMs, for various pre-ELM conditions, is just as important to predicting the effect of ELM pulses on the target surfaces of future devices as is predicting the characteristics of the ELM perturbation of the core plasma
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Configuration control, fluctuations, and transport in low-collisionality plasmas in the ATF Torsatron
In low-collisionality plasmas confined in tokamaks and stellarators, instabilities driven by particles trapped in inhomogeneities of the magnetic fields could be important in increasing plasma transport coefficients. In the Advanced Toroidal Facility (ATF), an {ell} = 2, M = 12 field-period stellarator device with major radius R = 2.1 m, average plasma minor radius a = 0.27 m, central and edge rotational transforms {chi}{sub 0} {approx} 0.3, {chi}{sub a} {approx} 1, the effects of electron trapping in the helical stellarator field are expected to be important in plasmas with {bar n}{sub e} {approx} 5 {times} 10{sup 12} cm{sup {minus}3}, T{sub e0} {approx} 1 keV. Such plasmas have already been sustained for long-pulses (20 s) using 150--400 kW of 53.2-GHz ECH power at B = 0.95 T. Transport analysis shows that for {rho} = r/a {le} 1/3, the electron anomalous transport is {le}10 times the neoclassical value, while at {rho} = 2/3 it is 10--100 times neoclassical; this is compatible with expectations for transport enhancement due to dissipative trapped-electron modes. 4 refs., 3 figs
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