[en] A new method to reduce plasma heat flow to a limiter head has been developed with the use of a Radio Frequency (RF) field barrier. The RF current drawn on the limiter surface produces a ponderomotive potential, Ψ_rf of the order of the temperature of the plasma near the limiter head in the frequency range ω_ci<ω<<ω_ce. The limiter head is protected from the plasma by this potential barrier. Experimental proof of the RF limiter concept was attempted using the PISCES-A facility with plasma parameters which are comparable to the edge plasma of typical tokamaks. Production of the RF potential (∼20 V) is confirmed by the reflection of an electron beam injected in vacuum with an RF power of ∼1 kW. Spectroscopic and Langmuir probe measurements show decrease and modification of the edge plasma density by the RF power. As a result of the RF field formation, a remarkable suppression of low frequency fluctuations has been observed at the plasma periphery, which is expected to reduce the plasma transport in the boundary layer. The experimental results are analyzed using numerical particle orbit and fluid drift calculations. Possible application of plasma flow due to ∇Ψ_rf x B_o drift tokamak a divertor configuration in confinement devices is discussed. (author)

[en] Deuterium plasma recycling and chemical erosion behavior of selected graphite materials have been investigated using the PISCES-A facility. These materials include: Pyro-graphite; 2D-graphite weave; 4D-graphite weave; and POCO-graphite. Deuterium plasma bombardment conditions are: fluxes around 7 x 10¹⁷ ions s^-1cm^-2; exposure time in the range from 10 to 100 s; bombarding energy of 300 eV; and graphite temperatures between 20 and 120 degree C. To reduce deuterium plasma recycling, several approaches have been investigated. Erosion due to high-fluence helium plasma conditioning significantly increases the surface porosity of POCO-graphite and 4D-graphite weave whereas little changes for 2D-graphite weave and Pyro-graphite. The increased pore openings and refreshed in-pore surface sites are found to reduce the deuterium plasma recycling and chemical erosion rates at transient stages. The steady state recycling rates for these graphite materials can be also correlated to the surface porosity. Surface topographical modification by machined-grooves noticeably reduces the steady state deuterium recycling rate and the impurity emission from the surface. These surface topography effects are attributed to codeposition of reemitted deuterium, chemically sputtered hydrocarbon and physically sputtered carbon under deuterium plasma bombardment. The co-deposited film is found to have a characteristic surface morphology with dendritic microstructures

[en] Deuterium plasma recycling and chemical erosion behavior of selected graphite materials have been investigated using the PISCES-A facility. These materials include: Pyro-graphite; 2D-graphite weave; 4D-graphite weave; and POCO-graphite. Deuterium plasma bombardment conditions are: fluxes around 7 /times/ 10¹⁷ ions s/sup /minus/1/cm/sup /minus/2/; exposure time in the range from 10 to 100 s; bombarding energy of 300 eV; and graphite temperatures between 20 and 120/degree/C. To reduce deuterium plasma recycling, several approaches have been investigated. Erosion due to high-fluence helium plasma conditioning significantly increases the surface porosity of POCO-graphite and 4D-graphite weave whereas little change for 2D-graphite weave and Pyro-graphite. The increased pore openings and refreshed in-pore surface sites are found to reduce the deuterium plasma recycling and chemical erosion rates at transient stages. The steady state recycling rates for these graphite materials can be also correlated to the surface porosity. Surface topographical modification by machined-grooves noticeably reduces the steady state deuterium recycling rate and the impurity emission from the surface. These surface topography effects are attributed to co-deposition of remitted deuterium, chemically sputtered hydrocarbon and physically sputtered carbon under deuterium plasma bombardment. The co-deposited film is found to have a characteristic surface morphology with dendritic microstructures. 18 ref., 4 figs., 1 tab

[en] The PISCES plasma surface interaction facility at UCLA generates plasmas with characteristics similar to those found in the edge plasmas of tokamaks. Steady state magnetized plasmas produced by this device are used to study plasma-wall interaction phenomena which are relevant to tokamak devices. We report here progress on some detailed investigations of the presheath region that extends from a wall surface into these /open quotes/simulated tokamak/close quotes/ edge plasma discharges along magnetic field lines

[en] The PISCES plasma surface interaction facility at UCLA generates plasmas with characteristics similar to those found in the edge plasmas of tokamaks (n = 10¹¹-10¹³ cm^-3, T_e = 3 - 30 eV). Steady state magnetized plasmas produced by this device are used to study plasma-wall interaction phenomena which are relevant to tokamak devices. We report here progress on some detailed investigations of the presheath region that extends form a wall surface into these simulated tokamak edge plasma discharges along magnetic field lines. A number of edge plasma diagnostics have been specially developed for this work and are now becoming fully operational including a fast-scanning multiple Langmuir/Emissive/Mach probe system and a CID camera imaging system. Both systems allow detailed diagnoses of a 10x10x10 cm³ region of magnetized plasma in contact with a wall surface. Measurements of plasma density, electron temperature, floating potential, space potential, and bulk plasma flow velocities have been obtained in hydrogen, helium, and argon plasmas with densities ranging from 10¹² to 10¹³ cm^-3, electron temperatures from 5 to 15 eV, and axial magnetic fields of 0.2 to 1.4 kG

[en] Measurements of equilibrium and turbulent plasma parameters in the far edge and scrape-off layer region of CCT H-mode discharges are presented. The results indicate a large reduction in cross-field particle transport rates during the transition from Ohmic to H-mode confinement. The reduced transport is due in part to changes in saturated fluctuation amplitudes as well as changes in the nature of the edge turbulence. In H-mode, the peak cross-field particle flux is reduced by a factor of 3-6 over the flux in an Ohmic discharge. These changes in edge transport rates are similar to changes in global particle confinement time. Analysis of the radial profiles indicates that poloidal and/or toroidal variations in transport play an important role in the overall particle transport. This paper presents measurements of equilibrium and fluctuating plasma parameters made during a CCT H-mode transition. (author) 3 refs., 2 figs

[en] Gas target or radiative divertors are under consideration for next generation tokamaks like ITER and TPX. A significant reduction of the parallel heat flux in a gas target has been demonstrated previously in PISCES and elsewhere. A reduction of the peak heat flux and the divertor electron temperature by gas puffing has been demonstrated in DIII-D. Energetic electrons can carry a substantial part of the parallel electron heat flux in a divertor plasma. We have experimentally investigated a regime where up to 90% of the total electron heat flux (Q_||^e ≤ 5 MW/m²) is carried by energetic electrons (n_eh/n_ec ≤ 0.1, U_eh ≤ 160 eV). The experiments were carried out in a hydrogen plasma in the PISCES-A linear plasma device at densities n ≤ 10¹⁹ m^-3. The heat flux to the (simulated) divertor target is measured with a biased calorimeter, allowing us to separately determine the target heat fluxes due to ions as well as cold and hot electrons. It is demonstrated that the energetic electron heat flux can be attenuated by nearly two orders of magnitude with active gas injection through the target. Neutral densities n_H2 ≥ 10²¹ m^-3 are required. Results from a 1 1/2-D fluid model based on coupled plasma/neutral equations are presented. (author) 8 refs., 4 figs

[en] A new plasma-surface interactions research facility: PISCES-B has been designed and constructed at University of California, Los Angeles. The entire vacuum chamber is bakable and a base pressure of the order of 10^-8 Torr is attainable using two turbo molecular pumps with a total pumping speed of 6000 l/s. The PISCES-B facility can generate continuous plasmas of argon, helium, hydrogen, deuterium and nitrogen. The density of these plasmas ranges from 1 x 10^-11 to 3 x 10^-13 cm^-3 and the electron temperature ranges from 3 to 51 eV. The plasma bombardment flux to a target surface inserted in the plasma column can be varied from 1 x 10¹⁷ to 8 x 10¹⁸ ions cm^-2 s^-1. Due to the high pumping speed, the neutral pressure of the working gas during plasma generation is controllable in the wide range from 3 x 10^-5 to 1 x 10^-3 Torr. These conditions are similar to those seen at the limiter and divertor areas in toroidal fusion devices. Using the PISCES-B facility, first materials erosion experiments have been conducted on 3% boronized graphites and iso-graphites as the reference materials. The chemical sputtering yield due to hydrogen plasma bombardment at 300 eV for 3% boronized graphite has been found to be about 30% smaller than that for iso-graphites at temperatures from room temperature to 900 degree C. Also, radiation enhanced sublimation due to hydrogen plasma bombardment at 400 eV is found to be suppressed by 20-30% for 3% boronized graphite at temperatures up to 1300 degree C. No significant surface composition change is observed after hydrogen plasma bombardment to a fluence of the order of 10²² ions cm^-2. 30 refs., 9 figs., 2 tabs

[en] The concept of the reentrant divertor (or gaseous divertor) has been suggested as a possible solution to the divertor heat load problem in next generation tokamaks. The idea of the reentrant divertor is to redistribute the divertor heat flux over a large surface area by radiation and/or elastic and inelastic collisions with neutral particles. Simulation experiments are performed in the PISCES-A linear plasma device to test the basic concept and to evaluate the axial and radial particle and heat transport. To data, data have been obtained in steady state hydrogen and argon plasmas at densities of up to 2x10¹³ cm^-3 and electron temperatures of 5-30 eV. With moderate gas feed (10 mTorr) to a simulated divertor slot (length 90 cm) we have observed the electron temperature to decrease axially from 25 eV to 3 eV. At higher neutral pressure (>25 mTorr) a neutralizer regime is found, where the plasma density at the simulated divertor target can be reduced by more than two orders of magnitude. Radial plasma loss is proportional to the neutral pressure and greatly enhanced as compared to the Bohm rate and the classical diffusion rate. The axial plasma heat flux to the divertor target is reduced by a factor of up to 2x10³. (orig.)

[en] The concept of the reentrant divertor (or gaseous divertor) has been suggested as a possible solution to the divertor heat load problem in next generation tokamaks. The idea of the reentrant divertor is to redistribute the divertor heat flux over a large surface area by radiation and/or elastic and inelastic collisions with neutral particles. Simulation experiments are performed in the PISCES-A linear plasma device to test the basic concept and to evaluate the axial and radial particle and heat transport. To date, data have been obtained in steady state hydrogen and argon plasmas at densities of up to 2x10¹³ cm^-3 and electron temperatures of 5-30eV. With moderate gas feed (10 mtorr) to a simulated divertor slot (length 90 cm) we have observed the electron temperature to decrease axially from 25 eV to 3 eV. At higher neutral pressure (>25 mtorr) a neutralizer regime is found, where the plasma density at the simulated divertor target can be reduced by up more than two orders of magnitude. Radial plasma loss is proportional to the neutral pressure and greatly enhanced as compared to the Bohm rate and the classical diffusion rate. The axial plasma heat flux to the divertor target is reduced by a factor of up to 10³