R. Lehmer

A fast scanning probe for DIII–D

A fast reciprocating probe has been developed for DIII–D which can penetrate the separatrix during H mode with up to 5 MW of NBI heating. The probe has been designed to carry various sensor tips into the scrape‐off layer at a velocity of 3 m/s and dwell motionless for a programmed period of time. The driving force is provided by a pneumatic cylinder charged with helium to facilitate greater mass flow. The first series of experiments have been done using a Langmuir probe head with five graphite tips to measure radial profiles of ne, Te, φf, ne, and φf. The amplitude and phase of the fluctuating quantities are measured by using specially constructed vacuum compatible 5‐kV coaxial transmission lines which allow us to extend the measurements into the MHz range. TTZ ceramic bearings and fast stroke bellows were also specially designed for the DIII–D probe. Initial measurements will be presented.

Plasma and neutral dynamics in a simulated tokamak gas target divertor

A stationary, detached ionization front is observed in an experimentally simulated divertor plasma (n≤3×1019 m−3, kTe≤20 eV) interacting with a hydrogen gas target. With a neutral hydrogen density, n0≊2×1021 m−3, the electron temperature at the simulated divertor target is reduced to kTeu2009target≊2.5 eV. Up to 97% of the electron heat flux (≤7 MW/m2) is dissipated by dissociation and ionization losses and hydrogen line radiation. The plasma pressure is observed to peak near the ionization front, and a plasma flow reversal is observed in the region of reversed pressure gradient. Classical momentum flow parallel to the magnetic field and anomalous cross‐field particle transport are found. The plasma flow is strongly damped by ion–neutral collisions and is subsonic. Numerical results from a one‐and‐one‐half dimensional (11/2‐D) coupled plasma–neutral fluid model (incorporating radial particle transport, recycling, and neutral gas injection) agree well with the experimental data, and indicate that the electron ...

Experimental simulation of the gaseous divertor concept in PISCES-A

Abstract 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 2 × 10 13 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 2 × 10 3 .

Presheath profiles in simulated tokamak edge plasmas

Abstract Steady state magnetized plasmas produced by the PISCES experiment are used to study plasma-wall interaction phenomena relevant to confinement devices such as tokamaks. An experimental investigation of the presheath region that extends from a wall surface into “simulated tokamak” edge plasmas along magnetic field lines is reported. Diagnostics especially developed for this work include a fast-scanning multiple Langmuir/Emissive/Mach probe system and a CID camera imaging system. Measurements of density, electron temperature, floating potential, space potential, and bulk plasma flow velocities have been obtained in 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. Plasma density profiles along the magnetic field typically show a characteristic factor of 2 decrease towards the wall surface. A plasma potential variation in the near presheath zone of order 0.5 T e is measured, consistent with the bulk plasma flow approaching the ion sound speed near the wall surface, as inferred from a simple “free fall” model. A Boltzmann model for the presheath density profile accuracy tracks the density profile measured both by the Mach probe and by spectroscopic means. Flow profiles are used as a consistency check on various magnetized Mach probe theories. Results suggest that cross-field transport of parallel momentum through viscosity is relatively unimportant in PISCES plasmas and thus may be unimportant in tokamak boundary layer plasmas. Discharges with non-thermal electrons display axial profiles of space potential and floating potential which indicate a “hotter” electron distribution function near the wall surface, consistent with “colder” electrons being reflected by the presheath potential drop.

E×B transport in the DIII-D boundary plasma

We have measured the electrostatic turbulence and associated particle transport in the DIII-D boundary plasma using a fast reciprocating Langmuir probe array located on the outboard midplane. Both the normalized rms fluctuation levels (density and floating potential) and the fluctuation-driven particle transport are altered by the L-H transition in the SOL. At the separatrix, the density fluctuation level is reduced a factor of 2, consistent with reflectometry results. There is a corresponding decrease in the turbulent particle flux. Deeper in the SOL, the turbulent particle transport in H-mode exceeds the L-mode value. The perpendicular diffusion coefficient D ⊥ and particle confinement time τ p have been estimated, assuming that the transport is purely turbulent and uniform on a flux surface. We find D ⊥ =0.7 D B (L) and 0.04 D B (ELM-free H), and τ p =54 ms (L) and 480 ms (ELM-free H).

Scrape-off layer measurements in DIII-D

In this paper, scrape-off layer measurements in DIII-D are presented as a function of the main discharge plasma parameters. A systematic study is under way to understand and predict the behavior of the edge and divertor plasma in DIII-D and this scaling behavior will be crucial for the design of ITER. To facilitate the studies, a fast reciprocating Langmuir probe incorporating five graphite tips was installed at the midplane of DIII-D which has the capability of performing multiple plunges 1 cm inside the separatrix during 5 MW of NBI. Density and temperature profiles in the midplane (reciprocating probe), near the top (Thomson scattering) and at the lower divertor plate (fixed Langmuir probe array) are compared by mapping the measurements into magnetic flux coordinates. Local pressure measurements are compared on different parts of a flux surface. The three different local measurements also indicate the spatial evolution of plasma conditions as plasma approaches the divertor plate. Ohmic and L-mode discharges exhibit similar (exponential) density and temperature decay in the scrape-off layer. H-mode discharges, however, display a faster spatial decay reflecting at least a factor of 3 decrease in the perpendicular diffusion coefficient. Consistency of the magnitude and scaling behavior of the edge profile parameters with models of the scrape-off layer is examined.

Impurity transport and retention in a gas target divertor: simulation experiments in PISCES-A and modeling results

Impurity retention in the gaseous divertor regime is investigated in the PISCES-A facility at UCLA. We report measurements and 1 1/2D fluid modeling results of impurity transport for typical tokamak divertor plasma parameters (10 18 ≤ n e ≤3×10 19 m −3 , kT e ≤20 eV). The neutral hydrogen density close to the (simulated) divertor target is 10 20 ≤ n 0 ≤3×10 21 m −3 . Gaseous trace impurities (argon, neon) as well as low- Z and high- Z materials sputtering carbon, tungsten) are studied. It is observed that the impurity retention in a gaseous divertor is substantially improved as compared to conventional divertor operating regimes. The modeling results suggest that the retention of neutral and ionized impurities is mainly due to collisions with hydrogen (deuterium) neutrals and ions streaming towards the divertor target a a velocity of 0.25–0.5 c s . A low level of residual impurity transport, observed at high neutral density, is attributed to a plasma flow reversal close to the radial boundary. Sputtering of a tungsten sample by intrinsic impurities has been shown to decrease substantially for target electron temperatures kT e

Plasma diagnostics for the DIII-D divertor upgrade

The DIII‐D tokamak is being upgraded to allow for divertor biasing, baffling, and pumping experiments. This paper gives an overview of the new diagnostics added to DIII‐D as part of this advanced divertor program. They include tile current monitors, fast reciprocating Langmuir probes, a fixed probe array in the divertor, fast neutral pressure gauges, and Hα measurements with TV cameras and fiber optics coupled to a high‐resolution spectrometer.

Low energy data on radiation enhanced sublimation of graphite

Abstract Erosion of POCO graphite by helium in PISCES-A was measured by carbon spectroscopy at temperatures from 550°-2000°C, energies of 30–250 eV and fluxes of 1−2×10 22 m −2 s −1 , Yields at low energies were higher than predicted in current models. The effect of redeposition is discussed.

Novel technique for edge plasma control by RF ponderomotive force

Abstract 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 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 eV) is confirmed by the reflection of an electron beam injected in vacuum with an RF power of ~1 kW. 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 Δ ⊥ Ψ rf × B drift to make a diverter configuration in confinement devices is discussed.