L.C. Emerson

Confinement improvement in beam heated ISX-B discharges with low-z impurity injection

Abstract Results are reported on improved confinement in the Impurity Study Experiment (ISX-B) neutral beam heated plasmas when a small amount of neon is injected shortly after the start of beam heating. The scaling of energy confinement is modified by the introduction of a dependence on line-averaged density. Calculations show the improvement is primarily caused by a reduction in electron heat conduction.

Studies of hydrogen recycle from the walls in tokamaks using a plasma-wall interaction simulator☆

Abstract The recycle of hydrogen from walls and limiters plays a fundamental role in the operation of tokamaks for several reasons: (1) the average proton confinement time in the plasma is short with respect to plasma pulse length, therefore each proton is, on the average, recycled several times to and from the walls; (2) the walls may elastically or inelastically reflect the hydrogen but, according to our measurements, half or more of the hydrogen incident on the walls may be buried in the wall, thermalized, and most will return to the plasma volume on a time scale which is on the order of 10−1 s. We have studied details of the hydrogen recycle phenomena from 304L stainless steel using a plasma-wall interaction simulator. Low energy (30–200 eV) atomic and molecular hydrogen ions were used. The evolution rate from surfaces bombarded with a flux of 6 × 1015ions/cm2 · s has been measured for several conditions and appears to be limited by complex diffusion phenomena and perhaps, in some cases, by surface recombination rates. The evolution rate reveals effects due to diffusion from the highly supercharged near surface region. Several mechanisms were discussed but it is not possible to identify which are responsible for the prompt desorption processes.

Surface impurities and “clean-up” techniques in ORMAK

Abstract The detrimental role played by impurities in thermonuclear devices is now well-known. Experimental studies to identify those impurities residing on the liner, or first wall, of the Oak Ridge Tokamak (ORMAK) have been carried out in the laboratory using Auger Electron (AES) and X-ray Photoelectron (XPS) spectroscopic techniques. Additionally, liner measurements have been made in situ using a small Soft X-ray Appearance Potential Spectrometer (SXAPS). Oxygen, iron and carbon were found to be the major surface impurities and, as confirmed by plasma diagnostics, also the major plasma impurities. Glow discharge cleaning of gold and stainless steel surfaces has been studied using various gases and gas mixtures. Oxygen discharges are very effective and hydrogen moderately effective in removing carbon and hydrocarbon deposits from both types of surfaces. Other parameters involved in the contamination-decontamination process, such as pressure and temperature, have been studied using techniques to controllably contaminate surfaces with hydrocarbons.

Particle removal with pump limiters in ISX-B

Abstract The first pump limiter experiments were performed on ISX-B. Two pump limiter modules were installed in the top and bottom of one toroidal sector of the tokamak. The modules consist of inertia cooled, TiC-coated graphite heads and ZrAl getter pumps each with a pumping speed of 1000–2000 l/s. The objective of the initial experiments was the demonstration of plasma particle control with pump limiters. The first set of experiments were performed in ohmic discharges (OH) in which the effect of the pump limiters on the plasma density was clearly demonstrated. In discharges characterized by Ip = 110 kA, B T = 15 kG , n e = 1−5 × 10 13 cm −3 and t = 0.3 s, the pressure rise in the pump limiters was typically 2 mTorr with the pumps off and 0.7 mTorr after activating the pumps. When the pumps were activated, the line-average plasma density decreased by up to a factor 2 at identical gas flow rates. The second set of measurements were performed in neutral beam heated discharges (NBI) with injected powers between 0.6 MW and 1.0 MW. Due to a cooling problem on one of the ZrAl pumps, the NBI experiments were carried out with one limiter only. The maximum pressure observed in NBI-discharges was 5 mTorr without activating the pumps, i.e., approximately twice as high as in OH-discharges. The exhaust efficiency, which is defined as the removed particle flux divided by the total particle flux in the scrape-off layer, is estimated to be 5%.

Surface impurities studies during the start-up and early operations of ISX-A using a surface analysis station with sample transfer

Abstract A surface analysis station was installed on ISX-A before start-up which provided data on samples of wall material and a variety of other materials exposed at positions between the first wall and the limiter inner radius. Samples were introduced, from air, into an ultra-high vacuum transport system without disturbing the vacuum in ISX-A or the analysis station. An Auger electron spectrometer in the analysis station along with other diagnostics provided information on the initial “clean up” as well as the subsequent routine operation of ISX-A. During the start-up period substantial sulfur contamination was evident but this was greatly reduced as hydrogen discharge cleaning proceeded. Samples of oxidized stainless steel were not cleaned to an oxygen-free status but the surface was reduced to a thin substoichiometric oxide. Carbon was significantly removed but not eliminated entirely. Routine tokamak operations were monitored by a means of a special composite sample which made possible estimates of both the state of cleanliness of the system and its rate of clean-up.

Initial testing of TiB2 and TiC coated limiters in ISX-B

Abstract Low-Z coatings on graphite substrates have been developed for testing as limiters in the Impurity Study Experiment (ISX-B) tokamak. Laboratory and tokamak testings have been accomplished. The laboratory tests included thermal shock experiments by means of pulsed e-beam irradiation, arcing experiments, and hydrogen and xenon ion erosion experiments. The tokamak testing consisted of ohmically heated plasma exposures with energy depositions up to 10 kJ/discharge on the limiters. The coatings, applied by chemical vapor deposition, consisted of TiB 2 and TiC deposited on POCO graphite substrates. The limiter samples were interchanged through the use of a transfer chamber without atmospheric exposure of the ISX-B tokamak. Limiter samples were baked out in the transfer chamber before use in the tokamak. Provisions for both heating and cooling the limiter during tokamak discharges were made. Initial testing of the limiter samples consisted of exposure to only ohmically heated plasmas; subsequent testing will be performed in neutral-beam-heated plasmas having up to 3 MW of injected power. Bulk and surface temperatures of the samples were measured to allow the determination of energy deposition. Extensive plasma and edge diagnostics were used to evaluate the effect of the limiter on the plasma (e.g. vacuum ultraviolet spectrometry to determine plasma impurity concentrations, Thomson scattering to determine Z effective, IR camera to measure limiter surface temperature, and laser fluorescence spectrometry to determine neutral impurity concentration and velocity distribution in the limiter region).

Coated-limiter testing in tokamaks

Over the past year, extensive operating experience has been gained with coated graphite limiters in the ISX-B and Doublet III tokamaks. A total of approximately 20,000 tokamak discharges have been performed with TiC coated limiters, and the plasma performance and capability with these limiters has been improved over that obtained with metallic limiters. While none of the limiters have failed while in service, the TiC coating and in some cases the graphite substrate have experienced damage. This damage consists of grooves formed in the coating and an area of coating melt and graphite cracking. The groove damage is believed to have been caused by disruptions and the melt region by the high heat loads during neutral beam heated discharges.

Arcing studies in ISX-B

Abstract Results of experimental measurements of arcing in ISX-B are described. Samples of stainless steel were exposed at the plasma edge and arcing currents to the sample were measured. Currents from several amperes up to 100 A were observed. It was possible to trigger arcs during well-behaved discharges by applying a high voltage to the sample. Elemental analysis of surface features of samples has shown that sulfur segregates to surfaces of arc craters in type 304 stainless steel. Detection of sulfur on surface samples may be related to arcing during cleanup processes in tokamaks.

Studies of mechanisms of hydrogen recycle using a plasma-wall interaction experiment

Abstract It is shown that for type 304L stainless steel at room temperature and ion energies of 100 eV a large part of the recycling occurs with hydrogen residence times in the wall of about 100 ms. The use of wall temperatures from 80 to 500 K permits differentiation between thermally activated processes and ion bombardment-induced phenomena. For low energy ions (100 eV) onto 300 K stainless steel walls recycling increases to more than 90% in a few tenths of a second at; 3 × 10 16 ions cm −2 s −1 . For ions up to 300eV at this flux there is little energy dependence. Both thermally- and ion-induced processes can be important at room temperature, but under the conditions of this study thermal processes greatly enhance the recycle rates at 300 and 500 K. Preliminary results indicate that recombination of atomic hydrogen to molecular hydrogen can be rate controlling in some practical regimes.

Time-resolved measurements of impurity deposition in ISX

Abstract Time-resolved measurements of impurity deposition were made in the plasma edge region of ISX-A. A cylindrical sample located 20 mm behind the limiter was exposed to the plasma in synchronization with the discharges to give a time resolution of ~8 ms. The cylinder was withdrawn to an analysis chamber where the impurities deposited as a function of time were determined by Auger electron spectroscopy. Later analysis by Rutherford ion backscattering was used to quantify the results. The primary metallic impurities were identified as titanium, iron and chromium. The time evolution of these impurities is correlated with conditions in the plasma during the discharge. All the impurities observed were found to accumulate on the sample at a minimum rate during the stable central portion of the discharge, and at a maximum rate at times when the plasma was unstable.