W. Poschenrieder

Physical and chemical sputtering of graphite and SiC by hydrogen and helium in the energy range of 600 to 7500 eV

The erosion of pyrolytic graphite and silicon carbide due to the bombardment with monoenergetic hydrogen ions with energies of 600 to 7500 eV has been investigated in the temperature range of near room temperature to 750°C. The erosion yield of SiC is about 10−2 and shows no pronounced temperature dependence. In contrast to SiC the erosion yield of pyrolytic graphite shows a maximum at a temperature of about 600°C. The ratio of the maximum erosion yield to that at room temperature depends on the energy of the hydrogen ions and increases from about 11 at 3000 eV to 32 at 670 eV. The production of CH4 during the bombardment of the graphite has been found proportional to the erosion yield. When graphite was bombarded with He ions no hydrocarbon production and no temperature dependence of the erosion yield could be observed. The results are compared with values for the erosion yields of carbon by thermal atomic hydrogen taken from literature.

Conditioning of asdex by glow discharge

Based on previous experience with conditioning of vessels for plasma experiments by a dc glow discharge, a very simple, safe, and effective technique was developed for ASDEX. It is characterized by a low working pressure of 5 × 10−3mb of H2 at which no violent arcing or damaging parasitic discharges occur and it utilizes anodes in a fixed position well within the limiter shadow. The efficiency of the glow discharge is monitored by a differentially pumped mass spectrometer. The results show a high efficiency in removing carbon from the vessel. Total amounts pumped out mainly as CO and CH4 are of the order of grams corresponding to hundreds of monolayers. Surface analysis of samples yield less than 30% of a monolayer after several hours glow discharge conditioning, but recontamination is seen to be significant. We also find a water production rate corresponding to the removal of one monolayer per hour during the glow discharge. An investigation of the current and pressure dependence of the impurity removal rates show that the chosen conditions are quite optimal.

Determination of ion temperatures in the edge plasma from ion flux transmission of apertures

Abstract The flow of ions through an aperture perpendicular to a strong magnetic field has been calculated for a Maxwellian velocity distribution. The results obtained are needed for the interpretation of time resolved analysis of fluxes using the collecting probe technique and also for in situ mass, energy, and charge state analysis with spectrometers. Moreover, the energy dependent transmission of the aperture can be used to determine ion temperatures. Deuterium, carbon and metal impurity ion temperatures in the limiter shadow plasma of TFR 600 have been evaluated from measured ion flux transmission. For deuterium a temperature of 100 eV was found on the electron side of the collector. At the ion side the temperatures are higher by a factor of two. Impurity ion temperatures are ambiguous to the extent of the knowledge on the charge state. Assuming singly charged ions for Ni + a temperature of 10 eV on the ion side and 4 eV on the electron side was found during high density discharges with an Inconel limiter. In case of a carbon limiter the C + ion temperature on the ion side was about 35 eV.

Divertor efficiency in ASDEX

Abstract The divertor efficiency in ASDEX is discussed for ohmically heated plasmas. The parameters of the boundary layer both in the torus midplane and the divertor chamber have been measured. The results are reasonably well understood in terms of parallel and perpendicular transport. A high pressure of neutral hydrogen builds up in the divertor chamber and Franck-Condon particles recycle back through the divertor throat. Due to dissociation processes the boundary plasma is effectively cooled before it reaches the neutralizer plates. The shielding property of the boundary layer against impurity influx is comparable to that of a limiter plasma. The transport of iron is numerically simulated for an iron influx produced by sputtering of charge exchange neutrals at the wall. The results are consistent with the measured iron concentration. First results from a comparison of the poloidal divertor with toroidally closed limiters (stainless steel, carbon) are given. Diverted discharges are considerably cleaner and easier to create.

Low energy neutral particle fluxes to the walls of ASDEX during He and D2 discharges

Neutral particle fluxes onto the walls of ASDEX have been investigated using a time-of-flight (TOF) method. The energy distributions of the neutrals could be determined in the range of 10–1000 eV/amu. Ohmic divertor and limiter discharges with equal plasma currents and densities have been compared for He and D2. The He0 outflux at ∼2000 eV from He discharges is 110 of the corresponding D0 flux in D2 discharges. At lower energies this difference is much smaller. In all cases many more He neutrals were observed than was anticipated from the CX rate-coefficients for He2+. The impurity fluxes due to sputtering by the CX-neutrals show no significant difference for He and D2 discharges. For divertor discharges CX-sputtering can fully account for the Fe impurity content determined spectroscopically.

Studies on Impurity Retainment in the ASDEX Divertor

Abstract The retainment and exhaust capability of the ASDEX divertor for neon and argon is investigated during ohmic heating and neutral injection heating. No pronounced influence of the scrape-off layer on the divertor particle outflux is observed, though ionization of neutrals in the divertor throats is most likely. The divertor outfluxes behave as if they are determined by the molecular flow conductances of the divertor throats. The plasma impurity outfluxes show strong top-bottom asymmetries which reverse with the direction of the toroidal field. The exhaust capability can be demonstrated by comparison with limiter discharges. The exhaust efficiency of the double-null configuration is about twice that of a single-null configuration.

Mass Spectrometry and Pressure Measurements as a Diagnostic Tool on ASDEX

Abstract Total and partial pressure measurements during tokamak experiments, which may be called neutral gas diagnostics, are a method of long standing. Still, several aspects, such as response optimization, more refined evaluation of outgassing measurements and data interpretation during discharges appear not to have found quite adequate coverage yet.

Test of a toroidal large area limiter in the ASDEX tokamak

Abstract A toroidal large area limiter was installed in the divertor tokamak ASDEX. The limiter basically consists of a plane annular stainless steel sheet which is tangential to the plasma and which is placed in the bottom of the main plasma chamber. In a second series of experiments this limiter was covered with graphite plates. With the steel limiter clean discharges could not be obtained. Energy deposition on the limiter was only 12% of the input. With the graphite limiter Z eff was below 3, energy deposition on the limiter was 25% (up to 40% in low density operation). Energy deposition was toroidally symmetrical. Results, including some experiments with a single null divertor are presented.

Retention of gaseous and target produced impurities in the ASDEX divertor chamber

This letter reports on two experiments undertaken to evaluate the retention of gaseous and target produced impurities in the ASDEX divertor. The retention for gaseous impurities was determined by puffing Ar into the main chamber and simulating the time behaviour of the Ar XVI line intensity with a time dependent impurity transport code including a simple divertor model. During Ohmic heating a factor of 3 and 4.5 enhancement of impurity retention if found relative to the vacuum time constant (90 ms) of the divertor chamber, for ne = 2 × 1013 cm−3 and ne = 3.5 × 1013 cm−3, respectively, while a drastic breakdown of the retention occurs during high power NI heating. – To deduce the retention of impurities generated at the divertor plates, a segment (3.5%) of the plates was covered with copper, a metal previously not used in ASDEX. By measuring the Cu influx at the target plates and the line intensity of the Cu XX line (11.38 A) in the core plasma and by using the transport code, it is found that during NI heating (ne ≤ 2 × 1013 cm−3) Cu atoms originating from the target plates have a ≤ 3.5 times higher probability to penetrate into the core plasma than if they had when originating from the main chamber walls.

Confinement and stability in ASDEX discharges close to the beta limit

The highest β-values derived from diamagnetic flux measurements in H-discharges are found to be close to the β-limit due to kink and ideal ballooning modes. Both energy and particle confinement are degraded in these discharges and do not recover during the injection period. The energy confinement time decreases by a typical factor of two, the electron thermal diffusivity being correspondingly enhanced. Electron heat conduction is again found to be the dominant energy loss channel. The degradation of confinement close to the ideal-MHD limit suggests that kink and ballooning instabilities occur and are responsible for the enhanced transport. This conclusion is supported by the correlation with the pressure profile and by the result that global and local transport do not change in a reference discharge which is stable against ideal-MHD modes.