Graphite Limiter Research Articles

Effect of atomizer surface type on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry

The effect of pyrocoated graphite, uncoated graphite, metal-carbide, and metal atomization surfaces on the quantitation of molybdenum and ytterbium by electrothermal atomization atomic absorption spectrometry was investigated. The peak shape was affected by heating rate and the different surfaces gave different shapes. Except for the case of uncoated graphite, the sensitivities and detection limits were similar for all surfaces. In a sodium chloride matrix it is preferable to use uncoated graphite for molybdenum because an ashing stage greater than the boiling point of sodium chloride can be used without loss of molybdenum. Tube lifetime depended on atomization temperature, atomization time and the matrix.

Impurity origin during ICRF heating in the JIPP T-IIU tokamak

Replacing stainless steel limiters by graphite limiters, we found that radiations from iron and titanium ions were significantly reduced. Total radiation and loop voltage also decreased. This indicates that the limiters are the major impurity sources both in the ohmic and RF heating phases. Although titanium radiations increased with RF power injected by an antenna with a titanium Faraday shield, the maximum intensity was much smaller than in the experiment where the titanium-flashed stainless steel limiters were used. Thus it has been found that the Faraday shield is less important as an impurity source than limiters. Toroidal asymmetry observed for O II radiation suggest that the energetic charge-exchange neutrals play a role in releasing oxygen from the wall and that those energetic particles are relatively abundant in the toroidal sections near the antenna.The Hα + Dα radiation decreases during the RF pulse around the limiter, which may be due to the change in hydrogen/ deuterium recycling at the limiter. The reduction of Hα + Dα is greater with graphite limiters than with stainless steel limiters. The relation between recycling and impurity release is briefly discussed.

Graphite limiters use in TFR

Graphite limiters have been used in TFR. They prove to be satisfactory for tokamak operation, when fixed as tightly bolted tiles. However, strong efforts must be undertaken to cope with the problems related to their large hydrogen content as well as their mediocre surface structure. This has led us to install a limiter bakeable to 600°C in TFR. Radiofrequency antennae protection had also to be changed from inconel or titanium to graphite to avoid metallic redeposition on the main limiter. Conditions of recycling differ from those met with inconel limiters. The impurity content of the plasma is now dominated by carbon and oxygen. Moreover, large desorption occurs during the ICRH pulse. Strong toroidal asymmetries were noticed as far as the radiation and charge exchange power losses are concerned, with a large enhancement in the vicinity of the limiter. A high neutral pressure (one order of magnitude higher than elsewhere) is created around the limiter. A global energy balance of the discharge was deduced from both bolometric, spectroscopic and limiter surface temperature measurements. The radiative loss is increased when carbon limiters are used and may exceed 70% of the total input. ICRH experiments led to a noticeable power deposition in the scrape-off layer, independent of the mechanism of wave absorption in the bulk.

Graphite limiter damage due to runaway electrons and abrupt Ip termination in doublet III

Two kinds of plasma-related damage, spotty damage and wide area damage, have been photographically recognized on the TiC-coated surface of the new graphite primary limiter in Doublet III. TV and infrared-camera observations revealed the relationship of these damages to the plasma operations. The spotty damage was generated by a single runaway discharge. The runaway electron energy was deposited not only on the surface but also penetrated into the bulk graphite. Thermal analysis showed that runaway electron of 5–6 MeV penetrates into the limiter to approximately 1.5–2.0 cm. Wide area damage was caused by one or more successive discharges which exhibited rapid termination of the plasma current. In this case, the electron energy was deposited only on the limiter surface.

Surface composition and topography of the graphite limiter in ASDEX

After having been in use during about 150 discharges the graphite limiter of the ASDEX tokamak was removed and analyzed by means of SEM, EIXE, FIXE, RBS and Mössbauer Spectroscopy. The surface was seen to be covered by a nonuniform layer of 1016–1018 metal at/cm2, primarily in the form of droplets with diameters up to a few μm. Simulation experiments suggest that the formation of these droplets is caused by heating. The relative metal concentrations in the droplets are close to those for stainless steel. The Mössbauer spectroscopy analysis indicates, however, that the major part of the metal is present in carbide phases, most probably (Fe, Cr, Ni)3C.On parts of the limiter the metal coverage shows a sharp maximum at the points nearest the main plasma, flanked by two minima. The minimum on the ion drift side is much broader than the one on the electron drift side. However, these features are not common to all parts of the limiter.

Characterization of impurities deposited on the PDX graphite rail limiter

The surface contamination of a graphite rail limiter recently removed from the PDX tokamak has been characterized using ion-beam, electron-beam and X-ray analyses. The major impurities detected were Ti and Fe, with the Ti present in the form of a carbide. The distribution of these elements indicate that the erosion and deposition processes balance at a distance one cm from the apex of this limiter (the point of contact with the plasma) for Ti and at about two cm for Fe. The importance of TiC formation on the conditioning process is discussed.

An evaluation of limiter configurations in ZT-40M

Three limiter configurations were evaluated in the reversed-field pinch, ZT-40M. A bare graphite mushroom-shaped probe located in a pump port measured changes in the energy flux profiles resulting from three different limiter configurations. The first configuration was a graphite segmented poloidal ring limiter located 90° toroidally upstream from the mushroom probe, and it protruded into the plasma chamber ~ 0.95 cm beyond the bellows. The second configuration consisted of three graphite tiles which were positioned in the bottom of the convolution section immediately upstream from the mushroom probe. These tiles were electrically floating with respect to the metal liner and protruded radially inwards ~1.27 cm beyond the bellows. For the third configuration four graphite tiles were placed on each side of the pump port section where the mushroom probe was located. These tiles extended 25 cm in both toroidal directions from the pump port section and protruded ~ 0.95 cm into the plasma region. These tiles were also electrically floating. The introduction of the poloidal ring limiter resulted in the mushroom probe absorbing 28% less energy at the major radial position where the tip of the mushroom was even with the shadow of the liner bellows. The tile limiters produced an additional, but small, reduction in the energy absorbed at this location. The tiles produced an increase in the plasma resistance and radiation late in the discharge. Significant amounts of arcing occurred between the tiles and the liner. The experimental results led to a configuration which utilized four graphite poloidal ring limiters. These four limiters protected the pump port areas; however, the occasional increase in the liner temperature at a location approximately halfway between two limiters was not eliminated. The fraction of input energy radiated during the constant-current phase of the discharge increased from 20% to 30% with the four poloidal limiters, owing to an increase in electron density. During start-up, the energy radiated per electron was less for the four-ring case than for the case with no limiters, indicating that the discharge may have been cleaner with the four rings.

Surface and subsurface analysis of limiter and divertor plates by photoacoustic and photothermal techniques

The product kρc is the relevant thermophysical parameter for the heating up of material walls in contact with plasma and, apart form the emissivity, it is also the decisive thermal parameter for the quantitative IR thermography; here k, ρ, and c are the thermal conductivity, density and specific heat capacity, respectively, of the wall material. The Photoacoustic Calorimetry (PAC) provides the diagnostic means to measure this quantity kρc directly. First results of PAC measurements with graphite limiter plates exposed in ASDEX to about 600 discharges are presented and compared to the unused graphite material. Further prospects of the nondestructive photoacoustic techniques in plasma-surf ace interaction problems are discussed, in relation to coatings/claddings and redeposition layers.

Tritium inventory and permeation in TFTR

The control of tritium in TFTR has both important safety and environmental implications. Current modelling techniques allow realistic predictions of the tritium inventory in and permeation through in-torus components. The source of the tritium was computed using a three-dimensional description of the neutral particle flux on the TFTR vacuum vessel wall from the neutral transport code DEGAS, under plasma conditions modelled with the one-dimensional transport code BALDUR. The movable limiter (graphite) was modelled using an extension of the Local Mixing Model for hydrogen retention and isotope exchange. In particular, the calculations consider the inventory and recycling for the movable limiters (which are expected to experience transient temperatures up to 2000°C) as well as the bumper limiters and protective plates which will remain somewhat cooler. Transient effects in the bulk graphite limiter and in the stainless steel wall were modelled using the DIFFUSE code with materials parameters taken from the most recent literature. It appears that for the expected material properties, specifically the hydrogen recombination constant measured in a clean TFTR environment, and operating scenario, there will be essentially no tritium permeation through the stainless steel walls or bellows and less than 0.1 kCi of tritium inventory in the stainless steel first wall. The limiters may contain as much as 5 kCi of tritium.

Thermal energy confinement scaling in PDX limiter discharges

Experiments were performed on the PDX tokamak to study plasma heating and beta scaling with high-power, near-perpendicular neutral-beam injection. The data taken during these experiments were analysed, using a time-dependent data interpretation code (TRANSP), to study the transport and thermal confinement scaling over a wide range of plasma parameters. This study focuses on results from experiments with D0 injection into H+ plasmas using graphite rail limiters, a = 40–44 cm, R = 143 cm, Ip= 200–480 kA, BT = 0.7–2.2 T, and, typically, n̄e = (2.5–4.2) X 1013cm−3. The results of this study indicate that for both Ohmic and neutral-beam-heated discharges the energy flow out of the plasma is dominated by anomalous electron losses which are attributed to electron thermal conduction. The ion conduction losses are not inconsistent with neoclassical theory; however, the total ion loss influences the power balance significantly only at high toroidal fields and high plasma currents. Therefore, except for these cases, the total thermal energy confinement time for the neutral-beam-heated discharges follows the scaling of electron energy confinement. While the confinement is found to have little dependence on toroidal field, it is a strong function of plasma current, increasing with increasing Ip. In contrast, Ohmic confinement times are found to scale as n̄eq½. Because of the dominant effect of the electrons, the total confinement is found to depend heavily on the variation of the electron thermal diffusivity, χe, in the outer region of the plasma (a/2 ≤ r ≤ 7a/8). The values of χe in this region of the plasma show an approximately inverse dependence on the local value of the poloidal magnetic field.

Improved performance of TiC-Coated graphite limiters by surface texturing

TiC-coated graphite limiters are currently in wide use as tokamak limiters. These limiters usually suffer a groove type of damage caused by plasma disruptions. A surface texturing treatment is described that reduces the occurrence of this type of damage.

Conditioning and power-handling of the cooled graphite limiter on PDX

A single graphite rail limiter was used for high power neutral beam injection experiments on PDX from February to July, 1982. The uncoated water-cooled limiter, with a total area of 800 cm 2, performed well in discharges of up to 7 MW of injected power. It was initially conditioned with high power ohmic discharges which removed absorbed and adsorbed impurities. After conditioning, carbon was the primary impurity which evolved from the limiter during high power neutral beam heated discharges. Visible Bremsstrahlung measurements of Z eff were employed to determine the effectiveness of the limiter in maintaining clean plasmas during neutral beam heating and as a measure of the conditioning of the limiter with time. Operation for a period of several months was required to condition the limiter surface to the point where neutral beam injection of 7 MW for 250 ms resulted in only a small rise in Z eff. Thermocouple measurements and a nonlinear cooling model were used to estimate both the absorbed power and the corresponding surface temperatures of the limiter. For the series of discharges studied, the power absorbed by the limiter was found to increase with input power at lower plasma currents, but was almost constant at large plasma current and beam power.

Heat flux to the limiter during disruptions and neutral beam injection in Doublet-III

The heat flux to the Doublet-III primary limiter has been monitored during plasma disruptions and during neutral beam injection. The surface temperature of the movable TiC-coated graphite limiter was measured with an Inframetrics thermal imaging system and a suitably filtered silicon photodiode spot detector. In addition, the floating electric potential of the limiter with respect to the vacuum vessel was measured. The heat pulse duration to the limiter was measured by the spot detector with a time response of ∼ 10 μs and these times were correlated with the plasma parameters. In limiter discharges, 20% of the plasma kinetic stored energy goes to the limiter during disruptions. The power balance during disruptions is also discussed. During neutral beam injection, the limiter is not heated uniformly; the ion drift side receives much more thermal flux than the electron drift side. The fraction of beam power going to the limiter is as high as ∼ 35% in normal limiter discharges.

Initial conditioning of the TFTR vacuum vessel

We report on the initial conditioning of the Tokamak Fusion Test Reactor (TFTR) vacuum vessel prior to the initiation of first plasma discharges, and during subsequent operation with high power Ohmically heated plasmas. Following evacuation of the 86-m3 vessel with the 104 1/s high vacuum pumping system, the vessel was conditioned by a 15-A dc glow discharge in H2 at a pressure of 5 mTorr. Rapid-pulse discharge cleaning was used subsequently to preferentially condition the graphite plasma limiters. The effectiveness of the discharge cleaning was monitored by measuring the exhaust rates of the primary discharge products (CO/C2H4, CH4, and H2O). After 175 h of glow discharge treatment, the equivalent of 50 monolayers of C and O was removed from the vessel, and the partial pressures of impurity gases were reduced to the range of 10−9–10−10 Torr.

Graphite paste electrodes: Effects of paste composition and surface states on electron-transfer rates

A new understanding of the nature of electrode reactions at graphite paste electrodes has been obtained by studying electron-transfer rates of redox systems under conditions of carefully controlled electrode composition and pretreatment. Dry graphite gives electron-transfer rates which give an almost Nernstian response and approach those obtained with platinum. The addition of any pasting liquid decreases these rates materially. Both electrochemical and chemical oxidative pretreatments of such pastes increase the electron transfer in the direction of the “dry” graphite limit. This effect appears to correlate with the increased hydrophilic nature of the electrode surface and concomitant removal of organic layers from the electrode surface.

Gas recycling studies on the TFR tokamak

Gas recycling has been studied in a large number of tokamaks for several years. On TFR, a previous experiment with an inconel limiter has shown that, after many discharges in hydrogen, when we change the working gas to deuterium the wall adsorbed isotope accounts for less than 5% after a few shots. In order to improve our better knowledge of the gas recycling we have performed similar experiments with two different limiters — an inconel limiter and a graphite limiter — at room temperature and with hot walls (250 ° C < T< 300 ° C). With the graphite limiter, when the filling gas is changed from 100% H 2 to 100% D 2, the isotopic ration H (H + D) is between 25 and 30% after five shots and reaches the limit of ~12% after ten shots. This ratio decreases more rapidly with hot walls but the lower limit is higher: 10 and 20% with inconel and graphite limiters respectively, instead of 5% and 12% at room temperature. This recycling coefficient is always higher with hot walls; this consequently should reduce the plasma edge temperature. The sputtering and the thermal load on the wall and the limiters might then also decrease.

Test of a toroidal large area limiter in the ASDEX tokamak

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.

Changes in limiter surface temperature profiles for small radius plasmas in doublet III

Ohmically heated discharges with a minor radius of 44 cm, 33 cm, and 24 cm have been produced in Doublet III. The 44 cm minor radius plasmas interact with multiple Inconel limiters on the inner wall and a single titanium carbide coated graphite limiter on the outer wall. The 33 and 24 cm discharges are produced by inserting the outer limiter 10 and 20 cm respectively. For the smaller minor radius plasma discharges, the plasma interacts only with outer limiter and the total power flux to the limiters can be monitored. The confinement characteristics of these discharges is reported elsewhere.

A Semi-empirical Law for Sputtering in Plasma Devices

A general law is given for sputtering coefficients as a function of the energy of hydrogen and helium isotope ions incident on metallic and graphite walls and limiters. The average over a Maxwellian spectrum is performed analytically.

Thermal load on the TFR Tokamak limiter during additional heating pulses and major plasma disruptions

Inconel and graphite have been tested as limiter materials in the TFR Tokamak. Their behaviour during MHD activity and plasma current disruptions in high density low impurity content plasmas has been studied. The discharge energy balance with auxiliary heating has been established by using infrared measurements of the limiter temperature increase and bolometric techniques. Measurements of the temperature distribution on different limiters show that the maximum limiter temperature can be reduced by a large factor by an appropriate choice of the limiter shape and of its total area. The characteristics of the scrape-off layer in the limiter shadow has also been investigated in different limiter configurations. The experimental results and observations indicate a low physical and chemical sputtering of the graphite limiters during plasma discharges and their good properties for thermal shocks during major disruptions.