The investigation of structure, chemical composition, hydrogen isotope trapping and release processes in deposition layers on surfaces exposed to DIII-D divertor plasma

Abstract

The exposure of ATG graphite sample to DIII-D divertor plasma was provided by the DiMES ( Divertor Material Evaluation System) mechanism. The graphite sample arranged to receive the parallel heat flux on a small region of the surface was exposed to 600ms of outer strike point plasma. The sample was constructed to collect the eroded material directed downward into a trapping zone onto s Si disk collector. The average heat flux onto the graphite sample during the exposure was about 200W/cm2, and the parallel heat flux was about 10 KW/cm2. After the exposure the graphite sample and Si collector disk were analyzed using SEM, NRA, RBS, Auger spectroscopy, IR and Raman spectroscopy. The thermal desorption was studied also. The deposited coating on graphite sample is amorphous carbon layer. Just upstream of the high heat flux zone the -redeposition layer has a globular structure. The deposition layer on Si disk is composed also from carbon but has a diamond-like structure. The areal density of C and D in the deposited layer on Si disk varied in poloidal and toroidal directions. The maximum DIC areal density ratio is about 0.23, maximum carbon density is about 3.8*10'8cm-2, maximum D areal density is about 3*1017~m2. The thermal desorption spectrum had a peak at 1250K.

Modification of the graphite tile surface in tokamaks

“…The modification kinetics of the divertor tile surface under irradiation have been well observed on an ATJ graphite sample exposed in the DIII-D tokamak at extremely high cyclic loading [11]. The ATJ graphite sample was exposed in the DIII-D divertor plasma for 3 s in each of four consecutive discharges using the DIMES-DUST system [10]. The graphite sample with a protruding edge 1 mm high was installed in the bottom part of the DIII-D tokamak divertor at a major radius of 148.5 cm.…”

“…These fine-grained graphites have a high density of about 1.8 g cm −3 and a thermal conductivity of about 100-140 W m −1 K −1 . The research carried out on mock-ups [4,5] in real conditions of operating tokamaks [6][7][8][9][10][11] reveals the considerable modification of the irradiated surface after exposure (figures 1(a) and (b)). The surface modification essentially depends on the load value, on its duration, on the temperature and, in particular, on the presence of oxygen molecules in the vacuum volume [4].…”

“…No appreciable modification of its surface was observed during tests in the DIII-D divertor plasma, even for tests in the DIMES-DUST system [5,12,13]. ATJ graphite in these conditions undergoes considerable modification [10]. On the RGT sample surface, only very fine particles of titanium carbide up to 1 µm in size are recorded.…”

Application of RGT graphite in fusion devices

“…Samples from the RGT-type graphite were tested under the conditions of the divertor plasma of the DIII-D tokamak in divertor material evaluation system (DiMES) dust experiments [7], and also under ion bombardment of the gas-discharge plasma [3,4]. During the exposure under divertor conditions, a sample was installed on the bottom of the DIII-D tokamak divertor at a major radius of 148.5 cm.…”

“…During the exposure under divertor conditions, a sample was installed on the bottom of the DIII-D tokamak divertor at a major radius of 148.5 cm. The DiMES sample (figure 1) had a leading edge and was arranged in the divertor region to take up the parallel heat flux only on the leading edge, which is a small part of the sample surface [7]. The sample also had a slot and a disc collector located under the slot to collect an ablated material sample during the exposure.…”