In the Pre-Fusion Power Operation phase, ITER will operate both hydrogen and helium plasmas for commissioning and demonstration of high confinement mode without nuclear activation of the vacuum vessel components 1. However, there are some concerns about the evolution of the performance of the tungsten (W) divertor under intense He flux since interaction between He and W Plasma Facing Compenents (PFCs) is known to induce specific structural changes, impacting fuel recycling properties and degrading thermal properties of PFCs.
In this context, the superconducting tokamak WEST ran its first helium plasma operations campaign to study plasma-wall interactions in a full tungsten environment at the end of the last experimental campaign with the inertially-cooled lower divertor with W-coated PFCs. The changeover from deuterium to helium plasma operation was conducted by operating repetitive plasmas in lower single null configuration (Ip=500kA, nl=3-4.1019 m-2), either in ohmic or in L-mode using lower hybrid (PLH = 3MW). The changeover was diagnosed by poloidally resolved visible spectroscopy, Langmuir probes and infrared surface temperature measurements providing information on the dependence of the recycling flux on the ion fluence and surface temperature. Outgassing after each discharge was monitored by penning gauge spectroscopy and Threshold Ionisation Mass Spectrometry (TIMS) 2 providing a global particle balance for each pulse.
Fig. 1 shows the D2 (blue dots) and He (red dots) fraction for three experimental days, comprising four changeovers between D and He operation. Fractions for each shot are obtained from partial pressures measured by penning gauge spectroscopy in the divertor, integrated over both the discharge phase and 25 sec. post-discharge durations for each shot. A 4h He-Glow Discharge Cleaning (GDC) was operated overnight twice (dashed lines) to complete the changeover. Preliminary gas balance analysis indicates that up to 90% of the injected deuterium remains trapped in the plasma facing surfaces, compared to 20% for helium. Although plasma breakdown could be achieved in He, it was attempted in D first. Density control in He plasma was however made difficult by the lower He fuelling needed compared to D. Control of W contamination and MHD, as well as LH coupling, required He-specific tuning of the scenarios during the first changeover, initially limiting the measured He fraction in the exhausted gas to 70% (until shot #55258). The radiated power fraction is also found higher in He plasma compared to D plasmas (70% vs 60%). Whereas impurities are the eroding species in D plasmas, sputtering by He+ and He2+ ions in He plasmas starts to be important, due to their larger impact energy. As a consequence differences in impurity production are observed in He and D plasmas. Hence the intensity of the 400.9 nm WI line at the outer divertor is found higher in He plasmas than in D plasmas. A similar observation is made for the boron radiation (BII at 412 nm) at the outer divertor, whereas the relative intensity of OII line at 435 nm is on the contrary found lower in He discharges.
Changeover from D to He exhibits different characteristic times in the recycling flux and in the exhaust. In the first He plasmas, the helium concentration in the recycling flux, estimated from the DI (434 nm) and HeI (388 nm) line intensities, was found to increase above 75% within the first 10 seconds at the strike line position on the outer divertor. Further away on the outer divertor, at the inner divertor and at the upper divertor, more than 25 seconds are required to achieve similar concentrations. As already mentioned the characteristic time for helium concentration changes in the exhausted neutral gas is longer and seems rather correlated with slow changeover rates of remote surfaces rather than with recycling flux at the strike points. This indicates that changeover is mainly driven by wall pumping during the discharges and partial release of gas after the pulse.
TIMS measurements in the gas released in the post-discharge revealed that the time scale for He and D outgassing was markedly different. An example is given in Fig. 2, where the He and D2 signals are plotted vs. time for shot # 55275. Whereas He outgassing occurs instantaneously at the end of the plasma (t=10 sec.) and decays within 30 seconds until the He signal gets below detection level, D outgassing is delayed by several seconds and lasts for about 10 minutes. These striking differences may be related to different retention and outgassing mechanisms from WEST plasma facing components, with a starting temperature of 350 K that can reach temperature above 950 K at the strike line. The observation that the D outgassing time scale increases with the cumulated He fluence, up to tens of seconds, may suggest that D retention occurs deeper than He in the bulk of plasma facing components. However it rather confirms that deuterium retention and subsequent outgassing occur in remote deposition areas, away from strike lines, as indicated by spectroscopic observations.
1 ITER Research Plan within the Staged Approach, ITR-18-003, 2018
2 C.C. Klepper, T.M. Biewer, U. Kruezi, S. Vartanian, D. Douai, D.L. Hillis, and C. Marcus, Review of Scientific Instruments 87, 11D442 (2016)
“This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.”
|Affiliation||Institute for Magnetic Fusion Research - French Alternative Energies and Atomic Energy Commission|
|Country or International Organization||France|