Thermalized collisional pre-sheath detected in dense plasma with coherent and incoherent Thomson scattering

Abstract

In the direct vicinity of plasma-facing surfaces, the incident plasma particles interact with surface-recombined neutrals. Remarkably high near-surface plasma pressure losses were observed in the high-flux linear plasma generator Magnum-PSI. Combining the incoherent and coherent Thomson scattering diagnostics, we directly measured particle, momentum and energy fluxes down to 3 mm from the plasma target surface. At the surface, the particle and total heat flux were also measured, using respectively an in-target Langmuir probe and thermographic methods. The near-surface momentum and energy losses scale with density, and amount to at least 50 % and 20 %, respectively, at ne = 8•10 20 m −3 . These losses are attributed to the efficient exchange of charge, momentum and energy between incident plasma and surface-recombined neutrals. In low-temperature plasmas with sufficient density, incident particles go through several cycles of interaction and surface deposition before leaving the plasma, thereby providing an effective alternative dissipation channel to the incident plasma. Parallel plasma parameter profiles exhibit a transition with increasing plasma density. In lowdensity conditions, the plasma temperature is constant and near-surface ion acceleration is observed, attributed to the ambipolar electric field. Conversely, deceleration and plasma cooling are observed in dense conditions. These results are explained by the combined effect of ionneutral friction and electron-ion thermal equilibration in the so-called thermalized collisional pre-sheath. The energy available for ambipolar acceleration is thus reduced, as well as the upstream flow velocity. In the ITER divertor, enhanced near-surface p-n interaction is expected as well, given the overlap in plasma conditions. Including these effects in finite-element scrape-off layer models requires a near-surface resolution smaller than the neutral mean free path. This amounts to 1 mm in Magnum-PSI, and possibly an order of magnitude smaller in ITER.

Advances in Magnum-PSI probe diagnosis in support of plasma–surface interaction studies

“…Also, it has been shown that the polytropic coefficient γ can have a significant variation in the presheath and sheath regions [29], from γ = 3 in the sheath to much lower values in the presheath. In the current estimation, formula (2) was used with γ = 1, assumption that worked well in previous studies [19]. It turns out that the decrease of the electron density and temperature in the presheath is compensated by the increase of the polytropic coefficient.…”

“…This is because the EEDF at the target is perturbed by strong temperature and density gradients parallel to the magnetic field lines, mostly in the highenergy tale because the hot electrons have a lower collision frequency. The probe characteristics plotted in figure 10 were not taken under detached but significant axial temperature and density gradients were recently reported [18,19] over 1-1.5 cm in front the target of Magnum-PSI.…”

“…The influence of the potential fluctuations on the T e overestimation by probes was discussed in [36] for two other linear plasma generators, NAGDIS-II and Pilot-PSI. In addition to plasma fluctuations, even the plasma flow (this is the case with Magnum-PSI [19]) can distort the probe characteristic [35]. It is a difficult task to discern the individual contribution of the above-mentioned reasons, but their combination leads to the overestimation of the electron temperature measured by probes at the target of Magnum-PSI.…”

“…In the divergent magnetic field configuration of the Magnum-PSI, plasma parameters have been measured by incoherent Thomson scattering (TS) [12,13] and electrical probes [13,17]. Plasma sustained by the superconducting magnet has been investigated so far by incoherent [11,18,19] and coherent [19] TS, optical emission spectroscopy [7], resistive bolometry [20], reciprocating probe [9] and angled-tip Langmuir probes [21]. However, the database containing plasma parameters is continuously updated in order to cover a wide range of operating conditions, in support of plasma-surface interaction studies.…”

Influence of porosity and blistering on the thermal fatigue behavior of tungsten

“…Table 2 summarizes the key loading parameters, in which the H + ion fluence was calculated with a collisionless sheath model using the measured TS data [32]. In the calculation, T e = T i was assumed as justified by coherent TS measurements [33,34].…”