High-performance plasmas after pellet injections in Wendelstein 7-X

Abstract

A significant improvement of plasma parameters in the optimized stellarator W7-X is found after injections of frozen hydrogen pellets. The ion temperature in the post-pellet phase exceeds 3 keV with 5 MW of electron heating and the global energy confinement time surpasses the empirical ISS04-scaling. The plasma parameters realized in such experiments are significantly above those in comparable gas-fuelled discharges. In this paper, we present details of these pellet experiments and discuss the main plasma properties during the enhanced confinement phases. Local power balance is applied to show that the heat transport in post-pellet phases is close to the neoclassical level for the ion channel and is about a factor of two above that level for the combined losses. In comparable gas-fuelled discharges, the heat transport is by about ten times larger than the neoclassical level, and thus is largely anomalous. It is further observed that the improvement in the transport is related to the peaked density profiles that lead to a stabilization of the ion-scale turbulence.

Nonlinear gyrokinetic PIC simulations in stellarators with the code EUTERPE

“…For this purpose, we have selected an experimental program carried out in the standard magnetic configuration of W7-X (configuration Ref_170_EIM). We choose the W7-X experimental program 20181016_37, which has been extensively studied in (Bozhenkov et al 2020), is well diagnosed, and was used for power balance analysis. In this program, ECRH heating in the X2-mode was used, reaching an injected power of 5 MW.…”

“…The focus of previous gyrokinetic analysis (Xanthopoulos et al 2020) has mainly been FIGURE 18. Temperature and density profiles from W7-X program 20181016_37 (Bozhenkov et al 2020), for times around t = 4 s. The experimental density profile and another one with double the density are shown.…”

“…put on the external region of the plasma, r/a > 0.5, where significant changes in profiles are produced in the post-pellet phase which lead to increased confinement (Bozhenkov et al 2020). However, we are now interested in testing the use of the NCSTs using experimentally realistic parameters with affordable resources.…”

“…The stability of these profiles to ITG modes was confirmed with linear simulations (not shown here). In a later time after the pellet injection, the plasma relaxes to a poorer confinement state, with a flat density profile (with n < 3.5 × 10 19 m −3 ) and peaked ion and electron temperature profiles at the same time (with central values T i ∼ 1.5 keV and T e ∼ 3 keV), characteristic of ECH heated discharges in OP1.2, which features a situation with ITG instability (see Bozhenkov et al 2020). The profiles used in our simulations are shown in figure 18 and are characteristic of the normal confinement phase in that discharge,around t = 4s.…”

Turbulence Mechanisms of Enhanced Performance Stellarator Plasmas

“…(ii) The PEP phase t ¼ ð1.65 − 1.75Þ s, taking place shortly after the pellet injections. During this phase, the experimental heat transport is reduced almost to neoclassical levels, as a power balance analysis indicates [21], and the diamagnetic energy W dia reaches its maximum value following a sharp increase. In Fig.…”