Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators

Abstract

The edge magnetic structure in the Helically Symmetric eXperiment (HSX) and Wendelstein 7X (W7-X) stellarators has been shown to have a significant impact on the particle fueling and exhaust of the plasma main species (hydrogen) as well as impurity helium. For HSX, the plasma sourcing to exhaust ratio, quantified by the effective and global particle confinement times τ * p and τ p,H , respectively, increases when a magnetic island chain is located in the plasma edge. The fueling efficiency is reduced by 25% when the plasma boundary is deformed by the magnetic islands. The X-point geometry also yields higher plasma temperatures in front of the main recycling region. When the island is moved radially inward, both τ * p and τ p decrease by 10 − 25% depending on plasma density. The τ p,H results rely heavily on EMC3-EIRENE modeling which confirms reduced fueling efficiency due to more rapid ionization in the outward shifted island position. These findings suggest that for a helically optimized system like HSX, the plasma fueling from the recycling source, as well as from active gas injection, can be controlled by the magnetic island chain in the plasma edge-which is a basic requirement for a divertor system. This process is also effective for the control of effective helium exhaust times, as τ * p,He measured by perturbative gas puff experiments is reduced by up to 40% when the islands are shifted inwards. For Wendelstein 7-X, a similar reduction of τ * p,He was inferred when magnetic islands were moved from the far plasma edge into the confined plasma region. However, the effective confinement features of H as the main plasma species were not affected due to the non-optimal position of the magnetic islands with respect to the highly localized ionization domain during the limiter startup campaign.

Radiative edge cooling experiments in Wendelstein 7-X start-up limiter campaign

“…In the increased iota OP1.1 configuration the 5/5 islands move inwards and become visible in the far SOL. The 5/6 island, located inside of the last closed flux surface, is moved inwards too and is now located in the particle source region, thus interacting with the recycling flux as main plasma particle source [23]. The plasma heating is provided in this initial phase only by electron cyclotron resonance heating (ECRH) with 6 gyrotrons, delivering a maximum power of 4.3 MW.…”

Limiter observations during W7-X first plasmas

“…These imperfection locations were present in pre-run visible light images, and behaved as if they had reduced thermal contact to the bulk material. Even though filtered line emission measurements looked directly at the limiter with good temporal resolution (100 kHz) (see figure 5(b)), and were used to determine neutral and impurity densities and particle fluxes [13], in this case we know the filterscope sightline did not include the carbon bloom locations. A further issue was that the 2.5 nm bandwidth interference filters were used, and in the case of H-alpha, there is a nearby competing C-II line at 657.8 nm, which could cause spectral pollution.…”

First attempt to quantify W7-X island divertor plasma by local experiment-model comparison

“…Therefore, it is expected that a combination of the code and local diagnostics is able to provide a more complete and quantitative physical picture of the 3D island divertor plasma. In fact, the EMC3-Eirene code has been routinely applied to W7-X to interpret experimental results, covering a variety of physical topics [1,[5][6][7][8][9][10][11][12][13]. This work is less physicsoriented.…”