Title: First lithium experiments in HIDRA and evidence of helium retention during quasi-steady-state stellarator plasma operations

Abstract

Recent experiments in Hybrid Illinois Device for Research and Applications (HIDRA) have had operational discharges between t_discharge = 60 and 1000 s using electron cyclotron resonant heating (ECRH) of the plasma. This means that quasi-steady-state plasma discharges reach conditions to study long-pulse plasma material interactions (PMIs). The newly commissioned HIDRA-Material Analysis Test-stand PMI diagnostic is used to place a drop of lithium onto a heated tungsten surface, transfer the sample in-vacuo and expose it in a helium plasma. Helium is of interest as there is an open question to whether lithium will be able to remove helium ash in real fusion devices. The introduction of the W-Li sample in HIDRA resulted in evaporation of lithium into the helium plasma during a 600 s pulse and caused a reduction of over 90% in neutral pressure during the discharge. It was also observed that the plasma density and temperature increased by over 2.5 times. Using spectroscopy and a helium collisional radiative model, the peak temperature and density of the helium plasma can be monitored during the discharge. During lithium evaporation, as significant lithium ionization occurs, there is a 85% drop in the HIDRA vessel neutral pressure, despite a constant flow rate ofmore » He gas. This reduction in neutral pressure is supported by spectroscopy data with corresponding reductions in He I line intensities (587 nm, 667 nm, 706 nm, and 728 nm), as well as those of other impurities. At one point in the discharge a lithium plasma is created, as indicated by an increase in Li⁺ emission and a complete reduction in He⁺ emission, but the electron density jumps from ne = 3 × 10¹⁸ m^–3 to over ne = 8 × 10¹⁸ m^–3 while the core temperature stays relatively constant between Te = 16 eV and 20 eV. Once lithium has completely evaporated from the sample and the majority of the ionized lithium has diffused from the plasma to the vessel walls, pressure and spectroscopy data paired with He collisional radiative model calculations shows a re-establishment of a helium plasma in a low recycling regime. In this regime, the density drops down to ne = 2 × 10¹⁸ m^–3 and the electron temperature increases from T_e = 20 eV to over T_e = 50 eV indicating an increase in helium heating efficiency. This is also indicated by the He+ emission re-establishing and having a higher intensity. Here, we show the results from the first lithium campaign in HIDRA. In the presence of lithium, and in particular when lithium ions are present, the helium disappears from the plasma via an as of yet unknown complex relationship that needs to be further studied. The most likely explanation is that the lithium ions are distributed around the vessel and able to trap helium to the surface turning HIDRA into a large gettering surface. These results have potential implications on future plasma facing component design using liquid lithium for impurity and recycling control using limiters and divertors.« less

Authors:

Andruczyk, Daniel  ^[1];

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Shone, Andrew ^[1]; Koyn, Zachariah ^[2]; Allain, Jean Paul ^[3]

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+ Show Author Affiliations

1. Univ. of Illinois at Urbana-Champaign, IL (United States)
2. Energy Driven Technologies LLC, Champaign, IL (United States)
3. Pennsylvania State Univ., University Park, PA (United States)

Publication Date:

Wed Jul 06 00:00:00 EDT 2022

Research Org.:

Energy Driven Technologies LLC, Champaign, IL (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1905582

Grant/Contract Number:  

SC0017719

Resource Type:

Accepted Manuscript

Journal Name:

Plasma Physics and Controlled Fusion

Additional Journal Information:

Journal Volume: 64; Journal Issue: 8; Journal ID: ISSN 0741-3335

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; HIDRA; helium; lithium; fusion; PMI; plasma