TJ-II:Imaging of pelet cloud dynamics in TJ-II using Halpha and bremsstraahlung filters and a fast-frame camera

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Experimental campaign

Spring 2022

Proposal title

Imaging of pellet cloud dynamics in TJ-II using Halpha and bremsstrahlung filters and a fast-frame camera

Name and affiliation of proponent

Gabor Kocsis, CRC-Budapest and Tamás Szepesi, CRC-Budapest

Details of contact person at LNF

Kieran J McCarthy

Description of the activity

This aim of this proposal is to investigate the interaction of cryogenic hydrogen and impurity pellets (TESPEL) with stellarator plasma by evaluating fast framing video data. In early 2020, LFS TESPEL and hydrogen pellet injection experiments were conducted on TJ-II but these were cut short by the pandemic. Movies (frame-rates up to 700 kHz) with TOP and TANGENTAL views were recorded using different filters (C I, C III, Hα, Li II). It was seen that the drifting pellet clouds extend broadly along the magnetic field lines. In the case of the impurity pellet, which travels inwardly into the plasma at a constant speed, the C+2 cloud surrounding a TESPEL is small and spherical. In was determined that plasmoids (partialy ionized clouds) are launched frequently (every 5-10µs) - as expected - and drift outwards from the plasma at a speed of approximately 5 km/s. In the case of cryogenic pellets, it was difficult to recognize the individual drifting clouds, despite the 700 kHz frame rate, so perhaps the time resolution was not high enough. Nonetheless, it was concluded that the cloud is hollow around the pellet and drifts outwards. Further experiments are needed in this area to achieve a better understanding of the cloud dynamics. Such information will be compared with cloud dynamics for similar pellets on the stellarator W7-X in order to achieve a more comprehensive understanding. It is proposed to continue these studies in TJ-II with the W7-X fast camera during 2022 with higher frame rates and with Hα and bremmstrahlung filters. Again, TOP and TANGENTIAL views will be needed and pellets will be injected into both ECRH and NBI plasmas. The proponents have significant experience in using fast-frame cameras on tokamaks and stellarators. Recently, they studied pellet cloud drifting (cryogenic and TESPEL) on the stellarator W7-X [1]. Also, the local team have significant experience with pellet injectiona nd fast frame image analysis [2].

International or National funding project or entity

Include funding here: Hungarian and EUROfusion grants plus Spanish PN: PID2020-116599RB-I00.

Description of required resources

Required resources:

  • Number of plasma discharges or days of operation: 4 (2 in March and 2 in May or June)
  • Essential diagnostic systems:Pellet Injector and Fast frame imaging camera with fibre bundle connected to A8:TANGENTIAL and B2:TOP
  • Type of plasmas (heating configuration): ECH and NBI (standard configuration is best)
  • Specific requirements on wall conditioning if any: Recent lithium and boron coatings
  • External users: No
  • Any external equipment to be integrated? Fast-frame camera as in spring 2020

Preferred dates and degree of flexibility

Preferred dates: March 2022 and May or June 2022


References

  1. G. Kocsis et al, Investigation of TESPEL cloud dynamics in Wendelstein 7X stellarator, Nucl Fusion 61 016006 (2021)
  2. N. Panadero et al, Experimental studies and simulations of hydrogen pellet ablation in the stellarator TJ-II, Nucl Fusion 58 026025 (2018)

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