# TJ-II: Spectroscopic studies of TESPEL-shell ablation clouds

Spring 2022

## Proposal title

Spectroscopic studies of TESPEL-shell ablation clouds

## Name and affiliation of proponent

Nerea Panadero, Kieran McCarthy, Belén López-Miranda, Isabel García Cortés, Daniel Medina, Julio Hernández

Laboratorio Nacional de Fusión, CIEMAT

N/A

## Description of the activity

Tracer-encapsulated solid pellets (TESPELs) are used to inject a known amount of tracer impurity particles inside the plasma at a well-localised radius. This is done with the aim of investigating impurity transport and accumulation in helical plasmas [1]. Over the past few years, quite an effort has been put into adapting the Hydrogen Pellet Injection 2 (HPI2) code [2] for impurity pellets such as TESPEL (C8H8 shells). The ablation module of this code has already been successfully modified. However, simulation of the homogenisation processes for impurities is not straightforward. A first step in the calculation would be estimating plasmoid initial parameters, such as size, density, temperature, or ionization state, using the value of local ablation. Still, since obtaining the ionization state of the C is a significantly complicated task, some simplifications are mandatory. For instance, the problem could be considerably reduced if including in the model one or two ionization states were sufficient. Prior theoretical analysis (although solely for carbon pellets) point out to C II, C III and C IV states, at least in the early stage. However, C IV has not been detected so far as suitably intense emission lines need to be detected in machine sector close to the TESPEL injection port. To support such simplifications, further experimental input would be necessary [3]. With this in mind, TESPEL injection experiments using C8H8 shells only are proposed for TJ-II. The idea is to inject ≤300 μm diameter TESPELs without tracers (C8H8 only) into ECRH plasmas. The evolution of the TESPEL shell ablation cloud would be followed with photodiodes and a fast camera to localize C ionization states during the early phase of the homogenization phase. Prior to that, emission lines of interest must be identified using a visible spectrometer that views the initial ablation process in order to identify interference filters for use with the fast camera. In the second phase, injections can be made to obtain fast-frame images of the ablation processes of interest. Reproducible plasmas with line-densities of the order $4.5 x 10^{18} m^{-3}$ are needed. Injections can be made close to the end of discharges to avoid problems due to the increased plasma density that occurs after a TESPEL injection.

## International or National funding project or entity

Studies of fuelling and impurity control in the stellarators TJ-II and W7-X through the use of cryogenic and TESPEl pellets (Estudios de abastacimiento y control de impurezas en los stellarators mediante el uso de perdigones criogenicos y TESPEL", Ref: PID2020-116599RB-I00

## Description of required resources

Required resources:

• Number of plasma discharges or days of operation: 3
• Essential diagnostic systems: TESPEL injector, visible spectrometer, VUV spectrometer, fast camera, TS
• Type of plasmas (heating configuration): ECRH
• Specific requirements on wall conditioning if any:
• External users: need a local computer account for data access: No
• Any external equipment to be integrated? No

## References

1. N. Tamura et al., Plasma Fusion Res. 10 (2015) 1402056
2. B. Pegourié et al., Nuclear Fusion 47 (2007) 44
3. M. Goto et al., J. Phys. B 43 (2010) 144023