# TJ-II:Search for physical mechanisms that lead to increase of turbulence following pellet injection

2017 Spring

## Proposal title

Search for physical mechanisms that lead to increase of turbulence following pellet injection

## Name and affiliation of proponent

Alexander Zhezhera and the HIBP team Naoki Tamura, K. McCarthy and the TESPEL / pellet team Thomson, reflectometry, edge teams

Kieran McCarthy

## Description of the activity, including motivation/objectives and experience of the proponent (typically one-two pages)

Motivation. Core plasma fuelling experiments, using cryogenic hydrogen pellets in the TJ-II stellarator, have shown that the radial redistribution of particles can be understood qualitatively from neoclassical predictions. In particular, a density peaking due to ablation is initially observed outside the core followed by the peaking moving inwards.[1][2]

The influence of impurity pellets (TESPEL) on core plasma turbulence and plasma profiles has been recently investigated using the dual HIBP system now in operation in TJ-II. Experiments show a transition from the electron (inwards $E_r$) to ion (outwards $E_r$) root as well as a change in the density profile (reduction in the level of hollowness) and temperature (with a significant drop from 1 keV to 300 eV in the central temperature) once TESPEL is injected in ECRH plasmas. Interestingly, in parallel, broadband turbulence (up to 800 kHz) is strongly amplified while low frequency fluctuations (below 20 kHz as proxy of ZFs) decrease (see figure).

HIBP TESPEL example

Objectives. The current proposal intends to investigate and identify possible physical mechanisms that could lead to the increase of broadband fluctuations after TESPEL/cryogenic pellet injection and its role on transport:

1. Is the drop in low frequency fluctuations a consequence of the collisional damping of ZFs with a concomitant increase of broadband fluctuations?
2. Is the increase in broadband fluctuations a consequence of the change in density / temperature profiles with the consequent triggering of plasma instabilities like TEM? [3]

In addition, turbulence spreading from the plasma core (triggered by TESPEL / pellet) to the plasma boundary region will be investigated.

## If applicable, International or National funding project or entity

Enter funding here or N/A

## Description of required resources

Required resources:

• Number of plasma discharges or days of operation:
• Essential diagnostic systems:

The dual HIBP system is a key diagnostic to measure density, potential and temperature fluctuations amplitudes as well as phase relations to measure LRC. Large cryogenic pellets (Type-3 o 4) or 300 μm TESPELs to modify plasma profiles sufficiently (large $n_e$ increase with reduced $T_e$). Doppler reflectometry to characterize plasma fluctuations and radial electric fields in the gradient region. Thomson profiles to characterize the influence of TESPEL / pellets on plasma profiles. Langmuir probes to measure density and potential in the plasma boundary region.

• Type of plasmas (heating configuration):
• Specific requirements on wall conditioning if any:
• External users: need a local computer account for data access: yes/no
• Any external equipment to be integrated? Provide description and integration needs:

## Preferred dates and degree of flexibility

Preferred dates: (format dd-mm-yyyy)

## References

1. J L Velasco et al., Plasma Phys. Cont. Fusion (2016)
2. K. McCarthy et al., 26th IAEA Int. Conf. on Fusion Energy (Kyoto, 2016)
3. M. Shats, J. H. Harris, K. M. Likin et al., Physics of Plasmas (1995)