TJ-II:Impurity transport studies by LILA-TOF detection. A Lithium Laser-Ablation based Time-of-Flight (LILA-TOF) diagnostic for measuring plasma edge ion temperature and toroidal plasma rotation

Experimental campaign

Autumn 2021

Proposal title

Impurity transport studies by LILA-TOF detection: A Lithium Laser-Ablation based Time-of-Flight (LILA-TOF) diagnostic for measuring plasma edge ion temperature and toroidal plasma rotation

Name and affiliation of proponent

López-Miranda, B., Tabarés, F., Baciero, A., Ochando, M. A., Medina, F., Tafalla. D., McCarthy, K. J., Laboratorio Nacional de Fusion, CIEMAT (Spain)

Details of contact person at LNF

belen.lopez.miranda@ciemat.es

Description of the activity

Background:

Although several methods for the measurement of Ti at the edge of TJ-II have been tried (RFA, He beam, Doppler spectroscopy…) [1-7] no systematic recording of this important parameter has been achieved so far. A new method for studying the thermalization and transport of injected impurities at the edge of hot plasma, (considering the last closed magnetic surface, the free path is between 1 to 2 cm approx.) under no perturbative conditions, was developed [8] using a Nd:YAG laser called Lithium Laser-Ablation based Time-of-Flight (LILA-TOF) technique. Ion temperatures deduced from the application of this technique (25 eV in the edge) are in good agreement with previous measurements in TJ-II by Retarding Field Analyzer (RFA) (17-21 eV in the SOL) [9].

Description:

In the proposed technique, a Nd:YAG laser is used to ablate Li from the lithiated wall of the stellarator TJ-II. While the laser pulse allows for the analysis of the released species through Laser-Induced Breakdown Spectroscopy (LIBS) [10], its laser pulse also provides a time reference for the Time-of-Flight (TOF) measurements of the Li+ ions performed. This is done by positioning light detection systems sensitive to an intense Li II spectral line at different toroidal locations away from such a source. TOF times of tens to hundreds of microseconds are recorded. Fast recording of the TOF of the Li+ emission toroidally away from the source will provide the energy distribution of the thermalized particles. Then, by de-convolving the shape of the recorded light pulse, the velocity distribution of the lithium-ion during its thermalization with the background plasma can be extracted. From this velocity distribution, the ion temperature of the background ions and the toroidal rotation at the plasma periphery can be deduced. The viability of injecting Li using LIBS and recording the lithium traces of the plasma-generated Li+ ions to study ion toroidal, and, consequently, deducing the TOF using its propagation has been demonstrated [10]. The preliminary values of Ti and vrot deduced are consistent with those measured with other diagnostics in TJ-II. Several limitations have been identified as the influence of plasma toroidal rotation. This parameter has a direct impact on the shape of the TOF trace. Consequently, this proposal is oriented to obtain a more accurate estimation of the rotation and Ti would be performed by simultaneously recording the lithium traces at two toroidal locations, on opposite sides of the injection point whenever available. Reproducible plasmas are required to compare the Ti obtained using TOF measurements in the different TJ-II monitors located in opposite sectors.

International or National funding project or entity

This work was funded by the projects from the Spanish Ministerio de Ciencia e Innovación RTI2018-100835-B-I00 (MCIU/AEI/FEDER, UE) and PID2020-116599RB-I00.

Description of required resources

Required resources:

 Number of plasma discharges or days of operation: 3 days

 Essential diagnostic systems: Nd:YAG laser, spectroscopic system, Bolometric arrays, X-Ray detectors, VUV spectrometer, Thomson Scattering, Doppler reflectometer.

 Type of plasmas (heating configuration): standard configuration (100_44_64), ECRH, this scenario requires a constant high line averaged density, similar to March 24th, 2021.

 Specific requirements on wall conditioning if any: fresh-lithiated wall.

 External users: need a local computer account for data access: 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] Kocan, K., Gunn, J. P., Komm, M., et al. 2008. Rev. Sci. Instr. 79 073502

[2] Katsumata, I. and Okazaki, M., 1967. Jpn. J. Appl. Phys 6 123

[3] Zurro, B., Vega, J., Castejón, F., et al. 1992. Phys Rev Lett. 69 2919

[4] McCarthy, K. J., Zurro, B., Balbín R., et al., 2003. Europhys. Lett., 63 (1), 49-55

[5] Pitcher, C. S. and Stangeby, P. C., 1981. Plasma Phys. Control. Fusion 31 1305

[6] Tabarés, F. L., Tafalla, D., Ferreira J. A., et al. 2010. Rev. Sci. Instr. 81 10D708

[7] Tabarés, F. L. and Tafalla, D., 2015. Nucl. Fus. 55 013020

[8] López-Miranda, B., Tabarés, F. L., McCarthy, K. J., Baciero, A., D. Tafalla, F. L., et al. 2021. Plasma Phys. Control. Fusion, submitted.

[9] Nedzelskiy I. S., Silva C., Fernández H., et al., 2009. Probl. Atom. Sci. Tech. 1, 174

[10] López-Miranda, B., Zurro, B., Baciero, et al., (2018). Investigation of the toroidal propagation of lithium injected by LIBS into TJ-II plasmas to measure edge ion temperature. EPS Conference Prague.

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