TJ-II: Impurity injection by laser blow-off (LBO): Confinement and transport studies of high Z impurity injection by LBO in ion-root regime discharges

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Experimental campaign

Autumn 2021

Proposal title

Impurity injection by laser blow-off (LBO): Confinement and transport studies of high Z Impurity injection by LBO in ion-root regime discharges

Name and affiliation of proponent

López-Miranda, B., Baciero, A., Ochando, M. A., Medina, F., McCarthy, K. J., García-Regaña, J. M., Velasco, J. L., Pastor, I. HIBP group and the TJ-II team, Laboratorio Nacional de Fusion, CIEMAT (Spain)


Details of contact person at LNF

belen.lopez.miranda@ciemat.es


Description of the activity

Motivation.

In previous works [1, 2, 3], we have studied impurity (BC, LiF, BN, Fe) transport mainly in ECRH plasmas heated by ECR [2]. Transport has been measured in low-density regimes (injecting controlled amounts of B or Fe impurities), where the radial electric field is positive, Er > 0, and the plasma is in the electron root. Several limitations were found to inject heavy impurities (Fe, W), particularly in high-density regimes, due to the intrinsic limitation of the cut-off density that interrupts the discharge and the lack of density control due to the absence of a true plateau. Near the value of the density where the transition to a positive electric field occurs (from electron-root to ion-root), an increase in confinement time was observed, but in those discharges with higher densities, the estimated confinement time was much longer than the duration of the discharge and it was not possible to deduce any value for the confinement time. Here we try to study the confinement time in ion-root regimes using LBO in order to investigate the behaviour of heavy impurities injected both in electron-root regimes and in ion-root regimes TJ-II plasmas. We also would compare the experimental results with the predictions of neoclassical transport in order to figure out the mechanisms involved in these processes.


Proposal.

We plan to inject heavy impurities (Fe, W) by means of LBO into TJ-II discharges. This scenario requires a constant high line averaged density (0.9×1019 m-3), similar to 48223. For this, the confinement times of the impurities and the transport coefficients will be estimated after injecting impurities of different masses. The confinement times of the impurities will be deduced from the decay of different radiation signals. Thanks to the reconstructions of bolometric and X-ray radiation profiles [4, 5], an attempt will be made to deduce the diffusion (D) and transport (v) coefficients that account for these emissions by using the STRAHL [6] transport code. Finally, since neoclassical transport simulations also predict differences in transport in different regimes [7], we would compare these results with the simulations of the EUTERPE [8] code used to estimate neoclassical transport.


Discharges and diagnostics.

We need discharges with good control of plasma density discharges in a fixed magnetic configuration (100_44_64). We wish plasma electron densities sufficiently high to obtain good Thomson Scattering profiles before and after impurity injection. Main diagnostics, apart from standard monitors, will be radiation diagnostics (bolometers, X-ray detectors, VUV spectrometer, etc.), and we also need the values of Er with HIBP system and Doppler reflectometer. It is necessary to obtain the ion temperature profiles by means of NPA system. A good plasma-wall condition is also required, so we prefer a fresh-lithiated wall. The transport properties of a selected group of discharges will be analyzed with the transport code STRAHL. The values of D and v coefficients obtained by STRAHL will be compared with neoclassical simulations in DKES/EUTERPE or similar codes performed by the TJ-II theoretical group.


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, NPA system, Doppler reflectometer, HIBP.
  • Type of plasmas (heating configuration): standard configuration (100_44_64), ECRH and NBI, this scenario requires a constant high line averaged density (0.9×1019 m-3), similar to 48223.
  • 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] Zurro B, Hollmann E, Baciero A, Ochando MA, Medina F, McCarthy KJ et al., (2011). Transport studies using laser blow-off injection of low-Z trace impurities injected into the TJ-II stellarator, Nuclear Fusion 51 (2011) 063015 (9pp).

[2] Zurro B., Hollmann, E. M., Baciero, A., Ochando, M. A., Medina, F. et al., (2014). Studying the impurity charge and main ion mass dependence of impurity confinement in ECR-heated TJ-II stellarator, Plasma Phys. Control. Fusion 56, [124007].

https://doi.org/10.1088/0741-3335/56/12/124007

[3] Zurro, B., Velasco, J. L., Hollmann, E. M., Baciero, A., et al, (2015). Transport analysis of impurities injected by laser blow-off in ECRH and NBI heated plasmas of TJ-II, Proc. EPS 2015. Lisbon.

[4] Medina, F., Pedrosa, M. A., Ochando, M. A., Rodríguez-Rodrigo, L., Hidalgo, C. et al., (2001). Filamentary current detection in stellarator plasmas. Rev. Sci. Instrum. 72, [471]. https://doi.org/10.1063/1.1310579

[5] Ochando, M. A., Medina, F., Zurro, B., Baciero, A., McCarthy, K. J. et al., (2006). Up-down and in-out asymmetry monitoring based on broadband radiation detectors. Fusion Sci. Tech. 50, [31]. https://doi.org/10.13182/FST06-A1252

[6] Dux, R., Neu, R., Peeters, A. G., Pereverzev, G., Mück, A. et al., (2003). Plasma Phys. Control. Fusion 45, [1815]. https,//doi.org/10.1088/0741-3335/45/9/317

[7] Velasco J. L., Calvo, I., Satake, S., Alonso, A., Nunami, M. et al., (2017). Moderation of neoclassical impurity accumulation in high-temperature plasmas of helical devices, Nucl. Fusion, 57, [016016]. https://doi.org/10.1088/0029-5515/57/1/016016

[8] García-Regaña, J. M., Beidler, C. D., Kleiber, R., Helander, P., Mollen, A. et al., (2017). Electrostatic potential variation on the flux surface and its impact on impurity transport. Nucl. Fusion 57, [056004]. https://doi.org/10.1088/1741-4326/aa5fd5



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