TJ-II:Investigating the Alfvén Wave damping

Experimental campaign

2017 Spring

Proposal title

Investigating the Alfvén Wave damping (2).

Name and affiliation of proponent

F. Castejón, E. Ascasíbar, A. Cappa

Details of contact person at LNF (if applicable)

N/A

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

Investigating the Alfvén Wave damping (2).

F. Castejón, A. Cappa, E. Ascasíbar, J. M. Fontdecaba, TJ-II Team, NIFS Team and HIBP team, and U. Kyoto et al.

Experiments on TJ-II plasmas have demonstrated the possibility of mitigating AEs by using ECRH. In fact, when ECRH power is applied, the AE disappears or its frequency evolves producing chirping. In both cases the ion losses are reduced . Nevertheless, it is not clear yet if the mitigation is due to the change in the plasma profiles induced by ECRH or by the damping increase. On the one hand, the changes in collisionality profiles can modify the fast ion distribution and produce a non-linear interaction between the fast ions and the mode , giving rise to the chirping. On the other hand, it is possible that the action of quasiperpendicular ECRH waves increases the fraction of trapped particles and produces the damping of the mode .

We suggested to explore the effect of EC waves on AEs in TJ-II. The proposal is to take the 100_44_64 configuration, in which AEs in steady and chirping states are observed and to inject EC waves at different values of |N|||, with opposite signs in both gyrotrons in order to compensate the current and change the pitch of resonant electrons. The location of the power deposition profile should be approximately the same during the experiment and the current should be compensated.

As |N||| increases, the heated electrons have smaller pitch and, hence, the fraction of trapped particles is reduced. One expects that compensating the current with the two gyrotrons, its effect will be small, as well as the effect of changing the profiles since the heating position is approximately the same.

Some experiments have been already performed, but the lack of calibration of ECRH mirrors prevented the successful operation. A second experimental day, after a first calibration, has shown a different behaviour of Alfvén modes where varying |N|||, showing a clear effect of this parameter. We need more experiments to understand the final dynamics.

We propose:

5 values of |N||| (0, 0.1, 0.15, 0.25, 0.4) x 3 Radial positions (Rho=0, 0.2, 0.4) x 2 repetitions x 2 densities= 2 days operation.

1 Nagaoka et al. NF 2013

2 Fontdecaba et al. EPS2014
3 Berk et al. PL-A 1997
4 Kolesnicheno et al. PPCF 2011

If applicable, International or National funding project or entity

Eurofusion Enabling Research 2017 - 2018 (5 ppy)

Description of required resources

Required resources:

• Number of plasma discharges or days of operation: 2
• Essential diagnostic systems: Mirnov, CNPAS, HIBP, Bolometers
• Type of plasmas (heating configuration): ECRH + NBI
• Specific requirements on wall conditioning if any:
• 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

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