TJ-II:Excitation of zonal flow oscillations by energetic particles

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Experimental campaign

2017 Spring

Proposal title

Excitation of zonal flow oscillations by energetic particles

Name and affiliation of proponent

Edilberto Sánchez, José Luis Velasco, Iván Calvo, José Manuel García-Regaña

Details of contact person at LNF (if applicable)

Edilberto Sánchez (edi.sanchez@ciemat.es)

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

Motivation

In stellarators the zonal flows (ZF) undergo several oscillations depending on the magnetic geometry and the plasma conditions. First, a Geodesic Acoustic Mode (GAM) oscillation, similar to that in tokamaks, appears, whose collisionless damping strongly depends on the rotational transform. A new oscillation, at a lower frequency, recently discovered theoretically [1], appears in stelallarators that is not present in quasisymmetric devices. In TJ-II, both oscillations have been found in simulations [2] and the low frequency one has been identified for the first time during pellet injection experiments [3]. Other experimental evidences appear to be related to zonal flow oscillations, both low frequency [4] and GAM [5].

According to gyrokinetic simulations, the GAM and LFO oscillations are damped in the TJ-II geometry (even collisionlessly) and the necessary driving mechanism has not been identified so far. The turbulence can drive (non-linearly) these oscillations, but it is not the only option as driving mechanism. In tokamaks it has been found that energetic particles can drive the GAM oscillation (EGAMs) [6], but in stellarators there are less evidences [7] and the oscillation fauna is wider than in tokamaks.


Objectives

In this experiment we plan to study the excitation of ZF oscillations by energetic particles in TJ-II.

We will work in NBI plasmas and will look for LFO by means of the Doppler reflectometry system and the dual HIBP. We will change the energy and intensity of the neutral beam in order to identify changes in the LFO, that can be reproduced in GK simulations, if these oscillations are generated.

As an alternative we could work in ECRH heated plasmas and generate fast ions by means of the diagnostic neutral beam injector (DNBI) and try to measure oscillations in the zonal potential, if we are able to generate them. If so, we will characterize their spectrum and study the dependency with parameters: beam intensity, beam energy, and ion and electron temperatures.

The experimental results will be compared to dedicated gyrokinetic simulations.

If applicable, International or National funding project or entity

N/A

Description of required resources

The experiments will be carried out in ECRH plasmas that will provide a ground state to add the fast ions from the neutral beam. At this stage, no change in the magnetic configuraton is required. We will work in the standard 100_44_64.

A basic characterization of the plasmas will require the interferometer and Thomson Scattering diagnostics. A good estimation of experimental electron density and temperature profiles at times of interest will be required for the simulations. In addition to the Thomson measurements, the He beam will help in fitting the profiles in the plasma edge and the NPA will be required to have some information about the ion temperature. The Doppler reflectometer and the dual HIBP are basic for the characterization of the potential oscillations.

A characterization of the fast ions populations by means of the CNPA and passive spectrroscopy and/or luminiscent probes will be very convenient.


Required resources:

  • Number of plasma discharges or days of operation: one experimental day.
  • Essential diagnostic systems:

- Interferometry - Thomson Scattering - Neutral Particle Analyzer - Helium beam - The double Heavy Ion Beam probe - Doppler reflectometer - [ CNPA - passive spectrroscopy and/or luminiscent probes]

  • Type of plasmas (heating configuration): stable NBI (and/or ECRH) plasmas in the standard configuration.
  • 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

  1. Mishchenko PoP 2008
  2. Sánchez PPCF 2013
  3. Alonso PRL 2017
  4. Pedrosa PoP 2008
  5. Castejón PPCF 2016
  6. Zarzoso PRL 2013 and reference therein
  7. Sun EFL 2016

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