TJ-II:Validation of ECCD predictions in TJ-II ECRH plasmas

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Experimental campaign

2018 Autumn

Proposal title

Validation of ECCD predictions in TJ-II ECRH plasmas

Name and affiliation of proponents

J. M García Regaña1, Á. Cappa1, A. Gozález-Jeréz2, A. López-Fraguas1, N. B. Marushchenko, ... 3 and TJ-II team1.

Background

Figure 1. Total toroidal current ECCD as a function of $ N_{\|} $ calculated for the TJ-II transmission line QTL2 with the code TRAVIS.

The current driven by means of EC waves, so-called Electron Cyclotron Current Drive (ECCD), has been proposed as a tool for non-inductive current induction in tokamaks, as well as a mean of current control and tailoring of the rotational transform $ \iota $ in stellarators. In particular, it can modify through its impact on $ \iota $ the map of Alfvén eigenmode (AE) and with it the fast ion dynamics. In view of future studies of AE-induced fast ion losses in TJ-II the present proposal focuses on extending the present the validation of the ECCD predictions comparing these with the experimental values. The available comparison considering past campaigns data and recent TRAVIS simulations find strong disagreements that suggest that corrections on the polarization of the incident wave, beam focusing and other factors leads to a numerical overestimation on the ECH power source on the numerical estimate. Hence the present experiment aims at measuring the induced ECCD together with the estimation of the experiment power source measured by means of modulated EC heating and confirming the modifications on the rotational transform obtained with VMEC.

Experiment planning and required resources

The main experiment conditioning requirements are:

  • ECH is the only heating system that will be employed. The characterization of the current driven will be done according the following strategy:
  1. Characterization of the current driven using only the transmission line QTL1, injecting on-axis and with $ N_{\|}=\left\{-0.3, -0.15, 0.0, 0.15, 0.3\right\} $. Considering 2 discharges per value of $ N_{\|}\ \Rightarrow $ 10 discharges.
  2. Characterization of the current driven using only the transmission line QTL2, injecting on-axis and with $ N_{\|}=\left\{-0.3, -0.15, 0.0, 0.15, 0.3\right\} $. Considering 2 discharges per value of $ N_{\|}\ \Rightarrow $ 10 discharges.
  3. Characterization of the current driven using only the transmission line QTL1, injecting on-axis and with $ N_{\|}=\left\{-0.3, -0.15, 0.0, 0.15, 0.3\right\} $, simultaneously to the employment of QTL2 with $ N_{\|}=0.0 $ on-axis for auxiliary heating the background plasma. Considering 2 discharges per value of $ N_{\|}\ \Rightarrow $ 10 discharges.
  4. Characterization of the current driven using only the transmission line QTL2, injecting on-axis and with $ N_{\|}=\left\{-0.3, -0.15, 0.0, 0.15, 0.3\right\} $, simultaneously to the employment of QTL1 with $ N_{\|}=0.0 $ on-axis for auxiliary heating the background plasma. Considering 2 discharges per value of $ N_{\|}\ \Rightarrow $ 10 discharges.
  5. Finally, for large $ N_{\|} $ the partial loss of X-mode polarization and the presence of spurious O-mode is expected at the resonance layer. Then, the current induction and power absorption may be sub-obtimal, and a scan on the ellipticity of the beam polarization at its launching position is foreseen in order to correct this effect. 10 discharges needed.
  • The discharges should be sufficiently long and the electron collisionality sufficiently high to capture the exponential growth of the induced toroidal plasma current and estimate the asymptotic value of it and the L/R time. Thus, time pulses of 250 ms. are initially planned and electron density as high as ECH conditions allow.
  • The magnetic configuration run during the experiments will be the standard configuration.
  • Good stationarity of plasma parameters is needed for the analysis.

The required signals to perform the analysis are:

  • The time evolution of the toroidal plasma current with the Rogowski coils.
  • The Motional Stark Effect looking at an inner core radial position $ r/a\approx 0.2 $, to check for possible changes on the rotational transform.
  • The time evolution of the line-averaged density $ \left<n_e(t)\right> $ with interferometry.
  • The radial profiles of electron density $ n_{e}(r, t_0) $ and temperature at one time instant $ t_0 $ using Thomson Scattering (TS).
  • The time evolution of the electron temperature profile $ T_{e}(r,t) $ with Electron Cyclotron Emission (ECE), when available, calibrated with TS.
  • The time evolution of the main ion temperature profile $ T_{i}(r,t) $ with the CNPA are two positions representative of the inner and mid core.

Preferred dates and degree of flexibility

Preferred dates: due to the availability on-site of the proponents the experimental days should fall out of the calender week 14.

A valuable input for this experiment regarding the L/R time at similar conditions in the absence of ECCD may result from the conduction of the experimental proposal "Validation of bootstap predictions" (http://fusionwiki.ciemat.es/wiki/TJ-II:Validation_of_bootstrap_predictions). Thus, the present proposal shall be preferably scheduled after that proposal.


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