Behler, K.

Bock, A.

Giannone, L.

Maraschek, M.

Poli, E.

Reich, M.

Stober, J.

Treutterer, W.

English

MPG.PuRe

Progress on ECCD-based NTM rt-control at ASDEX UpgradeM. Reich, A. Bock, K. Behler, L. Giannone, M. Maraschek, E. Poli,J. Stober, W. Treutterer and ASDEX Upgrade teamMax-Planck-Institut für Plasmaphysik, EURATOM association, D-85748 Garching, GermanyIn order to efficiently run high performance plasmas, Neoclassical Tearing Modes (NTMs), which are regularly observed at reactor-grade β-values, have to be controlled. Suppression of performance-limiting  magnetic  islands  is  the  prime  requisite,  but  controlled  triggering  of benign  modes  in  order  to  avoid  more  deleterious  ones  is  desirable  as  well.  At  ASDEX Upgrade,  a  steerable  mirror  is  used  to  direct  the  ECRH (or  ECCD) beam to the  desired deposition location in the plasma. Since the deposition can thus be changed during plasma operation and in real-time, a feed-back control of magnetic islands is feasible.The individual components of the full feedback loop for NTM control have been discussed previously [1]. Several sub-projects have since made considerable progress towards the fully integrated project. The ASDEX Upgrade real-time plasma equilibrium, which serves as the basis of many real-time diagnostic systems, and its progress are discussed separately [2]. The other  two  main  ingredients,  the  TORBEAM  real-time  diagnostic  and  the  mirror  control application  have  recently  reached  production status and are the main topic of this paper.The  plasma  scenario  chosen  for  demonstration has a plasma current of 1 MA and a toroidal field of 2.6 T on axis. This configuration allows to use ECRH  at  140 GHz  for  both  central  heating (impurity  control)  and  current  drive  across  a range of  minor  radii  with the  beam directed as shown  in  figure 1.  Safety  margins  for  the movement  of  the  mirror  were  chosen such that deposition  is  possible  between  ρpol ~  0.2  and ~ 0.7, a range of radii known to contain the q=1.5 surface.  Up to 5 sources  (12.5 MW) of  neutral beam injection (NBI) are necessary to  push the plasma β above the threshold for NTM onset and typically  excite  an  m=3,  n=2  magnetic  island, which is visible in the ECE radiation temperature Figure 1: Accessible deposition locations38th EPS Conference on Plasma Physics (2011) P5.102profile (figure 2).The  core  component  of  the  NTM  control project is the process which interfaces directly with the mirror. The desired mirror angle (αset) is determined by extrapolating from the current deposition  location  (ρtbeam)  and  the  expected change of the deposition (δρ)  with respect to a small  change of the mirror angle (δα) at  the current setting of the launcher (αmeas).  So the requested  mirror  angle  change  can  be expressed as:∆α=αset−αmeas=ρtbeam⋅δαδραmeas−αmeasThe input to this process comes directly from the TORBEAM code, which is calculating ρtbeam and δρ /δα αmeas using the numerical derivative at lowest order. The necessary inversion of the  derivative is  expected to  be well-defined in  the  range of  possible  mirror  angles.  The implementation of this controller also takes into account the possibility of changes smaller than the accuracy of the mirror steering mechanism and doesn't forward such requests to the actual launcher, thus avoiding continuous stresses for the system's mechanics. In order to deposit the ECCD beam at the location of the magnetic island (at the relevant rational  surface),  it  is  important  to  know the  expected  deposition  location.  The real-time capable TORBEAM code uses the magnetic equilibrium, the density profile and the mirror angle  (which  are  measured  by  different  real-time  systems  concurrently)  to  calculate  this position by solving the ray tracing equations. It has been possible to optimize the code such that calculating one beam trace takes on average less than 50 ms on the designated 3.33 GHz x86 machine. In the chosen scenario (with a well behaved density profile and only mildly varying plasma equilibrium) the execution time averages at 35 ms with occasional peaks up to 40 ms. A major step forward was achieved by parallel execution of several such calculations using the MPI (message passing interface) protocol. It is now possible to calculate two beam traces simultaneously on one dual-core machine (already done in practice) and as many traces as needed on a suitable multi-core machine. The speed-up for up to 8 such nodes has been shown to scale nearly linear (i.e. each beam trace uses an extra CPU core and the increase of the  total  execution  time  is  negligible).  By  using  a  fast  interconnection  network  (e.g. Figure 2: ECE temperature profile before  (1.9s) and after (2.2s) NTM onset (2.0s)38th EPS Conference on Plasma Physics (2011) P5.102infiniband), alternate beam traces can also be run on a cluster of machines, which will become necessary as soon as more than one ECCD beam are to be controlled in the same discharge.Before relying on the calculation of deposition location using the real-time TORBEAM, a thorough validation of the results was performed. Within a single plasma discharge, current, field, plasma shape and density were kept  as  constant  as  possible  and the mirror  was tilted in  steps  to  provide several different deposition locations. By modulating the ECRH at 250 Hz, we  determine  the  actual  deposition location from the ECE spectral power at  the  ECRH  modulation  frequency (cf.  figure 3) in each channel. When comparing  this  measurement  to  the  calculated  ECCD  deposition  using  TORBEAM,  the agreement is quite good. Occasional deviations of up to 0.08 in ρpol are thought to be caused by inaccuracies of the equilibrium and/or the resolution of the ECE system which is resolving only ∆ρpol ~ 0.04. It is thus possible to use the calculated deposition for the experiments (no modulation for real-time deposition measurement necessary).The most recent plasma experiment was aimed at triggering a magnetic island, measuring its amplitude and starting counter-measures as soon as the amplitude increases above a pre-set threshold  value.  The counter-measures  consisted  of  sweeping the  ECCD beam (in  a  pre-programmed  motion)  across  the  island  position  and  observe  the  reaction  of  the  mode-amplitude. We would have stopped the mirror and kept the beam depositing on the rational surface  to  prevent  the  island from reappearing,  if  the  amplitude  had  shrunk to  a  second threshold. Mode creation and detection was successful, however, due to lack of ECCD power (and driven current) the island was not completely stabilized, the “mode stable”-threshold was not  reached  and  the  ECCD  was  moved  further.  Consequently,  with  the  stabilizing  term removed, the island increased in size again, triggering the mirror sweep a second time. Using  the  fast  (1 MHz)  ECE  measurements,  which  are  acquired  by  a  real-time  capable system, we can attempt to localize the island with a correlation technique. By correlating the N=2  magnetic  signal  (also  available  on  the  same  system)  with  each  ECE  channel,  a correlation  amplitude  spectrogram can be  plotted.  While  the  maximum of  the  correlation amplitude in flux label coordinates (which each ECE channel is mapped to) apparently doesn't reflect the magnetic island's position, the phase between each ECE channel and the magnetic Figure 3: ECE spectral power of deposited ECRH 38th EPS Conference on Plasma Physics (2011) P5.102reference exhibits a jump of  π which moves through a number of channels and can thus be unambiguously identified as the island's O-point. The smaller step in phase located around ρpol ~ 0.35 is due to a group of ECE channels located at slightly different height and thus seeing an explicit phase offset with respect to the island. One can now compare the calculated deposition location of the ECCD beam (as calculated by TORBEAM) with the measured ECE-magnetics correlation amplitude and phase as done in figure 4.  The most stabilizing effect (mode amplitude shrinks) is achieved when the deposition crosses approx.  ρpol  ~ 0.45 (O-point location at t=3.4s). As soon as we have a robust algorithm for real-time detection of this phase jump of π (work already underway) we will close the NTM control loop.We have established a plasma scenario in which β-driven NTMs are triggered and have shown that our real-time systems can detect and respond to the occurrence of such instabilities. The real-time TORBEAM code performs very well  in its parallelized version and can provide useful deposition locations plus the numerical derivative for mirror movements for a number of beams every ~ 50 ms. The MHD controller has been commissioned and is ready for the proof-of-principle  demonstration.  Closing  the  full  stabilization  loop  depends  only  on  the available experiment time for this project in the ongoing campaign.[1] M. Reich et al., ECCD-based NTM control using the ASDEX Upgrade real-time system, EPS 2010[2] L. Giannone et al., Real time magnetic equilibria for NTM stabilisation experiments , EPS 2011Figure 4: Magnetics & ECE temperature correlation (amplitude, phase and N=2 amplitude)                 ECCD deposition calculated by real-time TORBEAM is indicated in black circles38th EPS Conference on Plasma Physics (2011) P5.102

Progress on ECCD-based NTM rt-control at ASDEX Upgrade

https://pure.mpg.de/pubman/item/item_2139999_3/component/file_3325503/P5.102.pdf

Progress on ECCD-based NTM rt-control at ASDEX Upgrade

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