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Improvements in the fast vertical control systems in KSTAR, EAST, NSTX and NSTX-U

D. Mueller1, N.W. Eidietis2, D. A. Gates1, S. Gerhardt1, S.H. Hahn3, E. Kolemen1, L. Liu5, J. Menard1, Y.S. Park4, S.A. Sabbagh4, S.W. Yoon3, and B.J. Xiao5

1 Princeton Plasma Physics Laboratory, Princeton Univ., Princeton, N.J. 08543, U.S.A. 2 General Atomics, San Diego, CA, 92186-5608, U.S.A. 3 National Fusion Research Institute, Daejon, Korea 4 Columbia University, New York, NY, 10027, U.S.A. 5 Institute of Plasma Physics, Chinese Academy of Sciences, Hefei Anhui, China

 E-mail contact of main author: mueller@pppl.gov

Abstract. The control of the vertical position in tokamak plasmas is essential for exploring the operational control space. In particular, the vertical control system must be capable of responding to transients as quickly as possible and controlling plasmas near the vertical stability limit. The design of the coils systems constrains the ultimate response frequency that is available, but in order to realize the potential of the coil design it is essential to have adequate diagnostics of the position and a feedback control system capable of that.

 

1. Introduction

The realization of a wide variety of plasma shapes with varying plasma current density profiles places challenging demands on the vertical control system in a tokamak. In particular the bootstrap current scales quadratically with elongation at fixed normalized  βN  =  βTaBT/Ip where Ip is the plasma current, BT is the toroidal field, and  βT =   P/(BT2/2µ0)   and P is the volume averaged plasma pressure. The plasma elongation is controlled by the action of coils that generally act to produce a field shape with an index of curvature that becomes closer to vertical instability as the elongation is increased. In devices such as the Korean Superconducting Tokamak Advanced Research (KSTAR), The Experimental Advanced Superconducting Tokamak (EAST) and ITER, the superconducting coils are separated from the plasma by conducting structures that increase the response time of the plasma to changes in the coil currents. Fast control of the plasma vertical motion, essential for stable operation at high elongation and disruption avoidance, can be accomplished with coils internal to the vacuum vessel in these devices. Each of the four devices, NSTX, EAST, KSTAR and NSTX-U, uses its own version of the General Atomics (GA) plasma control system (PCS)[1],[2],[3],[4] to control the coil power supplies with feedback on plasma parameters. The shape control systems in these tokamaks are generally based upon equilibrium analysis that does not lend itself well to fast control and in particular   cannot produce a reliable time derivative of the vertical position which is necessary for fast response. The simple analysis of standard integrated magnetic signals to

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produce a vertical position (z) signal can be incapable of yielding a derivative term (dz/dt) with sufficient signal-to-noise ratio to be adequate for control. Furthermore, for the two existing superconducting tokamaks, the slow response of the plasma to the distant superconducting coils necessitates the introduction of coils internal to the vacuum vessel in order to provide faster control of vertical instabilities.

2. Discussion

The difference of the signal from up-down symmetric pairs of voltage loops can be used to provide a better signal for the dz/dt in the PD control loop of z. The NSTX control system used the analog difference of a pair of voltage loops, with 2 kHz low-pass filters to remove signals due to n=1 plasma instabilities, to provide the dz/dt signal. This signal was used in the  control  of  the  difference  in  the  rectifier  power  supplies  for  up/down  difference  in  a  pair  of  outer  PF  coils.    Use of this improved signal-to-noise ratio signal realized an improvement in vertical control that allowed elongation (κ)  to be increased by about 0.2.[5] NSTX-U will have 6 pairs of voltage loops instrumented to provide dz/dt for feedback control. Use of the RMP saddle coils with 7 kHz power supplies for vertical control will be tested for use in vertical control if the use of the slower rectifier supplies on the poloidal field coils proves inadequate at high elongation with high internal plasma inductance (li). Analysis of these signals indicate they will provide an excellent representation of dz/dt for NSTX-U.[6]

The KSTAR control system uses coils internal to the vacuum vessel to provide control of vertical instabilities and has instrumented 2 pairs of voltage loops, whose positions are indicated in Figure 1, to provide a fast dz/dt that have better signal to noise than the derivative of signals derived from flux loops or Mirnov coils as can be seen in Figure 2. Relay feedback[7] was used to optimize the vertical PD control gains. The use of the voltage loop

 Figure 1. The geometry of the EAST and KSTAR devices showing the locations of the control coils and the sensors discussed in the text.  

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pairs in the control system has been successfully tested, and proven to have better signal to noise ratio than is provided by the derivative of integrated signals. Insufficient operational experience exists so far to quantify the expansion of the available operational space with the improved control response. The EAST control system has similarly been modified to include two pairs of loop voltage difference signals as indicated in Figure 1. Furthermore, the internal coil power supply has been modified to use voltage control rather than current control for faster response. The improvement in the signal to noise ratio for dz/dt based on the loop voltage pairs compared that derived from the derivative of the z position derived from the integrated magnetics is shown in Figure 3.

 Figure 2. The derivative of the fast z signal derived from integrated magnetics sensors shown in black is exhibits much more noise that the dz/dt term from the difference in a pair of up-down symmetric voltage loops shown in blue.  

Figure 3. EAST data showing the dz/dt signal derived from the loop voltage signals compared to that from the derivative of z derived from the integrated magnetics signals.

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In each device, the ability of the fast vertical control is affected by the slower shape control so the target values for the fast and slow control should be compatible in order to provide good control. In Figure 4 an example of a DND plasma in KSTAR for which the z request in the fast control loop was adjusted to match the z provided by the slower shape control is shown in blue. This results in more symmetrical plasma with higher kappa than the discharge shown in black without such an adjustment. Small differences in the vertical position derived from simple, but fast systems demanded for vertical control and that derived from the shape based on real-time equilibrium analysis (rtEFIT)[8] used for control of the superconducting coils are almost unavoidable. If that difference exceeds some value, the internal coils will be controlled to their maximum value and the fast control response will be lost. A remedy for this conflict is to employ a high-pass fast filter on the error in the z measurement, the target value minus the measurement, used in the control of the internal coils. This solution uses the fast internal coil response to operate only on the high frequency vertical movement of the plasma while the slower superconducting coils control slower motion. This avoids the necessity of empirically tuning the z reference from shot-to-shot as agreement between the z signal and the shape control changes.

         Figure 4. Comparison of plasma current, elongation, IVC current, feedback terms and z position in KSTAR as a function of time for two discharges. In shot 8870 shown in black, the z reference in the IVC control loop was zero. For shot 8872, the z-position reference was adjusted beginning at 3 s to achieve IVC current ~0 and zp ~ 0 after 4 s in plasmas with otherwise identical control. The implications of this discrepancy is discussed in the text.  

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This remedy has been employed in the EAST PCS system and has proved reliable to avoid saturation of the internal coil current while providing fast vertical control. The high-pass filter used in the EAST discharge shown in Figure 5 maintained stable control of the fast motion of the plasma while keeping the internal coil’s current near zero while the shape control of DRSEP was used to move the plasma vertical position under control of the superconducting coils.

3. Summary

Use of the voltage loop-based sensors for dz/dt in NSTX, KSTAR and EAST was successful in improving the signal to noise ratio of dz/dt used for vertical control and allowed operation closer to the vertical stability limit with an increase in achievable elongation in NSTX of 0.2. Use of multiple loop voltage pairs on NSTX-U will be commissioned in upcoming experimental campaigns. The use of a high-pass filter in the fast control loop was successful in both the superconducting tokamaks to eliminate the contention between control from the internal coils and the slower, but more powerful superconducting PF coils while maintaining control of fast transients with the internal coils.

4. Acknowledgements

This research was supported by U.S. DOE contract DE-AC02-09CH11466.

 Figure  5.    The  plasma  vertical  motion  was  ramped  by  controlling  DRSEP  shown  in  the  second  frame  which  permitted  zp  to  be  ramped  0.1  m  while  maintaining  fast  vertical  stability  and  avoided  large  current  in  the  internal  coil  (IC).    

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[6] GERHARDT, S.P., et al., “Magnetics For Equilibrium Reconstruction and Realtime Plasma Control in NSTX-Upgrade”, Review of Scientific Instruments 20th Topical Conference Proceedings on High-Temperature Plasma Diagnostics (2014) (in press).

[7]     HAHN, S.H., et al., “Progress and improvement of KSTAR plasma control using model-based control simulators”, Fusion Engineering and Design 89 (2014) 542–547.

[8] FERRON, J.R., et al., “Real time equilibrium reconstruction for tokamak discharge control”, Nucl. Fusion 38 (1998) 1055–1066.