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13-17 May 2019
Daejeon, Republic of Korea
Europe/Vienna timezone
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Low-risk Beginning of the Density Feedback Control in KSTAR

13 May 2019, 15:05
2h
Daejeon, Republic of Korea

Daejeon, Republic of Korea

Board: P/1-3
Poster Plasma Control Poster

Speaker

Dr June-Woo Juhn (National Fusion Research Institute)

Description

During the early campaigns of the KSTAR projgect, feedback control of plasma density has been successfully commissioned at the very first attempt by using a transfer function analysis. A stable and robust discharge was chosen as a test-bed i.e. a 300 kA ($I_p$) 2.0 T ($B_t$) ohmic circular limited plasma. Before direct feedback control, pre-programmed fueling modulation was carried out by puffing the deuterium gas. Line-averaged plasma density was measured in real-time by a 280 GHz interferometer system. From the open-loop experiments, both the density decay time ($\tau_i^*$) and the external fueling efficiency ($f_{ex}$) were obtained approximately : 3.0 to 5.0 s and 10 to 20 % respectively. By transfer function analysis, several transient responses such as rising time, settling time and overshoot ratio were estimated in a certain range by the measured ranges of $\tau_i^*$ and $f_{ex}$. It is found that $\tau_i^*$ has little effect on those response characeristics while $f_{ex}$ plays primary role together with magnitude of the proportional gain $K_p$. This is due to predominance of valve response whose characteristic time $\tau_v$ was approximately determined as 60 ms by dusing $D_{\alpha}$ signal, which is much shorter than $\tau_i^*$. Considering these values, $K_p$ for closed-loop control were initially set 2.5 as minimum and followed by stepwise increment to reduce steady-state error without any integral gain $K_i$ to avoid any uncertainty. The small $K_p$ was chosen being concerned on excessive fueling if any unexpected result happens. Similarly the target density waveform was also set low initally liniearly increasing until a flattop period for one second before current ramp-down. In this way the first density feedback control was successfully finished although the transient responses were far different from the experimental result while the predicted steady-state error was in good agreement with the experimental undershoot. By replacing $\tau_v$ with arbitrary characteristic time $\tau_a$ two different settling time in the two subsequent feedback experiments were both matched well with a single $\tau_a\sim120ms$. This is due to a digital low-pass-filter included by a plasma control system (PCS) acting as 50 ms delay of response. Including the filter, transfer function becomes 3-pole system and no more simple analytic expression of response characteristics were avaliable. Instead, they were fully numerically computed. The changed settling times including the digital filter matched well with $\tau_a\sim50ms$ which became much closer to the original $\tau_v$. In summary, response characteristics in longer period (settling time and steady-state error) are evaluated well with the transfer functions in considering simple particle balance model with fixed $\tau_i^*$ and $f_{ex}$ and fueling delay estimated by $D_\alpha$ signal including digital filter. However rising time and overshoot still does not agree with any values of $\tau_a$, which implies the density feedback system is not simply the second or third order or even linear system. For more accurate prediction of response, therefore, nonlinear or time-varying numerical model will be necessary especially in dealing with the recycling coefficient $R$ that underlies in $\tau_i^*\equiv\tau_i/(1-R)$ where $\tau_i$ is particle confinement time.

Primary author

Dr June-Woo Juhn (National Fusion Research Institute)

Co-authors

Sang-hee Hahn (National Fusion Research Institute) Yong-Seok Hwang (Seoul National University) Mr K. P. Kim (National Fusion Research Institute) Mr Y. O. Kim Kim (National Fusion Research Institute) Dr Y. U. Nam (National Fusion Research Insitute) Mr J. I. Song (National Fusion Research Institute)

Presentation Materials