Progress and Plans for SUNIST Spherical Tokamak

1. 4th IAEA TCM on ST and 14th ISTW Frascati, October 7-10,2008 Present status and research plan of the SUNIST spherical tokamak* Zhe Gao,1Yexi He,1 Yi Tan,1 Wenhao Wang,1 Huiqiao Xie,1 Lifeng Xie,1 Long Zeng,1 Liang Zhang,1 Chunhuan Feng,2 Long Wang2 and Xuanzong Yang2 1）Department of Engineering Physics, Tsinghua University, Beijing, China 2）Institute of Physics, Chinese Academic of Science, Beijing, China Electronic mail: gaozhe@mail.tsinghua.edu.cn *This work is supported by the Major State Basic Research Development Program from MOST of China under Grant No. 2008CB717804, NSFC under Grant No. 10535020 , as well as the Foundation for the Author of National Excellent Doctoral Dissertation of PR China under Grant No. 200456.

2. In memory of Professor Yexi He

3. OUTLINES • Progress of ECR startup • Plan and progress of Alfven wave current drive • Upgrade plan of the device and present status

4. Turbo pump 8 mm interferometer Cryogenic pump Photo diode Fast camera (<20kfps) Microwave injection Piezo-valve Experimental system for ECR startup • Microwave source • 2.45GHz (nc=7*1016m-3) , <100kW, ~10ms (normal direction injection) • Diagnostics • Rogowski coil (damaged in 2007), Flux loop • 8mm interferometer (transferred form SWIP, cannot work in 2006) • Photo diode and Fast camera

5. Initial results in 2006 Cut-off density for 2.45GHz neis roughly characterized by the emission of visible light (since low Te and low ionization ratio) The microwave is not absorbed by plasmas It is absorbed but no plasma current can be efficiently driven “The microwave output power has been kept around 20 kW because the plasma current quality could no longer be improved with the higher injected power.” Y X He, et al. 2006

6. PH2 ~ 5x10-3 Pa Bv ~ 20 G Oscillation Short delay, but 0.3kA (a) (b) (c) (d) (e) (b) (c) (e) (f) (a) (d) 1st ECR launcher (l) 1st ECR (m) (g) (n) (h) (i) (g) (f) (h) (i) (k) (j) Improved results due to low filling pressure discharges in 2007 • Long time discharge conditioning • PH2 ~ 1x10-3 Pa • Bv ~ 15G • No oscillations • Long delay, 2 kA Ip obtained ~0.5 ms ~2 ms Tan, ISTW 2007

7. Plasma current is estimated from the change of poloidal flux when Rogowski coil is damaged Inverse-proportinal relations of Ip to Bv shown Further study in 2008: the light emission signal as a criteria for determining the operation regime Smooth heads: low filling Spiky heads: high filling Yoshinaga 2006

8. Further study in 2008: analysis of the behavior of spiky Light r0=RRES-R0 Reflection Large fraction of microwave power is reflected. The reflection ratio is with respect to the position of ECR layer. Light (a.u.) Delay Delay is mainly affected by the power density at the ECR layer . t t

9. Change the lauching angle Plasma density control New ECR Source Future Plan on ECR startup ECR layer Vacuum Vessel Limiter New antenna planned 新天线部件图？ Current antenna Flexibilities for the radial position of the launching face

10. OUTLINES • Progress of ECR startup • Plan and progress of Alfven wave current drive • Upgrade plan and status of the device

11. Background: AWCD experiments Low-frequency waves were considered as an attractive mechanism of driving plasma current because of its potential high efficiency, no density limit and the convenience of high power RF sources. But electron trapping dramatically decrease its efficiency Both experiments shows the CD efficiency obtained is consistent with the Fisch-Carney theory. However, both are in the plateau regime, therefore, the trapped electron (neoclassical) effects cannot be verified. Wukitch 1995, Intrator 1996 at Phaedrus-T Ruchko 2002 at TCABR

12. Motivation Low aspect ratio tokamak may be a good platform to study the behavior or contribution of trapped electrons in low frequency wave current drive. • To investigate the possibility of Alfvén wave current drive (or increasing its efficiency) in low aspect ratio toroidal plasmas. • In physics, to evaluate the contribution of nonresonant drive (exist or not? Many previous work suggest “yes” but our recent theoretic work say “No” Gao, Fisch and Qin, 2006, 2007) and to investigate the role of trapped particles ( how the momentum returns from trapped electrons ) • Rf coupling physics

13. Experimental system setup: antenna design • Vacuum Vessel Return Limb , Straps , , , Plasma , , • four modules in toroidal direction and two antenna straps in poloidal direction for each module • referenced to the design for the ETE in Ruchko 2004 • Test and assembling Detail in the presentation by Dr Yi Tan， In Plenary 6, session 3,tomorrow afternoon • Resonant frequency and • impedance calculation

14. Experimental system setup: antenna shielding • BN side limiter ? • Metal side limiter? • BN front or BN coating needed? Sorensen 1996 at Phaedrus-T

15. Experimental system setup: rf generator • Four-phase oscillator (the phase shift between outputs does not depend on the variation of the antenna impedance) • 4x100kW/0.4~1MHz, • Status: four phase output tested for 4x10kW, but still stored in the factory referenced to Ruchko 2004 at TCABR

16. OUTLINES • Progress of ECR startup • Plan and progress of Alfven wave current drive • Upgrade plan and status of the device

17. Upgrade of diagnostics • New Mirnov probe system — Equilibrium, excited modes • New Rogowski coil and flux loop — Equilibrium, plasma current and loop voltage • Electrostatic probe — density and potential fluctuation, flow • Photo diode and Fast camera — Equilibrium, temperature • Microwave reflectometer (8~12mm) and interferometer (3mm) — density and its fluctuation • Radiometer (2~12GHz), Spectrometer ( ICCD, vacuum ultraviolet), X-array — electron temperature (Previously Existed; Now ready; budget ready; in application)

18. Improvement of control of equilibrium field (a set of capacitor bank with 1200V、270mF , IGBT-based control scheme) • rf power: 400kW/<1MHz/30ms • Microwave power: 100 kW/2.45GHz/10ms >40kW/>5GHz/>30ms Upgrade of power supply (Previously Existed; Now ready; budget ready; in application)

19. Modification of vacuum chamber A new manhole window is opened for convenience of in-vessel components installing and servicing

20. Status of the machine • Mission: to install AW antenna and new magnetic probe system, and to open a new manhole window in vessel Disassembled April 17, 2008 Fabrication May 26, 2008 Before March 27, 2008 expected to be re-assembled before November this year Vessel reassembled and start of leak detection Sep. 25, 2008 Cleaning Sep 11, 2008

21. Summary • ECR startup improved and analyzed, but limited by poor diagnostics and microwave power (low freq. and short pulse) • AWCD experiment system is preliminarily setuped, but lack of experiences • Diagnostic system and power-supply system will be upgraded in a few years

22. potential areas of collaboration Any comment is welcome and any help or collaboration is desirable, typically in • Design/improvement of antenna system using more realistic model • Antenna shielding and density control in future AW experiments • The coupling of rf to plasmas • Diagnostic of wave-particle interaction • Theory and simulation of the behaviors of the plasma with ECR/AW injection