Progress of Alfven wave experiment in SUNIST The 2 nd A3 Foresight Workshop on Spherical Torus, January, 2014, Beijing, China Y. Tan 1*, Z. Gao 1, W. H. Wang 1, H. Q. Xie 1, Y. Q. Liu 1, O. Pan 1, Y. Z. Jiang 1, R. Ke 1, S. Chai 1, L. Wang 2, X. Z. Yang 2, C. H. Feng 2 1 Department of Engineering Physics, Tsinghua University, Beijing, P. R. China 2 Institute of Physics, Chinese Academy of Sciences, Beijing, P. R. China *

Basic Principle Motivation Research Plan Recent Progress Future Work Summary Contents

Excite a rf B field in the vacuum –with a specific spatial structure (k) by a set of antenna Rf B field spreads into plasmas –becomes Compressional Alfven Wave (CAW) CAW  Shear Alfven Wave (SAW)  Kinetic Alfven Wave (KAW) –at Alfven resonance surfaces (where V A ~ wave phase velocity v p ) Transfer energy to electrons –and may drive current if v || ~ wave phase velocity v p Basic Principle Rf current Rf Generator Antenna Plasm a Z axis Resonance layer

Is AW a high efficiency current drive method? –Yes, if no trapped electrons –No, if most electrons are trapped (ST) –Unsure theoretically, if the moment can be recovered by some special process AWCD on SUNIST –Try to verify the possibilities of moment recovery experimentally Motivation Ehst and Karney, NF, 1991, Gao, A3 SS, 2013

Motivation (continued) Effective space Trapped ratio of 100 eV electrons in SUNIST L Zhang, PhD dissertation, 2009

Research Plan Hardware design, manufacture Coupling study (small power) Heating and current drive (high power) Current status upgrade

Setup of the experimental system Coupling study without antenna limiters Installation of the antenna limiters Coupling study with antenna limiters Setup of a new rf generator Recent Progress

The RF generator –Four phases outputs But only two phases are stable –20 ~ 50 kW, 0.4 ~ 1 MHz (not continuous) Antennas –No shielding, no limiters –Two of four pairs used –  phasing –|N|=1 ~ 60%, |M|=1 ~ 15% Plasma parameters –I P ： 30~50 kA –n e ： 0.5 ~ 3  19 m -3 –B T ： 800~1200 G antennas Plasmas and the antennas without protector Setup of the experimental system Toroidal arrangement of the antennas

Antenna impedance measurements –Low frequency  Z = U/I is viable –I: rf current transformer (Pearson Model 110) –U: rf voltage transformer (homemade) I RF measurement U RF measurement Vacuum results Coupling study

Have similar trends as 1-d calculations Coupling character without antenna limiters Experimental results (left) and theoretical results (right) of the impedances of antennas

Side limiters made by boron-nitride plates have been proven to be effective in Phaedrus-T Installation of the antenna limiters Impurity lines of the AWCD experiments in Phaedrus-T tokamak With Faraday screen With BN limiters Sorensen, NF, 1996

Two limiters have been installed now Installation of the antenna limiters (ctd.) Design sketch Boron-nitride protected antennas in the vacuum vessel Tan Y, et al. FED 13, 30

The effects of BN limiters are obvious Installation of the antenna limiters (ctd.) Effective protection Stable operation with BN limiters Shot , without BN limiters Not stable Stable operation

Plasma parameters –I P ~ 50 kA, n e L ~ 1E19 m -2 Coupling character with antenna limiters Antenna loading vs. B T in different discharges Stable operation Antenna loading decreased  coupling weakened  offset of the limiters may be too much

A new four phase rf generator has been installed on SUNIST –0.56 ~ 1.6 MHz / 4 X 25 kW Setup of a new rf generator The new rf generator of SUNIST Stable operation Arbitrary phase adjustment

Ohmic discharges need to be improved further –A stable density is required for AW experiments Future Work Typical ohmic discharge waveforms with different gas puffing Stable operation

Flexible configurations of aspect ratio –For better understanding of the shaping effects Future Work (continued) Stable operation plasma Low A High A Movable limiter Inner limiter Outer limiter

The coupling character of Alfven wave has been studied experimentally –Coupling is more strong without limiters than with limiters In the future –Re-investigate the coupling using new rf generator under higher and flexible plasma parameters Thanks Summary Stable operation