Plasma Current Start-up Experiments without the Central Solenoid in TST-2 and Future Plans Y. Takase Graduate School of Frontier Sciences, University of Tokyo for TST-2 and Groups Joint Meeting of the 3rd IAEA Technical Meeting on Spherical Tori and the 11th Internat Workshop on Spherical Torus St. Petersburg State University, Russia 3-6 October 2005

Compact, high  plasma with good confinement can be realized in ST  compact burning plasma experiment and fusion reactor Motivation for CS-less I p Start-up Lower aspect ratio and elimination of central solenoid (CS) improves economic competitiveness

TST-2 Spherical Tokamak Design parameters of TST-2: R = 0.38 m / a = 0.25 (A = 1.6) B t = 0.3 T (0.3 T achieved) I p = 0.2 MA (0.14 MA achieved) t pulse = 0.05 s (0.3 s achieved) Main research topics: Plasma start-up optimization RF heating and wave physics MHD instability and reconnection Control of turbulence and transport TST-2

Relocation of TST-2 (Twice in 2 Years) U. Tokyo Hongo ~2002 U. Tokyo Kashiwa 2004~ Kyushu U. Fukuoka 2003 TST-2

EBW Antenna Waveguides from 8.2GHz klystrons Typical plasma equilibrium TST-2 at Kyushu University (2003) Heating / current drive By ECW and/or EBW

Plasma Stored Energy Increase (Heating) Observed during RF Injection 15% increase in W total RF onno RF W total = W kin + W mag(pol) W kin

Plasma Current Formation by ECH 1 kA / 1 kW achieved by ECH (2.45 GHz) Higher current for low gas pressure  low collisionality is important Requires vertical field with positive curvature  trapped particles are important TST-2

Solenoidless Start-up Experiments Forest scenario –“pressure driven current” with mirror B v – 4kA maintained for 0.27 s (with static B v ~ 2 mT, no induction) – T e = 160 eV (plasma is collisionless) no  =  ce res. inside vac.ves.

Reconstructed Equilibrium of the RF Start-up Plasma (I) R [m] Z [m] 0 Plasma is limited by the outboard limiter, j  is truncated at top and inboard

Features of RF Start-up Plasma Equilibrium R [m] j  [kA/m 2 z=0 p z=0 0 high  p large outboard boundary current Outboard co-PS current is dominant, while inboard counter-PS current is truncted. Steep pressure gradient at the outboard boundary Soft X-ray flux and temperature are roughly consistent with the pressure deduced from equilibrium reconstruction.

Completely CS-less Start-up to I p = 10 kA Achieved in TST-2 PF1 PF2 PF4 PF3 PF5 CS 1 turn 6 turns 24 turns

New Start at the Univ. Tokyo Kashiwa Campus Resume operation at Kashiwa –Solenoidless start-up Based on results of JT-60U –Reconnection physics Reconnection Events Ion heating –Turbulence and transport Develop fluctuation diagnostics –HHFW heating / current drive 10-30MHz / 400 kW k || control (new antenna) –Prepare LHCD system 200MHz / 400kW (from JFT-2M) TST-2

100kW of RF Power Injected Successfully Full-wave calculation by TASK/WM HHFW Antenna TST-2 B t = 0.3 T, f = 21 MHz, n = 10, n e = 2  m -3, T e = 0.3 keV EE

Preparation in Progress for 200 MHz Experiments EE B t = 0.3 T, f = 200 MHz, n = 10, n e = 2  m -3, T e = 0.3 keV Full-wave calculation by TASK/WM TST-2 Combline antenna 200 MHz transmitters (from JFT-2M)

Reconnection Events TST-2

li W(J) radiation H_alpha Hard X ray mag. fluct. log(b^2) Ip Detailed Time Evolution q_0 TST-2 t=26.2ms t=26.6ms

CV intensity decreases while CIII, OIII intensities increase (loss of electron energy) CV (core ）, and OV, CIII, OIII （ edge ） ion temperatures increase at reconnection events Reconnection events Conversion of magnetic energy to ion kinetic energy Ion Heating Observed at Reconnection Events TST-2

A New Experiment to Explore Ultra-High  Plasma Formation by Plasma Merging UTST

TST-2 Magnetic Diagnostics

Megawatts of heating power can be obtained by plasma merging / reconnection Rapid Heating by Plasma Merging / Reconnection TS-3 Y. Ono, et al. a a time [msec] I tfc =0 dW/dt ~10[MW] I tfc =10kA dW/dt ~6[MW] I tfc =35kA dW/dt ~4[MW] Merging Single Merging Single Merging Single B+B text B p I z B+B text ST#1ST#2 B p B p I z B+B text High  ST Comparison of thermal energy evolution for merging (solid line) and single (dashed line) formation FRC High q ST Low q ST

Start-up without Center Solenoid Demonstrated VT in coil disconnected Use VR and VT out only Discharge duration increases with improvement of plasma position control JT-60U

Demonstration of Advanced Tokamak Operation without the Center Solenoid Start-up and initial ramp-up Noninductive ramp-up Transition to self-driven phase JT-60U

ITB + H-mode (I p = 0.7MA)   p = 3.6,  N = 1.6, H H = 1.6, f BS > 90% Profiles of CS-less Advanced Tokamak with Very High Self-Driven Current Fraction JT-60U transport barriers

Backup(I) R [m] Z [m] V.V 1st2nd3rd PF2 Coil Antennas The following flux function was tried, but parameters  and  were not efficient. External poloidal field This implies that the present magnetics cannot resolve edge fine structure.

Backup (II) Fitting was poor for inboard B z poloidal angle Flux loop Pickup coils Flux function (red) and area of large j  (blue).