RF Start-up, Heating and Current Drive Studies on TST-2 and UTST Y. Takase, TST-2 Team, UTST Team The University of Tokyo The 15th International Workshop on Spherical Tori October 2009 Madison, Wisconsin, U.S.A. TST-2 1 UTST R  0.38 m a  0.25 m B   0.3 T I p  0.2 MA

Outline I p start-up experiments on TST-2 High-harmonic fast wave (HHFW) experiments on TST-2 and UTST Plan for LHCD experiment on TST-2 2

I p Start-up Experiments on TST-2 In TST-2, I p start-up, ST plasma formation and sustainment have been achieved by EC power (up to 5 kW at 2.45 GHz). –When I p reaches a critical value, I p increases abruptly (current jump) and reaches a steady sustainment level I p sus which is proportional to B z. –Before current jump the field configuration is open. –After current jump an ST configuration with closed flux surfaces is sustained. Once initial plasma is formed, RF power (up to 30 kW at 21 MHz) injected using the HHFW loop antenna can induce a current jump and sustain the ST configuration with the same I p sus as the EC sustained case. 3

2-Strap HHFW Antenna (only 1 strap was used) 21MHz, up to 400 kW (up to 30 kW was used) TST-2 Spherical Tokamak and Heating Systems X-mode launch horn antenna for ECH 2.45 GHz, up to 5 kW R = 0.38 m a = 0.25 m A = 1.5 4

3 Phases of I p Start-up by ECH Open Field Lines Current Jump Closed Flux Surfaces z[m] x[m] y[m] z[m] x[m] y[m] I p increases rapidly once I p reaches a critical level determined by B v. 5 particle orbit

RF (21MHz) power can induce a current jump. Antenna excites a broad toroidal mode number spectrum, up to |n  | ~ 20. But only |n  | = 0, 1, 2 can propagate to the core. Ion absorption is not expected due to high  /  ci (> 10). Ion (H/D, C, O) heating was not observed (< 10 eV). Electron absorption is expected to be weak due to low  e. Soft X-rays (up to 3 keV) were observed at high RF power (~ 30 kW). Sustainment by RF Power Alone RF only RF RF sust. EC sust. I p can be sustained by RF power alone. 6

Truncated Equilibrium To treat finite p and j in the open field line region, “truncated equilibrium” is used. [A. Ejiri et. al., Nucl. Fusion 46, (2006).] Outboard limiter R Top limiter Bottom limiter LCFS Inboard limiter The following effects are not taken into account: anisotropic pressure parallel pressure gradient Truncation boundaries 7

Equilibrium Reconstruction Flux loops Pickup coils Saddle loops Vacuum field and locations of magnetic measurements x10 Red: measurements Black: fit Distribution of I eddy is pre-calculated for given I p (t). I eddy can become ~1/3 of I p during current jump. 8

Evolution of Equilibrium Pressure [Pa] j  [kA/m 2 ] Z [m] R [m] #53783, t=25ms  jfjf total ff’ p’ (a) j f [kA/m 2 ] Z [m] R [m] #53783, t=40ms  jj total ff’ p’ LCFS (b) j f [kA/m 2 ] Z [m] R [m] #53783, t=50ms  jj total ff’ p’ LCFS -4 (c)(a)(b)(c) appearance of closed flux surfaces 9

Comparison of Equilibria during Sustainment Truncated boundary LCFS  jj Inboard limiter LCFS Outboard limiter # ms EC sustained, I p = 0.6 kA Truncated boundary LCFS  jj Inboard limiter LCFS Outboard limiter # ms RF sustained, I p = 0.6 kA Truncated boundary LCFS  jj Inboard limiter LCFS Outboard limiter # ms EC sustained, I p = 1.3 kA 10

v || /v 0 v ^ /v (I) (II) (IV) (III) A D B C EF (a) Velocity Space Structure in Vacuum Field Velocity space for orbits starting from R = 0.38 m for PF2+PF5 configuration classes of particle orbits A. Ejiri et. al., Nucl. Fusion 47, (2007). 11

Banana orbits under the influence of self-generated E  are analyzed. –Angular momentum is conserved from axisymmetry –Banana particles are frozen to flux surfaces, and move with flux surfaces towards the low field side. –This movement causes kinetic energy and plasma current to decrease (inverse of Ware pinch). Passing particles have short energy decay times. They are accelerated in the direction to reduce I p. Movement of orbit is small.  I p stops increasing when closed flux surfaces are formed. Effect of E  on Particle Orbits in Start-up Plasma 12 – 14 mV/m Toroidal Field (t=35 – 40 ms) R [m] –3 mV/m

Condition for Flux Conservation 0 0 time [ms] 10 P , qRA  mRV  Angular momentum conservation  Flux conservation 13

 of Orbits in Velocity Space 14 Trapped region Inverse of Ware pinch;  = 0  < 0,  R,  Z ~ 0 Counter moving Acceleration  < 0,  R,  Z ~ 0 Co moving Deceleration E  -field dominated region Velocity normalized by V 0 =R  e pol Transition region Co  Trapped:  ~ 0 Mixed transition region Co  Counter :  < 0,  R < 0 Co  Trapped:  ~ 0,

Discussion of Current Drive Mechanism Pressure [Pa] j f [kA/m 2 ] Z [m] R [m] #53783, t=50ms  jj total ff’ p’ LCFS -4 4V/1.5 A=3  =400A Because of the V.V. current, the poloidal current is always in the diamagnetic direction. In addition to the precessional current of trapped particles, Pfirsch-Schülter current can give net toroidal current in the open field line region. The vertical drift current (I d ) returns partially through the plasma (I pZ ) and partially throught the vacuum vessel (I VV ). 15

HHFW Experiments on TST-2 and UTST Up to 300 kW of RF power at 21 MHz has been injected into TST-2 plasmas. Two-strap antenna excites HHFW with a wavenumber spectrum peaked at k  ~15 m -1 at the antenna. –When parametric decay instability (PDI) is observed, the T e increment becomes smaller, the edge density shows a rapid increase, and the impurity T i increment increases. –Wave measurements by microwave reflectometry, electrostatic and electromagnetic probes are consistent with the HHFW pump wave decaying into the ion Bernstein wave (IBW) or the HHFW lower sideband, and the low frequency ion-cyclotron quasi-mode (ICQM). –The lower sideband power varies approximately quadratically with the local pump wave power, which becomes smaller as absorption of the pump wave by the plasma increases. In UTST, direct wave measurements inside the plasma were made with a 2-D array of magnetic probes. –The measured wave field profile was roughly consistent with the result of TORIC full-wave calculation. 16

φ ＝ -60° φ ＝ -30° φ ＝ -55°φ ＝ -65° φ ＝ -115° φ ＝ -120° φ ＝ 155° φ ＝ 150° φ ＝ 65° φ ＝ 60° φ ＝ 55° φ ＝ 30° φ ＝ 0° 2cm Direction of B field to be measured Core (insulator) 1 turn loop S. S enclosure Slit Semi-rigid Cable φ ＝ -125° RF Diagnostics BφBφ BzBz Reflectometer φ ＝ 145° TOP VIEW center stack probes strap

Parametric Decay Observed by Reflectometer There is a threshold in pump wave power. → Parametric Decay Instability (PDI) Ion Cyclotron Quasi-Mode reflectometer f ci RF probe pump QM LS pump LS

Correlation Between PDI and Electron/Ion Heating Stronger PDI   Less electron heating  More ion heating inboard-shifted outboard-limited inboard-shifted

Spectral Broadening of the Pump Wave Spectral broadening can occur by scattering by density fluctuations parametric decay instability Spectral broadening becomes larger farther away from the antenna. Downshifted and broadened pump wave was observed at the inboard wall.

UTST Experiment (Univ. Tokyo and AIST) High-  ST formation by double-null merging (DNM) High-  ST sustainment by additional heating: NBI and RF Objectives: PF Pair Coils 0.7m HHFW antenna 2m Magnetic probe array located 45  away toroidally from the antenna

Magnetic Flux Surfaces During HHFW Injection 4.9ms

RF B 2 Profile Comparison HHFW field is stronger in the periphery for single-strap excitation. RF magnetic field strength is lower for double-strap excitation Single-strap excitationDouble-strap excitation

Wavenumber Measurement Radial coherence Vertical coherence Reference

Radial and Vertical Wavenumbers Frequency [MHz]

Plan for LHCD Experiment on TST-2 Preparation is underway for lower hybrid (LH) current drive experiments on TST-2. –Up to 400 kW of power at 200 MHz will be used to ramp-up I p from a very low current (~ 1 kA), very low density (< m -3 ) ST plasma. Wave propagation and absorption were investigated using the TORIC-LH full wave code. –Core absorption is expected initially, but absorption is predicted to move radially outward with the increase in I p and density. 26

Preparation for LHCD Experiment on TST MHz transmitters Combline antenna (11 elements) Initially, the combline antenna used on JFT-2M, adapted for use on TST-2, will be used to excite a unidirectional fast wave with n  = 12 (corresponding to n || = 5). Direct excitation of the LH wave is planned in the future. The fast wave can mode convert to the LH wave and drive current. (200 kW x 4, from JFT-2M)

TORICLH/TST2/101/O n e0 = 1 x m -3 T e0 = 1 keV I p = 10 kA n ||0 = 7  ant = 0  Collaboration with J. Wright, P. Bonoli (MIT)

TORICLH/TST2/101/U n e0 = 1 x m -3 T e0 = 1 keV I p = 30 kA

TORICLH/TST2/101/M n e0 = 1 x m -3 T e0 = 1 keV I p = 100 kA

TORICLH/TST2/102/A n e0 = 1 x m -3 T e0 = 1 keV I p = 100 kA n ||0 = +7  ant = 0 

TORICLH/TST2/102/B n e0 = 1 x m -3 T e0 = 1 keV n ||0 = +3

TORICLH/TST2/102/G n e0 = 1 x m -3 T e0 = 1 keV n ||0 = -3

TORICLH/TST2/102/F n e0 = 1 x m -3 T e0 = 1 keV n ||0 = -7

TORICLH/TST2/101/O n e0 = 1 x m -3 T e0 = 1 keV I p = 10 kA n ||0 = 7  ant = 0 

TORICLH/TST2/102/P n e0 = 1 x m -3 T e0 = 1 keV n ||0 = 7  ant = 90 

TORICLH/TST2/102/Q n e0 = 1 x m -3 T e0 = 1 keV n ||0 = 7  ant = -90 

TORICLH/TST2/102/O n e0 = 1 x m -3 T e0 = 1 keV n ||0 = 7  ant = 180 

TORICLH/TST2/101/A n e0 = 5 x m -3 T e0 = 1 keV LH res. at x = -5 cm FW cutoff at x = -14 cm