NSTXHHFW

1

R0a

R0 = 0.38 m a = 0.25 mA = R0/a = 1.5Bt = 0.3 TIp = 0.1 MA

Overview of TST-2 Experiment

Y. Takase for the TST-2 Group

The University of Tokyo, Kashiwa 277-8561 Japan

The Second A3 Foresight Workshop on Spherical TorusRoom 105, Liu-Qing BuildingTsinghua University, Beijing, China6-8 January 2014Motivation and Goal of ResearchEconomically competitive tokamak reactor may be realized at low aspect ratio (A = R0/a) by eliminating the central solenoid (CS)

S. Nishio, et al., in Proc. 20th IAEA Fusion Energy Conf., FT/P7-35 (Vilamoura , 2004). 2higher Btlower A higher btno CSCS

2002.06.21Start-up and initial ramp-upNoninductive ramp-up (LH)Transition to self-driven phase

advanced tokamakS. Shiraiwa, et al., Phys. Rev. Lett. 92 (2004) 035001. Formation of Advanced Tokamak Plasma without CS was Achieved on JT-60U

Is plasma current (Ip) ramp-up by LHW possible in ST? Demonstrate on TST-234Three Antennas used on TST-2

Combline Antennatraveling waveexcites traveling FWIp driven by SW (LHW)

requires mode conversion from FW to SWexcites traveling SW

travelingwave

traveling waveexcites traveling SW

sharper k spectrum & higher directivityGrill AntennaECC Antennan|| Dependence of Ip and HX Spectrum(Grill Antenna)

Highest plasma current was obtained for 1.5 < n|| < 4.5

Count rate of high energy photons was lower when n|| > 7.5[kA]n|| 5T. Wakatsuki

Calculated Electric Field (Ez)Ez (Without Plasma)6T. InadaRamp-up to 12 kA by LHW(ECC Antenna)

IpBvBtFWDREFtransECHnelLCFSlimiter#2 #4 #6 #9AXUVHaTST-2 Shot 983627T. ShinyaScaling of Ip with Bv and BtIp increases with Bv .Upper bound of Ip increases with Bt .co-drivecounter-drive8T. ShinyaHigher Ip is obtained with ECCA for the same PRF . Higher ne is obtained with ECCA. Scaling of Ip with PRF and ne1 A/W9T. ShinyaComparison of the Current Drive Figure of MeritHigher hCD is obtained with ECCA (mainly because of higher ne). An order of magnitude improvement in hCD may be possible by suppression of edge wave power loss and fast electron loss operation at higher Bt, ne, Ip , PRF, Te should help.10IP [kA]CD vs IP CD [1016 A/m2/W ]T. WakatsukiHard X-ray Spectra for Co/Ctr Current Drive Directions(Combline Antenna)Photon flux is an order of magnitude higher in the co direction.Photon temperature is higher in the co direction (60 keV vs. 40 keV).Consistent with acceleration of electrons by a uni-directional RF wave.

Ip coctr

antennaHX viewHX viewwave11T. Wakatsuki

CoCh1CtrCh2

SBD Be&SBD P.P SBD PP__ SBD Be

1721.51.00.50[kA][j/m^2][1/m^3]Analysis section[ms][ms][ms]

E[keV]:Ch1,Co:Ch2,CtrSimilar co and counter temperatures,but co flux is larger than counter flux.[a.u.]Hard X-ray Spectra for Co/Ctr Current Drive Directions(ECC Antenna)12K. Imamura

59 mm5.6 mm675.7 mm31Limiter Vf has a large gradient near the limiter radius. LCFSRadial Profile of the Floating Potential(Combline Antenna)13H. KakudaFloating potential (where Iprobe = 0) is sensitive to the presence of high energy electrons.R= 585 mmR=700 mmR=125 mmantennaprobewave

IpBtIon Temperature, Toroidal Flow and Poloidal Flow(Grill Antenna, Ip ~ 6kA)Lower sightlineUpper sightline

Right sightlineLeft sightline

14S. TsudaEmission intensity is small in the poloidal direction and was comparable to the case of ECH.Measurement was not possible amount of light is insufficient in the 35-60msec.In a measurable period of time, as well as when the ECH, poloidal flow was larger than the toroidal flow

14One-Fluid Equilibrium (EFIT)

Ip = 11 kAbp = 0.9Reconstructed equilibriumVisible light imagecurrent density profile peaked on the outboard side15A. Ejiri15Plasma pressure max ~40 Pa (?)Peaked toroidal current distribution (?)What is plasma sound speed?Large Er shear ion orbit compressionElectron: largely satisfies pe = -JBIon (outboard): roughly equal pi, centrifugal, and electrostatic forces balanced by -JB

Two-Fluid Equilibrium (Initial Result)TST-2 Shot 7546716M. Peng & A. IshidaFloating Potential Measurement at 200 MHz :ES Probe with Embedded High Impedance Resistor

1051041030.11101001000[][MHz]Chip resistor 100 kRed broken line : Sheath impedanceGreen solid line :Ordinary Langmuir ProbeBlack solid line :Electrostatic Probe witha 100 k Chip ResistorAbsolute Value of Impedance23452345Electrode 1, 2, 3 With 100 kElectrode 4 Magnetic ProbeElectrode 5Ordinary Langmuir Probe17H. KakudaPhase Difference and Wavenumber Measurement

Electrode Separation14.2 mm

2318H. KakudaFrequency Spectra Measured by RF Magnetic Probes(Combline Antenna)

Combline antenna excites the FW, but LHW generated by PDI?Pump wave (f = 200 MHz 1 kHz) has FW polarization (|Bt| > |Bp|). PDI sidebands have SW (LHW) polarization (|Bt| < |Bp|).Pump wave weakens when sidebands intensify.

poloidaltoroidalpumplower sidebandupper sideband19T. ShinyaRF Magnetic Probe Array for k Measurement(Grill Antenna, ~ 1kA)

radialtranslationabcderotation30 mm20k|| = 10 m-1 corresponds to n|| = 2.4 T. ShinyaNd:YAG laser1064nmLaser energy: 1.6 [J]Repetition rate: 10 [Hz]Pulse width: 10 [ns]~6mSpherical MirrorPolychromatorSpherical MirrorBeam DumpLensNd:YAG LaserTST-2

Brewster WindowOptical isolatorYAG laserTST-2TST-2 Double-Pass Thomson Scattering SystemLaser beam is focused in the plasma by a lens (f = 2000mm).Optical isolator is used to prevent laser damage by reflected light.21[1st pass][2nd pass]Scattering VectorP||PIncident LaserBSphericalMirrorObserverPlasmaBJ. Hiratsuka21Comparison with interferometerThomson: 5.7 1018 [m-2]Interferometer: 4.9 1018 [m-2]Thomson scatteringTypical ne and Te profiles(OH Plasma)reasonable agreementTiming of TS measurement

InterferometerneL time slice

ne profile

Te profileneL [1018 m-2]ne [1019m-3]Te [eV]Major Radius [mm]22J. Hiratsuka

R=220mm (plasma edge)R=389mm (plasma center)Electron Temperature Anisotropy(OH Plasma)Assuming Maxwellian velocity distribution, current densities are: plasma edge 300kA/m2plasma center 500kA/m2 (plasma current / plasma cross section ~ 300kA/m2)23J. Hiratsuka

Time evolution of electron temperature

Central and edge Te approach each otherTST-2Center of Plasma CoolingEdge of Plasma Heating

Evolution of Te anisotropy

CenterEdge

24K. Nakamura24Multi-Pass Thomson ScatteringTST-2f = 2500 mm Mirror #1f = 2000 mmMirror #2Pockels CellPolarizerHalf-wave plateYAG LaserBrewster WindowLaser pulse is confined between concave mirrors (red line)25H. Togashi25Probe System for Turbulence StudyPlasma flow, magnetic field, potential, and density fluctuations are measured simultaneously. coil1Bt , Ip

3-axis pickup coil

tiptip2tip3JupJdowncoil230mm

zonal flowGlobal modeMicro-turbulenceNonlinear energy transfer directionlocation26M. Soneharathis is the composite probe system used in the experiment.the size is 30 mm in diameter at the front, and it has 7 electrodes to measure floating potential or ion saturation current, probe tip is made of Molybdenum, and encased in boron nitride.each electrode is 1mm in diameter 5mm in lengththe two electrodes located in the shadowed area can be used to measure plasma flow as a mach probe. It also has a 3 axis pickup coil to measure 3 dimensional magnetic fluctuation simultaneously.this probe system is inserted at the midplane of the device.

this probe tip can be rotated with rotary stage. ant it is inserted at the midplane of the device.30mm5mm2mm

----- (2013/02/26 10:25) -----BT,Bp,Ip26Poloidal Flow and Stresses Poloidal flow and radial derivative of stress (Te=Ti is assumed).Profiles of poloidal flow and stress derivative are similar.

27M. SoneharaRogowski Probe

1-D pickup coil3-D pick up coilRogowski coil for measuring the local current was developed.Langmuir probeRogowski coil28H. Furui28Successful Measurement of Local Current(OH Plasma) Plasma current Local plasma current density Poloidal magnetic field generated by the plasma current Bp is estimated to be under 20 mT at the location of the Rogowski coil. Output voltage of the Rogowski coil due to Bp is estimated to be under 100 V.

Local current measurement was performed successfully with small pickup of magnetic fields.

29H. FuruiConclusionsST plasma initiation and Ip ramp-up by waves in the LH frequency range were demonstrated on TST-2.

Inductively-coupled combline antenna (FW launch), dielectric-loaded waveguide array (grill) antenna (SW launch), and electrostatically-coupled combline antenna (SW launch) were used.

Spontaneous formation of the tokamak configuration with closed flux surfaces was observed when the toroidal current in the open field line configuration exceeded a critical level (~ 1 kA in TST-2).

Similar Ip was obtained with different antennas for similar Bv and PRF, but hCD is higher with the ECC antenna because of higher ne.

At low Ip ( < 2 kA in TST-2), Ip is dominantly pressure-driven, and is proportional to Bv. In this regime, Ip is independent of the wave type. At higher Ip (> 5 kA in TST-2), Ip becomes mainly wave-driven. In this regime, control of the current density profile by externally excited waves should become possible.30ConclusionsVarious diagnostics and RF launchers are being developed.

Electrostatically-coupled combline antenna (LHW launch).

Hard X-ray spectroscopy and imaging.

UV spectroscopy for ion temperature and flow measurements.

Electrostatic probes for wave and turbulence measurements.

Magnetic probes for wave and current density measurements.

Double-pass Thomson scattering for Te anisotropy measurement.

Two-fluid equilibrium.31Need for Power Supply UpgradeIn order to demonstrate ramp-up to higher Ip, power supply upgrade is needed to sustain higher Bt (~ 0.3 T) for longer time (> 0.1 s).

32

32Near-Future PlansCoil Power Supply UpgradeSustained high field (Bt = 0.3 T for > 0.1 s) for further Ip ramp-up.

Wave/Turbulence DiagnosticsElectrostatic probe arrayReflectometer / interferometer-polarimeter

Plasma diagnosticsMulti-pass Thomson scatteringEBW emissionCurrent profile measurement by Rogowski probe / magnetic probeIon flow measurement33