17th International Stellarator / Heliotron Workshop Princeton, USA Overdense Plasma Operation in WEGA Stellarator Matthias Otte, H.P. Laqua, S. Marsen,

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Presentation transcript:

17th International Stellarator / Heliotron Workshop Princeton, USA Overdense Plasma Operation in WEGA Stellarator Matthias Otte, H.P. Laqua, S. Marsen, Y. Podoba, T. Stange, D. Zhang, F. Wagner and the WEGA-team Max-Planck-Institut für Plasmaphysik, Greifswald branch, EURATOM Ass. Wendelsteinstr. 1 D Greifswald, Germany

Matthias OttePrinceton, 13 October Outline Motivation on over-dense plasma generation Overview WEGA Setup of 28GHz ECRH system Results from OXB mode heating Summary and outlook Overview of actual WEGA results will be presented on poster P-D02 during workshop’s poster session from Monday till Wednesday.

Motivation Why OXB Mode Heating ? Electromagnetic waves used for resonant electron or ion cyclotron heating can not penetrate the plasma above associated cut-off density ➽ problem for high-density operation Electrostatic Bernstein waves (EBW) -no density limit but need a medium for propagating -excitation via mode conversion process (X-B or O-X-B) -damped on electrons at fundamental or harmonic Doppler-shifted electron cyclotron resonance Plasma is optically thick for EBWs even at low temperature (<10 eV) Interesting physics: -Wave field physics -Generation and confinement of fast electrons -Current drive in overdense plasmas Matthias OttePrinceton, 13 October 20093

The OXB-mode Conversion Process Matthias OttePrinceton, 13 October Requirements O-wave launched with θ opt in respect to the magnetic field vector - correct polarisation - angular window width ~ 1/k 0 L n (with L = n / grad n) Density n e above cut-off density n e,cut Existence of UHR (ω > ω c ) Example: Ray tracing calculation of slow X-mode conversion process at W7-AS θ op t y

Matthias OttePrinceton, 13 October WEGA = Wendelstein Experiment in Greifswald zur Ausbildung (for education) Hybrid tokamak/stellarator experiment in Grenoble/France in the 1970s and ´80s for development of LH heating Installation at IPP Greifswald in 2000/2001 as classical stellarator October 2009: about pulses with typical duration of s 0.5 T operation for 30 s possible Up to ~100 pulses / day Overview WEGA

Matthias OttePrinceton, 13 October B toroidal B poloidal B vertical Setup and Magnetic Properties Vessel Two half-tori with R = 0.72 m, r = 0.19 m 100 ports (  ≤ 92 mm) Magnetic field coils 40 toroidal field coils: B max (cw) = 0.34 T and 0.9T for pulsed operation Helical field coils: l = 2, m = 5 Rotational transform  = a max = 11 cm, V max = 0.15 m 3 (limiter configuration) a <= 5 cm for high iota (separatrix configuration) Vertical field and error field compensation coils Plasma heating kW 2.45 GHz (cw) 10 kW 28 GHz (cw ) 5-arm transformer with 0.44 Vs Working gases Helium, Argon, Hydrogen

Relevant 28GHz Diagnostics Interferometer (80 GHz, single channel) Sniffer probe (28 GHz) 12-channel bolometer array Radiometer with 12 channels (23 – 40 GHz) for ECE, EBE and Reflectometry Spectrum analyzer up to 40 GHz Soft X-ray (PHA) Langmuir probes Matthias OttePrinceton, 13 October Bolometer array Sniffer probe ECE antenna system Low field side High field side

Microwave Diagnostic Matthias OttePrinceton, 13 October Temperature measurements ECE: problem ➽ over-dense and optically thin Electron Bernstein emission EBE: accessible via BXO conversion, optically thick even at low temperature, no density limit ➽ oblique alignment of antenna necessary Oblique alignment of EBE antenna (55°) EBE horn HFS-ECE horn

Setup of 28GHz ECRH system

Matthias OttePrinceton, 13 October ECRH Components Installation of a 28GHz ECRH for B 0 =0.5T operation in cooperation with CIEMAT/Spain and IPF Stuttgart 4.3 m Gyrotron 10kW cw (20 kHz modulation) Transmission line Mirror system inside vessel for X2 and OXB mode 28 GHz Gyrotron (CPI VGA-8028) 0.1  10 kW CW 15 kW 10s 20 kW 1s 80 dB Directional Coupler TE 02 -TE 01 Converter DC Break & Mode Filter TE 01 -TE 11 Converter HE 11 Waveguide HE 11 Uptaper TE 11 -HE 11 Converter Vacuum Window WEGA Vacuum Vessel Arc Detector Arc Detector Antenna Forward Power Detector Reflected Power Detector Smooth Waveguide TE 02 TE 01 TE 11 Uptaper TE 11 HE 11 Ø41 mm Ø63.5 mm Ø44.45 mm Ø63.5 mm Ø32.6 mm

Matthias OttePrinceton, 13 October ECRH Components: In-Vessel Mirrors Switching between X2 and OXB mode heating possible by exchanging A-port mirror Quasi-optical Gaussian beam in equatorial plane at vertical elongated symmetry plane (steepest gradient expected) under angle of 55° to magnetic field line Steerable OXB-mirror for finding optimum OXB conversion (up to now venting of vessel necessary) flux surfaces Gaussian beam port toroidal coil elliptical focus OXB-Mirror Discharges at B 0 =0.5 T up to 30s at  <= 0.4 OXBA-portX2

Matthias OttePrinceton, 13 October OXB – Ray Propagation Ray-tracing calculations by J. Preinhaelter and J. Urban (IPP-Prague) predict high efficiency central heating at densities above 1 × m -3 and no Doppler-shift Challenge How to reach 1×10 19 m -3 (28 GHz O-cutoff) ? 0.5×10 19 m -3 is already reached with X2- mode (X-cutoff) Additional energy sources available 20 kW 2.45 GHz resistive R-wave heating at 0.5 T Ohmic heating with transformer mirror

Results from OXB mode heating

Matthias OttePrinceton, 13 October Reaching OXB mode Start OXB conversion above 1 × m -3 (O-mode cut-off): central deposition due to high single pass absorption EBE signal in keV – range (radiation but not electron temperature) ➽ supra-thermal electron component density peaking up to 1.4 × m -3 (33 GHz EBE signal) Switch off additional heating: Plasma sustained by OXB- heating only, strongly peaked radiation profiles Overcome power gap above 0.5 × m -3 (X2-mode cut-off) by additional resistive 2.45 GHz heating (20 kW) 1. 1.Plasma start-up: Plasma generation by 10% X2-mode at resonant field, multi-pass heating with broad deposition He, #30340

Further Results during OXB Phase Matthias OttePrinceton, 13 October Langmuir probe Peaked profiles with n e,0 = 1.3 × m -3 well above cut-off density Bulk parameters:  n e = 1 × m -3 at R = 75 cm  T e = eV 12-channel Bolometer camera Reconstructed radiation power signal: Peaked profile during OXB-phase due to central power deposition Broad, flat profile during O2-mode heating due to multi-pass absorption

Matthias OttePrinceton, 13 October Soft X-ray Pulse height analyzer with range of 0.5keV – 15keV Signals above 1keV detected exclusively during OXB phase Only Bremsstrahlung, but no characteristic radiation detected (e.g. wall material) except from argon from previous experiments ➽ supra-thermal electron component arises in center as expected by resonant absorption mechanism

Optimization Gas Analogue results in argon even without additional heating but no conversion reached in hydrogen (factor of 3 missing in density) Rotational transform iota (iota↑, confinement↑, but plasma size↓) best iota = 0.3 (still limiter configuration, higher iota not accessible) OXB mirror position October 2009: remotely steering of OXB-mirror possible, detailed study of emission pattern performed Matthias OttePrinceton, 13 October Thermal emission pattern for varying toroidal position on target installed at vessel high field side

Summary and Outlook WEGA has reached a new unexplored physics regime! Quasi-stationary overdense plasma heated by 28 GHz OXB mode conversion exclusively Supra-thermal electron component detected in EBE radiation, confirmed by X-rays detector, however quantitative fraction to be determined Acceleration mechanism of the supra-thermal electron component ➽ maybe auto-resonance cyclotron acceleration of electrons which is not limited Supra-thermal electrons Fast particle confinement Generation of current drive Ray tracing with non-Maxwellian ➽ do they exist and how could they be measured? Source for highly charged ions Matthias OttePrinceton, 13 October

Acknowledgement The authors gratefully acknowledge the work of D. Aßmus, R. Gerhardt, the colleagues of the W7- diagnostic group and the technical staff of the IPP. Thank you for your attention! Matthias OttePrinceton, 13 October

EBE Signal T rad before OXB-heating Chan GHz Chan GHz Chan GHz Chan GHz T rad during OXB-heating notch filter