Slide 1

MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China EBW Experiments on MAST V. Shevchenko 1, G. Cunningham 1, A. Gurchenko 2, E. Gusakov 2, A. Saveliev 2, A. Surkov 2, F. Volpe 1 1 EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon,Oxon, OX14 3DB, UK 2 Ioffe Institute, Politechnikheskaya 26, 194021, St. Petersburg, Russia Slide 2 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Long-term goals CTF or ST Power Plant will not allow the use of a central solenoid NBCD is considered as a main source to sustain steady state current in ST plasma EBW can provide a critical off- axis current drive to sustain ST plasma in equilibrium EBW H&CD can also assist plasma start-up Midplane topology of cut-offs and resonances in MAST (H-mode) EC harmonics are usually obscured by cut-offs in ST Introduction Slide 3 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China B total nene Plasma LH O-mode RH O-mode Antenna Configuration for O-X-B Conversion O-X-B conversion is possible when ce < RF < pe Angular width of mode conversion cone depends on n e Launch plane is determined by B tot and n e at the plasma edge Angle between n e and optimal direction depends only on | B tot | in the layer where RF = pe B tot at the plasma edge is the most crucial parameter for the optimal launch configuration sin 2 = N 2 ||,opt = Y/(Y+1), Y= ce / a)b) O-X-B mode (N || < 0) conversion window calculated for the 60 GHz launcher located 22.5 cm below the midplane in MAST: a) high density ELM-free H- mode, b) high density L-mode plasma. Contours indicate 10% steps in conversion efficiency, i.e. 0.5 means 50% mode conversion efficiency etc. Slide 4 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China EBW Modelling in MAST Midplane topology of cut-offs and resonances in sawtoothing H-mode plasma, shot #7798 in MAST. like O/X Power deposition radius against frequency within the range of fundamental EC resonance. 40 cm above midplane launch, N || < 0. 1 ce Slide 5 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China EBW Emission in MAST EBW emission spectrogram measured in sawtoothing H-mode plasma, shot #7798. Red areas correspond to higher emission intensity. ECRF power was injected at 0.21 - 0.24 s, I TF = 91 kA. EBW emission spectrogram in ELM-free H-mode plasma at optimised TF (I TF = 83 kA), shot #11156. ECRF power (0.5 MW) was injected at 0.22 - 0.29 s. From Ruby TS and EFIT Frequency, GHz Slide 6 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China EBW Steerable Launcher in MAST Final polarisation can be chosen from linear to circular Resultant beam divergence is less than +/-2.5 o (w = 25 mm) Poloidal steering range of +/-13 o, toroidal +/-24 o, accuracy of 0.5 o 21 mirrors 7 beams 200 kW each 60 GHz Slide 7 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China O-X-B mode conversion window is broad (about 5 at 50% efficiency). Relaxed launch configuration. EBW absorption is expected to be very peripheral, r/a ~ 0.8. Non-linear effects can be observed. Plasma Targets for EBWH ELM-free H-mode Sawtoothing H-mode Ohmic H-mode Mode conversion window is moderate (about 3 at 50% efficiency). Relatively stringent to launch parameters. EBW absorption is more central, can reach r/a ~ 0.6. Heating effects can be observed. Mode conversion window is narrow (about 1.5 at 50% efficiency). Very stringent to launch parameters EBW absorption can reach transiently r/a ~ 0.4. Heating effects must be detectable. EBW deposition Slide 8 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Lower Hybrid Probe Head Can be moved to a specified distance from the plasma in the midplane Allows axial rotation 45 o Loop (20x40 mm 2 ) & L shape antennas 76-545MHz spectrum analyser with 10 MHz resolution 50 mm Slide 9 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Parametric Effects in ELM-free H-mode A strong emission enhancement around 134 MHz has been observed during RF injection. This emission was identified as LH emission originating in the X-B mode conversion layer near UHR at 60 GHz Parametric Decay (subject to ~80 kW RF power threshold at UHR) indicates the mode conversion is no less than 50% Slide 10 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Parametric Decay Threshold where T eff =T e + 4T i is the radius of the heating beam L is the inhomogeneity scale: L -1 = grad(n e )/n e + 2( ce / pe ) 2 grad(B)/B For typical MAST parameters: f = 60 GHz, T i T e = 140eV, B =0.38 T, L = 3 cm P UHR */( 2 ) 260 W/cm 2 P UHR * 80 kW for 10 cm Slide 11 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Sawtoothing H-mode Ray-tracing Modelling UHR EBW power deposition profile UHR Poloidal projection of EBW ray-tracing results Slide 12 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China EBWH in Sawtoothing H-mode Plasma Average heating result. Shots #9262, #9263, #9267. EBW emission has been used to optimise magnetic field and launch angles EBE has a maximum when O-mode cut-off is about 2/3 of the gap between 5 ce and 6 ce mode coupling is strongly modulated by sawteeth and ELMs heating effect is better seen after averaging over a few shots no parametric decay has been observed in this scenario EBW radiative temperature measured from 2 ce during ELM-free intervals at optimal magnetic field. a) 60 kJ (~3 MW) 7 kJ (~0.25 MW) 250 kW Plasma Energy, kJ RF Power, kW Time, s 60 70 80 50 40 400 300 200 100 0 0.200.250.30 0.35 b) Before RF injection During RF injection Slide 13 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China EBE in High Density Ohmic H-mode Resonance topology for high density Ohmic H-mode EBW emission (60.5 GHz) in high density Ohmic H-mode during plasma compression Outer plasma radius, m #10638 #10639 ECE EBE 4 ce 3 ce 2 ce EBE 4 ce 3 ce As the plasma boundary moves into the higher magnetic field during compression, EBE comes first from 4 ce, then from 3 ce and finally from 2 ce Slide 14 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Angular window for O-X-B mode conversion measured at 60.5 GHz Launcher Aiming in Ohmic H-mode EBE has been used to optimise the launch configuration for EBWH at 3 ce Slide 15 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China SXR signal in the RF heated shot (red) and reference shot (black) SXR Signal During EBWH SXR signal from the RF heated shot with subtracted reference signal Plasma outer radius D signal 60 GHz EBE signal SXR differential signal RF power SXR signal was doubled during RF pulse Slide 16 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Plasma Energy Increase During EBWH Plasma energy (EFIT) during RF injection. Shots: #10704, #10706, #10707, #10709. Plasma Energy RF Power Plasma Density D signal Slide 17 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Electron temperature profiles measured by Thomson scattering in RF heated (red) and ohmic (blue) plasmas at 0.20 s. Electron Temperature Increase During EBWH Electron temperature profiles measured by Thomson scattering in RF heated (red) and ohmic (blue) plasmas at 0.18 s. Electron Temperature increased by 10-15% due to RF injection Slide 18 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Conclusions Proof-of-principle studies of the O-X-B scheme have been conducted on MAST at 60 GHz. Antenna aiming was performed using EBW emission measurements and mode coupling modelling: In ELM-free H-mode only peripheral absorption is possible but the mode coupling efficiency was proven to be high. Lower Hybrid emission (subject to ~80 kW RF power threshold at UHR) indicates the mode conversion is no less than 50%. Some evidence of EBW heating was observed in the sawtoothing H-mode target plasma. Total plasma energy shows 10% increase during RF pulse. In high density Ohmic H-mode the mode conversion window is very narrow. However, after careful antenna alignment using EBW plasma emission at 60 GHz 10% electron temperature increase has been measured during RF power injection. EBW heating has therefore clearly been observed via the O-X-B mode conversion process Slide 19 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Further Plans Conduct EBW assisted plasma start-up experiments with the recently installed 28 GHz, 200 kW start-up system. As seen in experiments EBW emission is strongly anisotropic. Angular co-ordinates of maximum EBE intensity are predominantly determined by the magnetic field pitch angle at the plasma edge. The magnetic field pitch angle experiences strong variations during the plasma shot due to L-H transitions, sawteeth and other plasma activities. To study the dynamics of the magnetic pitch angle we are planning to: Upgrade the FSR with a remotely controlled spinning mirror Make a real time angular scan over the range, expected due to pitch angle variations. Real time measurements of EBW emission angular dependence should give us a direct estimate of the magnetic pitch angle potential q-profile diagnostic. It would allow more accurate predictions of launch parameters for EBWH experiments and to clarify the edge plasma physics during L-H transition, ELM-free H-mode etc. Slide 20 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China 28 GHz EBW Assisted Plasma Start-up Modelling results. EBW driven current (trapping effects included) in the range of plasma temperatures and densities. Input power 150 kW, 28 GHz. Ray-tracing modelling (poloidal view) of the EBW CD plasma initiation. The RF beam pattern, as measured at low power, is well within the grooved area of the graphite mirror-polariser. Slide 21 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China 28 GHz Antenna Mock-up Assembly In-vessel mirrors of the 28 GHz launcher Mock-up assembly of the launcher (upside-down) Slide 22 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China EBE from plasma to radiometer Tilted spinning mirror for angular scan of EBW emission (red ellipses). Inclination of the contours of BXO conversion efficiency (colour ellipses) Inclination of field lines at cutoff location q-profile Spinning Mirror Principle Slide 23 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Manual replacement of mirrors of different tilt 4.5 o 1.1Kg 1.5 o 0.9Kg 3 o 1Kg Replacement Mirrors Slide 24 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Spinning Mirror Set-up on MAST Slide 25 MAST V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China Summary A dedicated complex of high power RF heating systems, EBW diagnostics, and modeling tools has been developed at Culham in order assess the potential of EBW assisted plasma start-up, EBW heating and CD in MAST. EBE measurements provide a powerful tool for EBW heating optimisation Proof-of-principle (60 GHz) EBWH experiments confirm modelling predictions Real time angular EBE measurement is a potential q-profile diagnostic 28 GHz EBW plasma start-up/assist system has been installed on MAST Low frequency (~18 GHz) 1 MW EBW system is considered for MAST