EBW Experiments on MAST

EBW Experiments on MAST V. Shevchenko1, G. Cunningham1, A. Gurchenko2, E. Gusakov2, A. Saveliev2, A. Surkov2, F. Volpe1 1EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon,Oxon, OX14 3DB, UK 2Ioffe Institute, Politechnikheskaya 26, 194021, St. Petersburg, Russia V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Introduction EC harmonics are usually obscured by cut-offs in ST • 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) V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

LH O-mode Plasma ne Btotal RH O-mode a) b) Antenna Configuration for O-X-B Conversion 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. sin2f=N2||,opt=Y/(Y+1), Y=wce/w • O-X-B conversion is possible when ce < RF < pe • Angular width of mode conversion cone depends on ne • Launch plane is determined by Btot andne at the plasma edge • Angle between ne and optimal direction depends only on | Btot | in the layer where RF = pe • Btot at the plasma edge is the most crucial parameter for the optimal launch configuration V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

likeO/X 1ce EBW Modelling in MAST Power deposition radius against frequency within the range of fundamental EC resonance. 40 cm above midplane launch, N|| < 0. Midplane topology of cut-offs and resonances in sawtoothing H-mode plasma, shot #7798 in MAST. V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

From Ruby TS and EFIT Frequency, GHz 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, ITF = 91 kA. EBW emission spectrogram in ELM-free H-mode plasma at optimised TF (ITF = 83 kA), shot #11156. ECRF power (0.5 MW) was injected at 0.22 - 0.29 s. V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

EBW Steerable Launcher in MAST 21 mirrors 7 beams 200 kW each 60 GHz • Final polarisation can be chosen from linear to circular • Resultant beam divergence is less than +/-2.5o (w = 25 mm) • Poloidal steering range of +/-13o, toroidal +/-24o, accuracy of 0.5o V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

EBW deposition Plasma Targets for EBWH Ohmic H-mode ELM-free H-mode Sawtoothing 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. • 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. • 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. V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Lower Hybrid Probe Head 50 mm • Can be moved to a specified distance from the plasma in the midplane • Allows axial rotation ± 45o • Loop (20x40 mm2)& L shape antennas • 76-545MHz spectrum analyser with 10 MHz resolution 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% V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Parametric Decay Threshold where Teff =Te + 4Ti ρ is the radius of the heating beam L is the inhomogeneity scale: L-1 = grad(ne)/ne + 2(ωce/ωpe)2grad(B)/B For typical MAST parameters: f = 60 GHz, Ti≈ Te = 140eV, B =0.38 T, L = 3 cm PUHR* ≈ 80 kW for ρ ≈ 10 cm PUHR*/(πρ2)  260 W/cm2 V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

UHR Sawtoothing H-mode Ray-tracing Modelling UHR EBW power deposition profile Poloidal projection of EBW ray-tracing results V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

During RF injection Before RF injection b) a) 80 7 kJ (~0.25 MW) 70 Plasma Energy, kJ 60 60 kJ (~3 MW) 50 40 400 RF Power, kW 250 kW 300 200 100 0 0.35 0.20 0.25 0.30 Time, s EBWH in Sawtoothing H-mode Plasma Average heating result.Shots #9262, #9263, #9267. EBW radiative temperature measured from 2ωce during ELM-free intervals at optimal magnetic field. • 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 5ωce and6ωce • 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 V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Outer plasma radius, m EBE ECE 2ce 3ce 4ce #10638 ECE EBE 3ce 4ce #10639 EBE in High Density Ohmic H-mode EBW emission (60.5 GHz) in high density Ohmic H-mode during plasma compression Resonance topology for high density Ohmic H-mode As the plasma boundary moves into the higher magnetic field during compression, EBE comes first from 4ωce, then from 3ωce and finally from 2ωce V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Launcher Aiming in Ohmic H-mode Angular window for O-X-B mode conversion measured at 60.5 GHz EBE has been used to optimise the launch configuration for EBWH at 3ωce V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Plasma outer radius D signal 60 GHz EBE signal SXR differential signal RF power SXR Signal During EBWH SXR signal in the RF heated shot (red) and reference shot (black) SXR signal from the RF heated shot with subtracted reference signal SXR signal was doubled during RF pulse V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Plasma Energy RF Power Plasma Density D signal Plasma Energy Increase During EBWH Plasma energy (EFIT) during RF injection. Shots: #10704, #10706, #10707, #10709. V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Electron Temperature Increase During EBWH Electron temperature profiles measured by Thomson scattering in RF heated (red) and ohmic (blue) plasmas at 0.20 s. 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 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 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. 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. V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

28 GHz Antenna Mock-up Assembly Mock-up assembly of the launcher (upside-down) In-vessel mirrors of the 28 GHz launcher V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

EBE from plasma to radiometer  Spinning Mirror Principle 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 V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Replacement Mirrors Manual replacement of mirrors of different tilt 1.5o 0.9Kg 3o 1Kg 4.5o 1.1Kg V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

Spinning Mirror Set-up on 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 V.Shevchenko et al, ISTW 2006, 11 -13 October 2006, Chengdu, China

EBW Experiments on MAST