Lower Hybrid RF: Results, Goals and Plans J.R. Wilson Alcator C-Mod Program Advisory Meeting January 27, 2010
ITER Needs and the RENEW Report Provide a Context for LH Research on C-Mod ITER Needs: Hea-ng and Current Drive Strategy Baseline (LH no) Upgrade (possibly) Integrated Scenarios Hybrid scenario (Can C Mod get something like this?) AT (Demonstrate that LHCD is necessary) ITER Physics Issues Elm Control H mode Pedestal Rota-on C Mod experiments have seen intriguing phenomena and will con-nue exploring this physics
LH Research Contributes to Progress on Several RENEW Thrusts Thrust 4 Qualify opera-onal scenarios and the suppor-ng physics basis for ITER Thrust 5 Expand the limits for controlling and sustaining fusion plasmas Ac-ve control Current profile control in steady state Stabiliza-on Can LHCD affect instabili-es in a controlable fashion Thrust 6 Develop predic-ve models for fusion plasmas, supported by theory and challenged with experimental measurement
Physics Issues being studied Density Limit LHCD efficiency falls as density is increased (~1/n e ) un-l the density limit is reached Both C Mod and FT U observe a lower density limit than expected from theory and earlier experiments Joint ITPA Experiment IOS 5.3: Assessment of lower hybrid current drive at high density for extrapola@on to ITER advanced scenarios Rota-on C Mod observes Counter I p plasma rota-on during LHCD Pedestal Modifica-on Modifica-on of H mode pedestal observed during LHCD Possible effect on ELM s??
Hard X-rays disappear at high density HXR emission drops much faster with density than 1/n e theoretical prediction Precipitous drop in emission above 1.0x10 20 m -3 Higher toroidal field and plasma current help at high density Changing antenna phasing (n ) has little effect 1/n e For AT operation need to operate in H-mode regime Accessibility limits do not explain fall-off Parametric decay does not explain fall-off
Computational modeling does not predict drop in HXR at high density GENRAY*: Follows ray trajectories in plasma CQL3D*: Calculates damping and current drive Predicts HXR emission similar to 1/n e CQL3D *codes courtesy of CompX
Large currents measured in the Scrape-Off-Layer during LHCD at high density LSN Jsol Bt Ip Parallel electric currents in the SOL flowing between inner and outer divertors Currents increase during LH at high ne, at similar density to loss of HXR, suggesting SOL absorption Current driven in SOL is larger for higher n Direction of SOL current does not change with n
What can be causing fall-off in emission at high density?? Computational models did not allow for propagation and absorption outside of the last closed flux surface A Scrape-off Layer (SOL) model has been added to the GENRAY code that allows for propagation and collisional absorption in the SOL Collisional damping calculated by replacing m e with m e (1 +iν/ω) in the wave equations where ν is the electron-ion collision frequency (ν ie ~ T e -3/2 - becomes strong in cold SOL plasma) For L-mode plasmas agreement between experiment and modeling improved
Addition of collisional damping in the SOL improves agreement with experiment Collisional damping in SOL calculated by GENRAY SOL parameters T min = 5 ev n min = 1e11 λ T = 5 mm λ n = ~2 cm Collisional damping increases at high density as rays propagate farther in the SOL 2D SOL w/ collisions
If model is correct can we find conditions where SOL absorption is less?? H-mode plasmas have significantly different SOL plasmas but high edge density Newly discovered I-mode has hot core plasma without edge density barrier Do I-mode plasmas look better as LHCD targets?? (High core temperature should increase single pass damping) Can this hypothesis of collisional damping be tested be comparing LHCD for different plasmas??
Increasing minimum edge temperature increases x-ray emission, decreases SOL absorption and increases CD ~1/n e T emin = 50 ev I LH = 157 ka P SOL /P core = 0.07 T emin = 20 ev T emin = 10 ev I LH = 86 ka P SOL /P core = 0.23 T emin = 5 ev I LH = 17 ka P SOL /P core = 0.7
Application of LHCD causes changes to core, pedestal and SOL in H-mode above n e limit Density decreases in core and top of pedestal Density increases at pedestal foot and in SOL Temperature increases in core plasma Increase in HXR emission Decrease in V loop, mostly due to changes in n e, T e ()* %###!### 6 78 6 9:;< # +!!"#$!"!!"!$!"%!"%$!"&!"&$!"'!"'$!"$ &,+!#%# + (-!" * (./0* (4"5"* % = /#! +!!"#$!"!!"!$!"%!"%$!"&!"&$!"'!"'$!"$ = />4?@ & %! + C:C+1 /#!!"#$!"!!"!$!"%!"%$!"&!"&$!"'!"'$!"$! #"$ # + %!+DBEB-/F/G!!"#$!"!!"!$!"%!"%$!"&!"&$!"'!"'$!"$ #$%&'()*+,&-!./ 12-/+(3* 1AB-3B=+1 /# + + +
In the Presence of LHCD a counter I p plasma rotation is observed Rota-on appears inside loca-on of CD Magnitude of rota-on velocity change is propor-onal to apparent current driven In H mode with edge CD rota-on change is seen to propagate inward [1.] A. Ince Cushman, et.al., Phys. Rev. Leh., 102, 035002 (2009) [2.] J. E. Rice, et. al., Nucl. Fusion 49, 025004 (2009)
LH driven rotation localized to Core R(cm)
Rate of injected wave momentum sufficient to explain plasma momentum buildup
Mechanism for rotation remains to be determined Inward pinch of trapped electrons? Deviation of energetic passing particles from flux surface? Other?
New Diagnostics Being added to aid understanding of LH Physics PCI Reflectometers for density measurements at launcher location Reflectrometry to directly detect 4.6 GHz component of density fluctuation
Direct detection of the LH wave by a reflectometor Mo-va-on To study the LH wave propaga-on close to LCFS (spectrum broadening, PDI, density limit etc ) Complementary to PCI (measurement is close to LCFS) To understand the reflectometer (measurement of known density perturba-on) Promising result was already obtained on the prototype instrument (2008 APS Dominguez)
Full wave simulation using COMSOL Phase response to Gaussian shaped density perturba-ons shows that the localized (dx ~ cm) reflectometer signal response. The either upshio or downshio is allows depending on the propaga-on direc-on of the reflectometer wave due to the fact that LH wave is the backward wave. density cutoff transient simulation shows the difference of time of flight of three wave components phase response upshift scattering dr (m) Maximum response happens not at but in front of the cutoff layer downshift
PCI setup in Alcator C Mod for LH waves detec-on R = 60 79 cm OPM M Phase plate M plasma M 32ch HgCdTe photoconduc-ve Detector extra phase π/2 M LASER CO2 60W CW Inside the cell OPM M M: mirror OPM: off axis parabolic mirror M Modulator
New Launcher to be installed this spring Reflectometer waveguides
In FY2010 will Assess the power handling limits of the new antenna Hard limit Using ten klystrons direct fed guides can fall between hard and weak limits split guides will be just below weak limit weak limit In FY11 with more sources can push beyond strong limit
Goal is to have 4 MW of klystron Power and 2 Launchers by end of FY2012 16 klystrons available in 2011 2 nd launcher in 2012 Need experience with present launcher Mesh with ICRF and Divertor upgrades
2010 LH Research Program Characterize new launcher performance Coupling, Power Handling, Effect of reflec-ons Study Density limit Use new reflectometer to to measure SOL wave fields Explore different configura-ons to modify SOL characteris-cs Con-nue pedestal modifica-on studies Apply to ELMing discharge 2011 Design 2 nd launcher begin fabrica-on Increased source power available to test power limit Con-nue studies on density limit and rota-on 2012 Add PCI diagnos-c for core wave detec-on Combine with ICRF for rota-on profile sculp-ng Complete fabrica-on of second launcher Full system capability by end of FY 12 Determine most promising LH scenario for AT physics studies
Progress on Advanced Scenarios can be made with Existing System U-lize current profile diagnos-cs to test ability to manipulate current profile and effect of MHD stability on profile modifica-on. Con-nue studies on LHCD during ramp up (ITER like scenarios) Explore I mode with its large core electron temperatures which yield strong absorp-on. Goal is to establish best candidate advanced scenarios in -me for full capability (FY12)