Recent results from lower hybrid current drive experiments on Alcator C-Mod

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1 Recent results from lower hybrid current drive experiments on Alcator C-Mod R. R. Parker, S.-G. Baek, C. Lau, Y. Ma, O. Meneghini, R. T. Mumgaard, Y. Podpaly, M. Porkolab, J.E. Rice, A. E. Schmidt, S. D. Scott, S. Shiraiwa, G.M. Wallace, S. Wukitch MIT-PSFC, J.R. Wilson PPPL A novel coupler designed to optimize LHRF power delivered to Alcator C-Mod plasmas is now in operation. New diagnostics have also been added to better understand the conversion of waveguide modes in the grill antenna to slow LH waves, and the penetration and damping of the waves as they pass beyond the separatrix. These include a set of 32 RF probes located near the plasma-grill interface, an X-mode reflectometer for measuring the density profile near the grill, and a two-frequency O-mode reflectometer for detecting LH waves beyond the separatrix via Bragg scattering. The latter, supplemented by X-ray and cyclotron emission profiles, is expected to be helpful in understanding the disparity in measured current drive efficiency at high density relative to predictions from ray-tracing and full-wave simulations. This paper surveys the results obtained with the new coupler, including coupling and current drive efficiency, plasma rotation and comparison with results of simulations. *Supported by USDoE awards DE-FC02-99ER54512, DE-AC02-09CH11466

2 Alcator C-Mod lower hybrid program Objective: The goal of the LHCD experiment on the Alcator C-Mod tokamak is to demonstrate and study full non-inductive high performance tokamak operation using the parameters close to that envisioned for ITER in terms of LHCD frequency, density, and magnetic field. Hardware installed: 4X16 waveguide grill, 1.5 < n < 3.5, driven by 10 klystrons operating up to 250 kw at 4.6 GHz for 5 s. P net 1 MW. Planned Upgrade: Additional grill, P net increased to 2 MW (2012) LH Diagnostics: 32 Channel x-ray camera Grill probes (reflection coefficients) Grating polychromators, Fabry Perot MSE (current profile) interferometer, radiometer (non-thermal ECE) X-mode reflectometer (n-profile at grill) O-mode reflectometer (wave penetration) New Simulation Tool: Full wave integrated FEM plus 3D Fokker-Planck codes, simulates wave physics from feeding waveguides to absorption region, including SOL

3 New, resonant 4-way power splitter simplifies grill and waveguide feed system See also J. R. Wilson, PO

4 The grill is formed by 4 rows of 16 waveguides P(n ) [a.u.] = 75 = 90 = 110 = 130 = 145 = n A wide range of n can be launched by electronically varying the phase of the klystrons

5 Design objective to increase reliable high-power power capability has been realized ~ 400 LHCD discharges since first operation in June Maximum average power increased up to 1.0 MW for 0.5 s, Peak power >1.1 MW 41 MW/m2 averaged across launcher, 47 MW/m2 in central waveguides Average LH power [kw] LH Discharge Number

6 Power reflection coefficient agrees with simulations when density-dependent vacuum gap is included degrees phasing 0.1mm behind LH limiter 1 mm behind LH limiter degrees phasing 0.1mm behind LH limiter 1 mm behind LH limiter degrees phasing 0.1mm behind LH limiter 1 mm behind LH limiter LH cutoff density 2.6E17 [m 3 ] Γ Coupling simulation x O. Meneghini, PO Average grill density (m 3 ) x x Experiments to test effects of posssible ponderomotive force to explain vacuum gap are planned before end of present campaign

7 A 3-beam X-mode reflectometer has been installed on the LH coupler to measure density and gradient in front of the grill Sample data resulting from an inner gap scan are shown Notice the higher density at the grill and steepening of the gradient during LH, relative to n n the Ohmic profiles. Both n grill and n are important in determining the coupling efficiency Cornwall Lau, TP

8 Discharges with full non-inductive current drive have been obtained Previous attempts to reach sustained zero loop voltage have been unsuccessful because product of density and current were too high; required power, P = nir/η By applying LH power to an upper null equilibrium with cryo- pumping to control density (n ~ 5e19 m-3), and operating at reduced current (Ip ~ 0.5 MA), the first discharges with V loop = 0 have been obtained. Engineering efficiencies η ~ 0.23 have been obtained, in line with previous estimates based on discharges with V loop small but nonzero. These discharges feature sawtooth stabilization. MSE constrained EFIT reconstructions with kinetic profiles indicate q(0) ~ 2 with flat shear.

9 Examples of zero loop voltage discharges Ip (MA) Ip (MA) V loop (V) V loop (V) 19 n 10 m 3 19 n 10 m 3 T e (0) (kev) T e (0) (kev) P RF (kw) P RF (kw) Ctr soft Xray (kw/m -2 ) Ctr soft Xray (kw/m -2 )

10 The central temperature peaks when sawteeth are stabilized Te Te (kev) [KeV] 3 2 Te (kev) Te [KeV] 2 1 OH s LH s 1 OH s LH s major radius [m] Major Radius (m) major radius [m] Major Radius (m)

11 Density profiles flatten slightly, increase at edge n (x1e20 ne [n20] m-3) 0.4 OH s LH s n (x1e20 ne [n20] m-3) OH s LH s major radius [m] Major Radius (m) major radius [m] Major Radius (m) Edge Thomson data (not shown) indicates increase in edge density Note: no edge data for shot

12 EFIT reconstruction using MSE indicates weakly reversed shear profile MSE pitch angle becomes stationary during the later half of LH pulse Equilibrium reconstruction constrained by 9 point MSE measurement and kinetic pressure profile, showing the shear reversal within the error bar due to the statistical error of pressure and MSE. Scott, TP Mumgaard, TP

13 A new full wave FEM-Fokker Planck code, LHEAF, simulates wavefields, current deposition and synthetic X-ray diagnostics for LH waves The code seamlessly treats the wave fields from inside grill to absorption Realistic geometry including X-Point, pedestal and SOL is incorporated Specular reflection at cutoff layers as used in ray tracing codes is not required Syun ichi Shiriawa, TP

14 Zero loop voltage has also been obtained in lower null discharges Ip (MA) V loop (V) 19 n 10 m 3 T e (0) (kev) P RF (kw) Ctr soft Xray (kw/m -2 )

15 Change in rotation due to LHCD is in opposite direction for these USN and LSN plasmas with v loop = 0 Coupling between ions and electrons in core also weakens, due to increase in Te and decrease in n e How does wave momentum dissipate in case of co-rotation? Change in rotation (km/s) What is role of E φ after lower hybrid turn off? Central ion temp (kev) Time from Lower Hybrid turn on

16 At slightly higher density and non-zero loop voltage, rotation in both USN and LSN plasmas is in counter-current direction However, dynamic response is different Counter current is direction of wave momentum In these plasmas, ions stay coupled to electrons Change in rotation (km/s) I P (MA) V loop (V) n( x m T e0 (kev) P LH (kw) Ctr Soft Xray (kw/m 2 ) ) Central ion temp (kev) USN See also Jungpyo Lee, TP LSN Time from Lower Hybrid turn on

17 Summary A new lower hybrid coupler with improved power handling capability has been installed on C-Mod and is routinely operating at the 1 MW level, corresponding to ~ 40 MW/m 2. Fully non-inductive plasmas have been obtained at densities in the mid m -3 range. MSE constrained EFIT analysis shows q(0) ~ 2 and flat central shear. Efficiency η = nir/p= , in line with ITER projection. A new full wave, Fokker-Planck code simulates RF fields and plasma response in realistic geometry, including grill region, SOL, pedestal and core LH-induced rotation is configuration dependent low density upper null plasmas with v loop = 0 rotate in co-curent direction, lower null in counter direction. Analysis of reflection coefficients suggest formation of a thin (~ 1 mm), low density or vacuum region between coupler and SOL plasma. Reflectometer measurements show that LH power modifies SOL profile in front of coupler, affecting coupling