Lower hybrid heating and current drive on the Alcator C-Mod tokamak

Abstract

Abstract.On the Alcator C-Mod tokamak, lower hybrid current drive (LHCD) is being used to modify the current profile with the aim of obtaining advanced tokamak (AT) performance in plasmas with parameters similar to those that would be required on ITER. To date, power levels in excess of 1 MW at a frequency of 4.6 GHz have been coupled into a variety of plasmas. Experiments have established that LHCD on C-Mod behaves globally as predicted by theory. Bulk current drive efficiencies, n 20 I lh R/P lh ~ 0.25, inferred from magnetics and MSE are in line with theory. Quantitative comparisons between local measurements, MSE, ECE and hard x-ray bremsstrahlung, and theory/simulation using the GENRAY, TORIC-LH CQL3D and TSC-LSC codes have been performed. These comparisons have demonstrated the off-axis localization of the current drive, its magnitude and location dependence on the launched n || spectrum, and the use of LHCD during the current ramp to save volt-seconds and delay the peaking of the current profile. Broadening of the x-ray emission profile during ICRF heating indicates that the current drive location can be controlled by the electron temperature, as expected. In addition, an alteration in the plasma toroidal rotation profile during LHCD has been observed with a significant rotation in the counter current direction. Notably, the rotation is accompanied by peaking of the density and temperature profiles on a current diffusion time scale inside of the half radius where the LH absorption is taking place. PACS: 52.50.Sw, 52.55.Wq IntroductionThe Alcator C-Mod program has as one of its goals the aim of producing advanced tokamak (AT) discharges. In order to provide the current profile control required for obtaining reduced or reverse shear a lower hybrid (LH) rf power system [1] has been installed on the tokamak. One of the advantages of exploring the applicability of LH current drive on Alcator C-Mod is the equivalence of many of the important parameters, such as plasma density, magnetic field, and rf frequency to those required for a reactor, and in particular ITER. In addition, C-Mod uses heating sources that provide low torque to the plasma, as would be true for a reactor, and the time constants involved allow for relaxation of the relevant profiles. Initial experiments [2,3] have demonstrated that the system behaves as expected with bulk current drive efficiencies η = n 20 I lh R/P lh ~ 0.25 in line with theory and expectations from previous experiments. Recent work, reported in this paper, concentrates on detailed measurements of the spatial location of the driven current, the role of fast electron diffusion in spreading the current and other effects of applying LH power to the plasma. In addition, to extrapolate the application of LHCD for use in future devices a program of model development and validation is underway. The plan of this paper is as follows. The local measurements of fast electrons and CD derived from hard x-ray measurements and MSE along with comparisons to modeling will be

On the toroidal plasma rotations induced by lower hybrid waves

“…Toroidal electric field on the right-hand side of Eqs. (17) and (18) is close to zero during LHCD according to experimental observations [24]. We can also safely neglect u r i because ions response much more slowly to the radial electric field than electrons do.…”

High density LHRF experiments in Alcator C-Mod and implications for reactor scale devices

“…It is expected that other parasitic edge-wave interactions are also minimized due to a reduced SOL density in the multi-pass damping regime. Improvements in the "density limit" and their correlation with PDI suppression have been reported in LHCD experiments in FTU [11] and EAST [12]. Both machines made use of lithium coating and special fueling methods to reduce wall recycling.…”

20 years of research on the Alcator C-Mod tokamak

“…[346][347][348] ICRF waves from nearby antennas can have a similar effect, lowering the density and increasing reflections. , the LH system on C-Mod can drive 100% of the plasma current (0.5 MA) for multiple L/R times, 347,350,351 as seen in Fig. 58(a).…”

“…347,351 • Developed the first full-wave LH codes, using these to explain the decrease in current drive efficiency at high densities. 304,358,361,366,399 • Showed the importance of spatial asymmetries and fast dynamics for disruption halo currents and disruption mitigation radiation.…”