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Radio frequency (RF) rectification of the plasma sheath is being actively studied on C-Mod as a likely mechanism that leads to prohibitively high molybdenum levels in the plasma core of ion cyclotron RF (ICRF) heated discharges. We installed emissive, ion sensitive, Langmuir, and 3-D B-dot probes to quantify the plasma potentials ({Phi}{sub P}) in ICRF and lower hybrid (LH) heated discharges. Two probe sets were mounted on fixed limiter surfaces and one set of probes was mounted on a reciprocating (along the major radius) probe. Initial results showed that RF rectification is strongly dependent on the local plasma density and not on the local RF fields. The RF sheaths had a threshold-like appearance at the local density of {approx}10{sup 16} m-{sup 3}. Radial probe scans revealed that the RF sheaths peaked in the vicinity of the ICRF limiter surface, agreeing with a recent theory. The highest {Phi}{sub P}'s were observed on magnetic field lines directly mapped to the active ICRF antenna. Measurements in LH heated plasmas showed a strong {Phi}{sub P} dependence on the parallel index of refraction n{sub ||} of the launched LH waves: {Phi}{sub P} is greater at lower n{sub ||}. Little dependence was observed on the localmore » plasma density.« less

Lower hybrid waves launched uni‐directionally into tokamak plasmas impart momentum to the electrons. This momentum can be transferred to the ions, leading to substantial counter current rotation. Observations of LH‐induced counter rotation have been previously reported [1], and the initial rate of increase has been found to be consistent with the calculated rate of wave momentum injection [2]. However, in recent experiments in Alcator C‐Mod it has been found that application of LH waves to relatively low current (Ip∼0.4–0.6 MA) plasmas can result in a co‐current change of rotation, which implies a different mechanism than that described above. This appears to be linked to the so‐called intrinsic rotation commonly observed in Alcator C‐Mod and other tokamaks [3]. In addition to the change in direction at low current, some dependence on the magnetic configuration (USL vs. LSN) has been observed.

Lower Hybrid Current Drive (LHCD) in the Alcator C‐Mod tokamak provides current profile control for the generation of Advanced Tokamak (AT) plasmas. Non‐thermal electron bremsstrahlung emission decreases dramatically at ne>1⋅1020[m−3] for diverted discharges, indicating low current drive efficiency. It is suggested that Scrape‐Off‐Layer (SOL) collisional absorption of LH waves is the cause for the absence of non‐thermal electrons at high density. VUV and visible spectroscopy in the SOL provide direct information on collision excitation processes. Deuterium Balmer‐, Lyman‐ and He‐I transition emission measurements were used for initial characterization of SOL electron‐neutral collisional absorption. Data from Helium and Deuterium LHCD discharges were characterized by an overall increase in the emissivity as well as an outward radial shift in the emissivity profile with increasing plasma density and applied LHCD power. High‐temperature, high‐field (Te = 5keV,Bt = 8T) helium discharges at high density displ...

Electron internal transport barrier (ITB) formation is observed during fully non‐inductive LHCD discharges on Alcator C‐Mod. The formation is characterized by an abrupt increase of the central electron temperature after a few current penetration time has passed following LHCD turn‐on. The ITB phase is often terminated by m = 2 mode activity. Kinetic equilibrium reconstruction and TRANSP/LSC analysis show shear reversal in the q profile during the ITB phase. TRANSP/LSC predictive analysis suggests that fBs∼60% is possible if a similar temperature gradient is maintained at r/a∼0.5 with H‐factor of 1.

Experimental observations of LHCD at high density (ne>1020m∼3) on the Alcator C‐Mod tokamak are presented in this paper. Bremsstrahlung emission from relativistic fast electrons in the core plasma drops sharply in single null discharges well below the density limit previously observed on limited tokamaks (ω/ωLH∼2). Modeling and experimental evidence suggest that the absence of LH driven fast electrons at high density may be due to collisional absorption in the scrape off layer. Experiments show that the expected current drive density dependence is recovered for inner wall limited discharges across the range of densities scanned (0.5×1020 m−3

Comparisons have been made between TOPLHA and ALOHA, two codes for Lower Hybrid antenna coupling to cold inhomogeneous plasmas, on the Tore‐Supra C2 multi‐junction and an ITER relevant Passive‐Active Multi‐junction (PAM). Calculation for several plasma scenarios of reflexion coefficients and launched spectra are in good agreement.

Recent results from the lower hybrid current drive experiments on Alcator C‐Mod are presented. These include i) MSE measurements of broadened LHCD current profiles; ii) development of counter rotation comparable to the rate of injected wave momentum; iii) modification of pedestals and rotation in H‐mode; and iv) development of a new FEM‐based code that models LH wave propagation from the RF source to absorption in the plasma. An improved antenna concept that will be used in the upcoming C‐Mod campaigns is also briefly described.

LHCD on Alcator C‐Mod is being used in plasmas with parameters similar to those expected on ITER for the purpose of tailoring the plasma current profile. LHCD experiments have also produced intriguing results related to momentum transport and edge pedestal physics that affect the toroidal rotation profile and the temperature and density profiles. Quantitative comparisons between local measurements and theory/simulation have been performed, confirming the off‐axis localization of the current drive, as well as its magnitude and location dependence on the launched n‖ spectrum and electron temperature. Applying LHCD during the current ramp saves volt‐seconds and delays the peaking of the current profile. Counter current toroidal rotation during LHCD has been observed in both L and H‐mode plasmas. In H‐mode plasmas the edge pedestal collisionality is reduced while the overall pressure in the pedestal increases slightly.

A numerical modeling of current profile modification by lower hybrid current drive (LHCD) using a fullwave/Fokker-Planck simulation code is presented. A MHD stable LHCD discharge on Alcator C-Mod was analyzed, and the current profile from full wave simulations was found to show better agreement with the experiment than a ray-tracing code. Comparison of full wave and ray-tracing simulation shows that, although ray-tracing can reproduce the stochastic wave spectrum broadening, the full wave calculation predicts even wider spectrum broadening, and the wave spectrum fills all of the kinematically allowed domain. This is the first demonstration of LHCD current profile modeling using a full wave simulation code in a multi-pass absorption regime, showing the clear impact of full wave effects on the LHCD driven current profile.