Alcator C-Mod Weekly Highlights April 7, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 10.5 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four run days were scheduled and completed, including two extended nine hour runs. One day was required for machine conditioning discharges to recover from the previous week's vacuum incident. The three research days supported experiments in the Lower Hybrid Physics and MHD areas. A total of 80 plasma discharges were produced with a reliability of 62%. One overnight boronization was carried out prior to Friday's experiment. Plasma operations are planned to continue this week. Operation Details ----------------- Tuesday's run was devoted to a series of conditioning plasmas in order to recover from the effects of a vacuum leak which resulted in disruptive runaway discharges the previous week. Initially only short resistive plasmas and unsuccessful startups (fizzles) were obtained. Unusually high levels of fluorine were observed spectroscopically during the discharges and on the residual gas analyzer (RGA), probably indicating that teflon insulation on an internal sensor lead had been compromised. Eventually startup reliability improved and full length discharges were produced. Flourine levels have remained at elevated but acceptable levels throughout the week. Research operation resumed on Wednesday, with several experiments supporting the Lower Hybrid Physics program. Tests of the effect of excitation of sets of 16 adjacent waveguides of the 22 waveguide LH coupler were carried out using 8 klystrons, in preparation for the next phase of lower hybrid operation in 2009, when we will be employing a new 4x16 guide coupler in addition to the present 4x22 waveguide one, each to be driven by eight klystrons. The test was successful, with no indications of problems associated with coupling to the undriven guides. There was little difference in current drive efficiency regarding which set of 16 waveguides were excited. A few shots on Wednesday were devoted to a test of the principle underlying MP#542a, which proposes to use the Langmuir probes mounted on the LH grill to determine the density gradient at the launcher mouth for equilibria which place the probes on different flux surfaces. Since the launcher and probes are typically shadowed by the local guard limiters as well as the main limiters, the proposition that the density measured in this region is approximately a function of flux requires demonstration. Probe measurements were made with the coupler in two positions relative to the fixed LH limiters, and for 2 different plasma shapes, one congruent to the plasma shape, the other strongly non-congruent. The results seemed to be at variance with previous studies, but need analysis before conclusions can be drawn. Also on Wednesday, in a continuation of the experiments supporting MP#515 "Effect of launched LH wave n-parallel spectrum on current profile", systematic MSE measurements of LHCD were carried out for 60, 75 and 90 degree phasing in ICRF-heated L-mode plasmas. The run on Thursday was devoted to MP#489 "Damping Rate of Toroidal Alfvén Eigenmodes at the Threshold of Stability in the Presence of ICRF Heated fast ions". This experiment comprises part of the doctoral research of an MIT graduate student. A goal of this run was to investigate anomalous non-monotonic stability behavior of TAE's at moderate ICRF power observed in some previous experiments, as opposed to the monotonic decrease in damping rate to the point of instability observed in other prior experiments. Anomalous behavior was not seen in the run; rather the damping rate was observed to decrease monotonically with ICRF power, independent of configuration (limited and diverted), density, or differences in the outer gap. The differences between these observations and those from last June still need to be explained. Slow frequency sweeps were performed in both diverted and limter configurations to enhance the analysis of the resonances. Wide frequency sweeps to examine the background antenna coupling to the probes were also performed. The compact neutral particle analyzer was operated in conjunction with the neutral beam to provide energetic particle distributions and profiles. Friday's run supported MP#543 "Exploration of LHCD effects on pedestal and core of H-mode plasmas". The goals of the MP are to document the conditions under which LHCD causes changes in the H-mode pedestal and core, to optimize the driven current in H-mode, and to measure the profile with MSE. The pumpout effect of LH on the H-mode density observed previously was not clearly reproduced on this run day, and there was no clear evidence of Lower HYbrid current drive in the H-mode discharges produced. Despite the preceding overnight boronization, steady USN H-modes were not accessible due to metallic injections. Much of this run was carried out in the lower single null configuration. It is conjectured that the LH effects observed in the previous experiment are topology-dependent. Some lower null H-mode discharges did exhibit small density decreases and pedestal relaxation, but the effects were smaller than those obtained previously. ICRF Systems ------------- All four FMIT transmitters were operational last week, with a 1MW final amplifier tube (#8973) installed in FMIT#1 (D-port). ICRF was used in support of all three research run days last week. Diagnostics ----------- A joint PPPL-PSFC-LLNL tele-conference was held this week to discuss possible re-designs of the C-Mod MSE optical system that would use curved mirrors rather than in-vessel lenses to provide focusing. Participants were Steve Scott (PPPL), Jinseok Ko and Earl Marmar (PSFC), Chris Holcomb, Mike Mikowski, Steve Allen, and Scott Lerner (LLNL). Scott Lerner reported results from numerical ray-tracing based on a tangential MSE view of the plasma, similar to the view that is currently used. The initial conclusion of the analysis is that spatial aberrations would be quite large with a curved mirror system due to the large angle-of-incidence (AOI ~ 45 degrees), and they cannot be adequately compensated by other optical elements. Therefore, a re-design based on curved mirrors is probably not feasible for the tangential plasma view. Separately, Steve Scott, Jinseok Ko, and Fred Levinton and Howard Yuh (Nova Photonics) evaluated the possibility of viewing the DNB from above (mainly poloidal), rather than from the side (mainly tangential). This approach would allow the use of mirrors at smaller AOI and so would alleviate the aberration problems. An optical arrangement of two curved mirrors for such a vertical view has been developed, which would be compatible with all mechanical interference limitations and provide spatial resolution comparable to the resolution of the current MSE system. However, the intensity of the spectroscopic line employed for MSE would be significantly reduced in this orientation. The tentative conclusion of this exercise is that a near-vertical MSE view is not feasible. Travel and Visitors -------------------- Miklos Porkolab visited Lawrence Livermore National Lab on March 31st where he had discussions on collaboration among C-Mod experimentalists and the BOUT code authors (Maxim Umansky and Bill Nevins). Paul Bonoli attended the Sherwood Fusion Theory Conference in Boulder, CO that was held from March 30 - April 2, 2008. he presented a poster on behalf of John Wright and other co-authors titled "Full-wave electromagnetic field simulations of lower hybrid wave propagation in ITER relevant regimes". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly