Alcator C-Mod Weekly Highlights Feb. 25, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 5.75 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, with four run days scheduled and completed. A total of 107 plasma discharges were produced with a reliability of 89%. The runs supported 7 experiments, including miniproposals from the Lower Hybrid Physics, H-mode and Advanced Tokamak Integrated Scenarios, Rotation, MHD, and Edge groups. An overnight boronization was performed prior to the plasma run on Thursday. The cryopump was successfully employed for density and particle control during several runs. Experimental time totaling about two hours was lost on Thursday due to consequences of a brief power outage (brown-out) from the electric utitlity just prior to the scheduled start time. Additional time (about two hours) was lost on Tuesday due to data acquisition and power system delays. Plasma operations are planned to continue this week. Operation Details ----------------- The new fixed phase shifters in the Lower Hybrid "jungle gym" were tested during plasma operation Tuesday morning. Fixed shifters replaced three of the older adjustable units, and the power levels were increased to determine if these components enabled transmission of higher power levels. This test was successful, with none of the new units exhibiting arcing up to the highest powers attempted, 185kW per klystron. This is a significant improvement over previous performance. 1MW net power was successfully coupled to the plasma, the highest power level attained during this campaign. Replacement of the remaining adjustable phase shifters is in progress. Tuesday afternoon's experiment was devoted to MP#526 "Accessing low density/collisionality H-mode with reduced particle inventory". The idea of this experiment is to produce low-density H-modes by preparing a very low-density L-mode target with minimal gas puff and then initiating the L-H transition by quickly increasing the target density above the low-density threshold limit, thereby minimizing the reservoir of neutrals available to fuel the H-mode. This technique, employing brief gas puffs to trigger the transition, was successful in producing low-density H-modes (nebar<1.5e20/m^3). For these unboronized wall conditions, the resulting discharges were ELM-free and exhibited high radiated power which restricted the duration of the H-mode phase. Attempts to use small lithium pellets to trigger the L-H transition were unsuccessful due to technical difficulties. The first half of Wednesday's run continued the investigation of sawtooth effects on rotation and ion temperature (MP#520). In addition to enhancing the dataset at 1MA plasma current, additional data were obtained at lower q95 and increased inversion radius with plasma current of 1.2MA. The second half of Wednesday's run began work on MP#530 "Disruption runaway generation with LH". The goal of this experiment was to determine the efficacy of lower hybrid current drive in creating a seed population of fast electrons which can be used for studies of runaway generation (and mitigation) during disruptions. Such runaway production is predicted for ITER, but has been difficult to study in existing devices because of the smaller ratio of current quench times to the runaway generation time. This is the first in a series of experiments that will concentrate on studying disruption runaway physics and mitigation as part of an ITPA MHD high priority task for ITER. We successfully produced substantial populations of fast electrons using LHCD prior to intentionally induced disruptions. Three sorts of disruptions were investigated during this half-day experiment: disruptions resulting from locked modes, VDE's, and Li pellet injection. The duration of the current quench, a signature of production of relativistic electrons during the termination phase, was observed to increase on some but not all of the locked mode and pellet-induced disruptions in which LHCD was applied. However, the results were not clear cut, and none of the hard-xray spectra taken during the quench showed evidence of high energy tails. Data from this run are being analyzed and will be used to guide the design of future efforts in this series. Thursday's run was devoted to MP#522 "Lower lp Long Pulse L-Mode and H-Mode Advanced Scenarios". Combinations of LHCD and ICRF during the current rise were employed to tailor the q profile in low current (450kA) discharges. The experiment relied on the slow current rise (flattop at 0.5sec) scenario developed previously. In Thursday's experiment the target density was also varied. LHCD in the current rise was found to be most effective when the target line-averaged density was kept below 9e19/m^3, which is also below the low-density limit for H-mode transitions. At these lower densities the application of high power ICRF (P>2MW) in the current ramp phase led to excess radiated power and collapse of the electron temperature. Optimal programming of the ICRF, LH, and density resulted in considerable delays in the onset of sawtoothing. Sawteeth which generally appear at 150 ms for the ohmic discharges were delayed up to 550 ms with LH alone, to 625 ms with L-mode and low power ICRF and LH, and out to 875 ms with high power ICRF and LH, although the high power ICRF cases were prone to radiative collapse. When high densities were used to get ICRF H-mode in the ramp, the sawteeth were delayed to 500 ms. Most of Friday's run was devoted to MP#495a "Alfvén Cascades studies with ICRH and LHCD". This experiment comprises part of the thesis research of an MIT graduate student. The initial goal was to reproduce current ramp parameters obtained in 2004 which exhibited strong reverse-shear Alfven eigenmode (cascade) activity, indicative of hollow current profiles, in plasmas with early ICRF heating and strong shaping. The plan was to then apply LHCD to these discharges in order to modify the current profile and document the variation in the RSAE activity. While the strong shaping (inner-nose limited, high triangularity) of the target discharge was largely reproduced, the resulting RSAE activity was much less apparent than in the 2004 discharges. The abrupt appearance of numerous TAEs without RSAE predecessors, as was seen in the 2004 runs, suggests that we are not generating the reversed shear. Most of the day was spent trying to enhance the RSAE modes using ICRF alone; the LHCD portion of the experiment was deferred to a later date. One hour on Friday was also devoted to complete the turbulence dataset for MP#429 "SOL turbulence in limited plasmas". The purpose was to obtain simultaneous data from the outer midplane and high-shear region views, under the same plasma conditions as the earlier experiment on this MP, for which the outer midplane 2D view was unavailable. The data are to be compared with GEM code simulations already carried out based on the previous experiment. Data were successfully obtained at 5.4 and 2.8T. Lower Hybrid System -------------------- Fabrication of additional fixed phase shifters to replace the adjustable phase shifters in the Jungle Gym is underway and completion is expected before operations this coming week. Testing of the first three fixed phase shifters during plasma operation last Tuesday demonstrated a substantial improvement in power handling capability. The lower hybrid system was successfully employed during three of the four run days this week. Travel and Visitors -------------------- Martin Greenwald traveled to Washington to participate in a meeting of the Fusion Energy Sciences Advisory Committee (FESAC). Ian Hutchinson and Miklos Porkolab also attended this meeting. Chuck Kessel (PPPL) was at MIT on Thursday to serve as Session Leader for the experiment that day. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly