Alcator C-Mod Weekly Highlights Feb. 04, 2008 FY2008 weeks of research operations: Target: TBD Completed: 4.2 weeks Operations ---------- Plasma operations continued last week, with three run days completed. A total of 72 plasma discharges were produced, with a reliability of 80%. Experiments proposed by the H-mode Integrated Scenarios and Rotation task forces were carried out. An overnight boronization was performed before Tuesday's run. A new machine protection circuit was installed, tested, and put into service this week. The hardwired circuit calculates the tension stress on the central solenoid components due to the other PF coils, and initiates a type 2 fault, causing a forced invert of all supplies, in the event the stress exceeds the OH coil preload. The run on Thursday was cancelled in order to troubleshoot a fault affecting the TF crowbar system. The problem was identified and successfully repaired by the end of the day. The planned experiment will be rescheduled. This week will be a maintenance week, with no plasma operations scheduled. Research operation is planned to resume next week. Run Planning ------------ An EPC meeting was held on Monday 1/28 at which seven new and revised proposals were considered and approved. George Sips (IPP Garching) presented remotely the joint ITER Rampup experiment which was run later in the week. Operation Details ----------------- Runs on Tuesday and Wednesday were devoted to MP#448a, "Comparison of Small ELM regimes on C-Mod, MAST and NSTX", which supports the C-Mod component of ITPA Joint Experiment PEP-6. Rajesh Mainji (PPPL) served as session leader, and Hendrik Meyer (UKAEA) participated by remote video-conferencing from Culham. An rf power/plasma beta scan was accomplished for a series of discharges with small ELMs. The rf power threshold for small ELMs was ~ 3.1 MW, and the ELMs were still observed at the max. rf power of 4.5 MW. The signatures of these small ELMs on soft X-rays, fast magnetics, and GPI were clearly observed. These ELMy discharges exhibited excellent confinement relative to the H98 scaling, and this will be an attractive regime for other experiments. ELMy H-mode access appeared to be remarkably sensitive to details of equilibrium shape, as was the pedestal temperature and the character of the quasi-coherent mode. The second half of the day on Wednesday continued MP#520 "Sawtooth evolution studies". Profiles of ion temperature and rotation were obtained using the high resolution x-ray spectroscopy diagnostic, which was operated at its maximum frame rate of 200 Hz. This experiment comprises part of the thesis research of an MIT graduate student. The run on Friday was devoted to MP#524 "Development of ITER relevant current rise phase", and was led by George Sips (ASDEX-Upgrade). The purpose of this experiment, which is part of a coordinated effort including ASDEX Upgrade, JET and DIII-D, is to obtain a dataset to be used in validating the proposed ITER Scenario 2 ramp-up phase. The ITER reference case calls for a ramp up to 15MA in 80 sec with 0.7< li < 1.0 throughout the ramp. The corresponding ramps on C-Mod have a flattop current of 1.35MA and ramp-up lasting between 0.4 and 0.6 seconds. Successful examples maintaining li<1 were obtained in the C-Mod experiments. The time of xpoint formation was also brought as early as 0.23sec, when the plasma current is at just over half its flattop value. In addition to ohmic ramp-ups, we also employed up to 3 MW of ICRF heating to increase the temperature during the current rise. The principal influence on li under all circumstances was dI/dt. Physics --------- Following application of LHCD, Alcator C-Mod plasmas are found to rotate in the counter-current direction in conjunction with the formation of ITBs. Transport barriers are seen in the density, momentum and energy channels along with a drop in the plasma internal inductance. These ITBs develop over a time scale similar to the current relaxation time but slow compared to the energy and momentum confinement times. For certain conditions, the ITB foot is located near r/a ~ 0.4 for electron density and temperature, and for ion (impurity) rotation, temperature and density. These discharges exhibit sawtooth oscillations throughout, with an inversion radius well inside the ITB foot. ICRF Systems ------------- We have integrated the Fast Ferrite Tuner optical arc detector into the RF fault system thus allowing for automatic retry after a fault. We have continued to have faulting at lower voltage than expected and will attempt to locate the arc location during upcoming maintenance time. In addition, we are investigating the cause of intermittent FFT computer "hangs" which occasionally result in delays in in the data acquisition system. Travel and Visitors -------------------- Greg Hanson (ORNL) visited to discuss the waveguide runs for the ICRF and LHRF reflectometer. We evaluated both E and J port locations for the ICRF system and have identified a simple run for the LH reflectometer as well. We discussed the waveguide run inside the ICRF antenna structure and access appears to be sufficient. Greg has completed testing about 80% of the microwave electronics and has begun some system testing as well. Rajesh Maingi (PPPL) was at MIT on Tuesday and Wednesday to serve as session leader for the runs devoted to his MP#448a. George Sips (IPP Garching, ASDEX Upgrade) began a visit to MIT last week. He served assession leader for the C-Mod experiment on Friday. Miklos Porkolab attended the first meeting of the newly formed multi-institutional group "Gyrokintic Simulation of Energetic Particle Turbulence and Transport", under the leadership of Prof. Zhihong Lin, UC Irvine. Porkolab is on the PAC of this group and the first meeting was organized on Jan 25, at UC Irvine, to discuss the goals of this program, including a strong element of validation by periodic comparisons with experiments from C-Mod, DIII-D and NSTX. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly