Alcator C-Mod Weekly Highlights August 15, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 16.2 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four run days were scheduled; Tuesday's run was stopped at 2:00PM to facilitate repair of the FMIT#2 crowbar circuit, and Wednesday's run was extended by two hours. The runs supported three experiments, in the Operations and MHD research areas. A total of 68 plasmas were produced with a startup reliability of 72%. Between shot boronizations were carried out during the experiments on Tuesday, Wednesday and Thursday. The frequency of the J-Port ICRF sytem will be changed from 50 to 78 MHz this week in preparation for upcoming experiments. Plasma operation will resume later in the week. Run Details ------------ The runs on Tuesday and Wednesday, and the last two hours on Thursday, were devoted to MP#430, "Localization of boronization effect". This experiment makes use of between-shot boronization to investigate the quality of the boronization, in terms of reduction in radiated power and plasma performance, as a function of the location of the ECDC resonance. This information is important both for understanding of the boronization effects and for operational optimization of machine performance. The experiments this week were carried out with a mixture of 20% deuterated diborane (B2D6) in helium, using a bottle loaned to us by the DIII-D group. All previous C-Mod boronizations have used a 10% diborane 90% helium mix. Scans of the sweep locations were carried out with sweep widths (resonance ranges) of 10 cm. Scans centered near R=0.7m, which intersects the top of the outer divertor modules, gave the best results in terms of radiated power and H-mode stored energy with 3MW of combined D(H) and D(He3) heating. This radius is also close to the He3 resonance location for the 5.2T field employed in these experiments. Additional experiments indicated that our usual procedure of modulating the ECDC discharge at about 1Hz was not as effective as using a 100% duty factor for the boronization. These experiments were conducted using a boronization duration of 10 minutes. Increasing the duration of the boronization to 20 minutes resulted in steadier H-modes with reduced radiated power, and also extended the effect for more than one tokamak pulse following the boronization; however, the peak plasma performance in these cases was not as good as that for the ten minute boronizations. The startup following the between-shot boronizations was problematic, with an increased frequency of fizzles. Substantial reduction of the initial gas puff was found to help improve startup reliability in these cases. Thursday's run was dedicated to MP#424 on disruption mitigation using high pressure gas jets. Dennis Whyte participated in these experiments as co-Session Leader, from the University of Wisconsin. These experiments support the C-Mod contribution to the ITPA Joint Experiment on the topic of disruption mitigation using this technique. For this experiment the gas jet injected helium at ~70bar into 1MA, 5.2T target discharges with stored energy up to 150kJ (~1 atm). Data obtained included IR camera images of the outer divertor during the disruptions, as well as Thomson scattering and radiated power profiles. An intended change of the working gas from helium to neon was not carried out because of an inadvertent overpressure of the gas jet pumping system during the changeover. The remainder of the run was turned over to the MP#430 boron localization experiments. The run on Friday was devoted to MP#404a on the parametric dependence of Alfven eigenmode damping, using the C-Mod Active MHD Spectroscopy system. Ambrogio Fasoli visited MIT to participate in these experiments, which are part of an ITPA Joint Experiment. Inner wall limited discharges were employed, with scans of triangularity from 0.27 to 0.75, and of toroidal field from 2 to 7 Tesla, at a constant q~5. The resonant TAE frequency of the drive was varied between 1, 1.5 and 2 in order to scan for different resonances. In addition the phase between the upper and lower antennas was inverted for some shots in order to excite a different mode spectrum. Clear resonances were obtained, with typical damping rates in the range of 2%. The data are being analyzed to determine the scaling of the damping with parameters. ICRF System ------------ The ICRF system (E-port at 80MHz and J-port at 50MHz) were used in experiments on Tuesday, Wednesday and Thursday last week. During the run on Tuesday the crowbar circuit on FMIT#2 (E-port) failed and could not be reset. Because the RF was necessary for that day's experiment and for others later in the week, the run was stopped and repair of the system begun. The problem was identified as a failed SCR in the crowbar trigger circuit. This component was replaced, the system was tested, and the transmitter returned to service for Wednesday's run. during the coming week, the J-port ICRF (FMIT#3 and 4) will be returned to 78 MHZ operation, which is appropriate for D(H) minority heating at B~5.3T and for D(He3) heating at B~8T, in preparation for planned experiments. Travel and Visitors, and Remote Participation ---------------------------------------------- Prof. Ambrogio Fasoli of the CRPP-Lausanne, Switzerland is visiting the PSFC as part of the collaboration on Alfven eigenmode active MHD studies between JET and C-Mod. He arrived on 10 August to participate in the experiments held on 12 August for MP404a Parametric Dependence of Moderate n Alfven Eigenmode Damping Rates. He also participated in discussions and electrical tests of a new amplifier that is being considered to drive the new JET moderate n TAE antennas. He will stay through 16 August. Dennis Whyte (U. Wisc.) participated remotely from Wisconsin in the boronization experiments on Tuesday and Wednesday, and served as co-Session Leader for the gas jet run on Thursday. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly