Alcator C-Mod Weekly Highlights August 29, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 17.9 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Five run days were scheduled and completed. A total of 106 plasma discharges were produced with a startup reliability over 82%. The five runs supported six experiments, including MiniProposals from the Operations, Transport, Divertor/Edge, MHD, and Advanced Tokamak groups. With the completion of last week's runs, we have achieved the level 1 DOE/OFES JOULE milestone for research operations on Alcator C-Mod (17 weeks, plus or minus 10%). One overnight boronization was performed prior to Tuesday's run. In addition, between-shot boronizations were carried out during operation on Monday and Wednesday. On Friday a cable connection to the commutation capacitor bank for the OH2U coil system required repair during the run. The connection was replaced and the system placed back into service after about onehour of down-time. Research operations are planned to continue this week. Run Details ----------- Monday's run began with a continuation of MP#430, aimed at optimization of between-shot boronization. A long (1 hour) boronization using the 20% B2D6 diborane mixture was carried out prior to the first shot of the day. The effects of this boronization were determined to be significantly degraded after six plasma discharges, which included two early disruptions. We then carried out two 20 minute boronizations with scans of the resonance location covering overlapping regions centered at small and large major radius. Shorter (ten minute) boronizations concentrating on narrow regions of the chamber were then carried out. The final three hours of the Monday run were devoted to MP#433 "Fueling in H-modes as a function of pedestal density", which was continued on Tuesday. This experiment tests the effects of a gas puff into an existing H-mode edge, intended to perturb the neutral source in the pedestal region. A series of discharges with plasma currents between 400kA and 1MA were run, with D2 puffs corresponding to manifold pressures of 10 to 40psi. The gas puffing had little effect on the density pedestals at the higher currents, while for these plasmas the temperature pedestal was shifted to larger major radii. By contrast, more pedestal fueling was evident at plasma currents of 0.6 and 0.4MA, for which the highest fueling rates resulted in termination of the H-modes. A comparison of poloidal location of the gas puff, varied between inboard and outboard capillaries, was carried out for 0.6 and 0.8MA plasmas, with no apparent difference in the effects of the different sources. The run was completed by experiments involving large puffs into ohmic (L-mode) discharges for the purpose of comparison of the fueling rate. Wednesday's run was devoted to MP#435, "Long Pulse AT Target Plasma Operation". The main goals of this MP are to produce a long pulse (multiple L/R times) discharge with parameters suitable for AT targets. Specifically, to examine the long pulse behavior of the divertor structure temperature and Mo and W impurity production, and to quantify the gas inventory and wall pumping effects during these long discharges. These experiments employed plasmas with Ip=600ka, Bt=5 tesla. We were successful in obtaining the longest plasma pulses ever produced in C-Mod, with end-of-flattop (EOF) at 3.5 seconds and end of plasma (EOP) up to 3.94 sec. We also produced sustained H-modes up to 1.7 seconds in length, also a record for C-Mod plasmas. Feedback control of the non-axisymmetric coils (A-coils) was successful in eliminating locked modes, which had been observed in previous long-pulse experiments. Plasma duration was limited by administrative limits on power supply/coil currents, in particular for the EF1 coils, not by magnet temperature. Molybdenum levels did not rise during long pulse L-mode plasmas, and did not increase with time within the H-mode phases. No tungsten was observed in the core. Divertor plate power fluxes are highly toroidally asymmetric, with leading edges getting hotter than the average. In these experiments, the strike points were not held fixed; after about 1.5 sec the outer strike moved up the divertor plate. IR camera data showed the effects of this movement. The points touched by the strike point cooled after the strike point passed, indicating that strike point sweeping could be used to mitigate divertor temperature rises. The experiment on Thursday morning was devoted to MP#429, "SOL turbulence in limited plasmas" proposed by Stewart Zweben (PPPL). The purpose of this MP is to obtain turbulence imaging data for relatively high-collisionality SOL plasmas in a near-circular plasma equilibrium, for comparison with the GEM simulation code. Data were obtained with the fast gas puff imaging (GPI) cameras and diode arrays for 800kA, 5.4T inner-wall limited discharges with kappa~1.2 and nebar~2e20/m3. These data will be supplied to B. Scott at IPP-Garching for comparison with the simulation code results. On Thursday afternoon we carried out a half-day experiment in support of MP#424 on disruption mitigation using the high pressure gas jet. This run employed krypton as the injected gas species, as a continuation of previous experiments which used helium, neon, and argon. Good data were obtained including Thomson scattering profiles, IR images and fast images of the gas jet. The radiated energy fraction obtained with krypton was found to be marginally higher than that with argon, and halo currents were also comparable to those with argon injection. These results imply that, while higher Z gases are beneficial, as shown in the prior experiments, the incremental improvement from argon to krypton is not large. Friday's run was dedicated to MP#409, "SOL Phase Space studies", which has the overall goal of expanding the database of SOL parameters as a function of plasma conditions. Friday's experiments were concentrated on the low-field, low-density regime, using 0.4MA, 2.7T discharges. A scan of density was performed in matched upper and lower null topologies (NL04 from 0.2 to 0.5e20 m^-2). Good profile and fluctuation data were obtained on high and low-field sides of the SOL with probes, fast diodes, and Chromex spectrometer (looking at D-beta). We took special care to target the probes (ASP, FSP, ISP) at the seperatrix for each set of conditions. The xybion camera was used to monitor D-alpha profiles on the low-field side. These data will be used to infer ionization profiles in the SOL. This experiment should help us understand the role of that the neutral ionization mean-free-path plays in affecting the SOL profiles, in particular the appearance of a 'shoulder', which appears in the far SOL at moderate n/nG. Folded into this story is the role that SOL flows might be playing. For example, we observed that the inner-wall probe persistently sees strong parallel plasma flows and a dependence on upper/lower null topology over the full density range. Some interesting changes in flows on the low field SOL (ASP) were also detected in the lowest density, upper single null discharges. Plasma potential profiles as well as perpendicular Mach flows appear to be affected. These changes are linked to upper versus lower-null topology. These data are also interesting in that they may allow us to cross-check the perpendicular Mach flow measurement against an ExB flow as inferred from plasma potential profile estimates. Long Pulse Diagnostic Neutral Beam ---------------------------------- Dr. Petr P. Deichuli and Mr. Grigory Shulzhenko from the Budker Institute for Nuclear Physics in Novosibirsk, Russia have been working with C-Mod personnel to improve beam reliability and beam current. We have found and corrected several problems including faulty IGBT's in the high voltage and arc power supplies. Some other faulty components were found in the beam current sensing circuitry, the cathode heat PWM controller, and the beam fault detection "STOP" module. The power engineer at the Budker Insitute believes that the inverter problems are due to a compromised control power supply. He is preparing a replacement supply for shipment to MIT. The beam is presently performing at 47-49kV, 6.6 amperes, 100 ms pulse width with 50% modulation for 0.5 sec. We plan to continue to increase the beam pulse length and beam current over the next week. ICRF Systems ------------ The ICRF system was operated in support of the runs on Monday-Thursday. E-port and J-port antennas were used, with J-port still configured as a dipole driven by FMIT#4 operating at 78 MHz. Planning for the repair of the FMIT#3 Uptegraff is underway. A crane has been contracted to remove the rectifier stack. A test of sample oil from the power supply found that the oil was free of carbon particulates which indicates the problem is probably localized to the rectifier stack. Lower Hybrid System ------------------- A 24-window braze test was carried out last week to validate the new molybdenum fixturing and other modifications that have been shown by the FEA work to reduce stress in the windows. The quality of the braze in the sample is high, and no cracks in the alumina have been seen. Less braze material was used in this test because of the smaller gaps expected from the stainless steel sample as compared with titanium samples. Travel and Visitors and Remote Participation -------------------------------------------- Stewart Zweben (PPPL) was at MIT all of last week for participation in C-Mod experiments. He served as session leader for the MP#429 experiment on Thursday and operated the fast imaging system during the MP#409 experiments on Friday. Dennis Whyte (U. Wisc.) participated remotely in the gas jet experiment MP#424 on Thursday afternoon. Petr Deichuli and Grigory Shulzhenko from the Budker Institute for Nuclear Physics in Novosibirsk, Russia, continued their work on the Diagnostic Neutral Beam. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly