Alcator C-Mod Weekly Highlights May 4, 2001 Alcator C-Mod is currently undergoing a 120C bake that is expected to last into early next week. Work continues on the ICRF, Lower Hybrid, DNB, and diagnostic systems. Physics ------- Significant progress has been made in the past two weeks analyzing the "two-frequency" ICRF heating experiments of internal transport barrier (ITB) modes in C-Mod. These experiments were analyzed using the TRANSP code and effort was concentrated on a shot on which 0.6 MW of 70 MHz heating power was used to heat inside the transport barrier and also to control the barrier. Off-axis heating was provided by 2 MW of ICRF power at 80 MHz. In this shot, peaking of the density profile and the accumulation of impurities were found to be arrested by application of the central heating power. TRANSP analysis of this discharge indicates the formation of a transport barrier in Chi_eff at r/a < 0.45, during the off-axis ICRF heating phase. After turn-on of the 70 MHz ICRF power, an increase in Chi_eff was found, concomitant with the abatement of the density peaking. However, a clear barrier in Chi_eff still persisted with Chi_eff remaining well below its pre-ITB value within the barrier region. The ICRF power deposition and energetic minority (H) tail evolution were computed in TRANSP using the FPP - TORIC ICRF module. The results of these analyses will be reported at the upcoming RF Topical Conference in Oxnard, CA and the Transport Task Force Meeting in Alaska. We have analyzed the ideal ballooning stability of the C-Mod edge in H-mode using kinetic EFIT runs based on Thomson scattering measurements of edge Te and ne profiles. This analysis is part of a concentrated effort to analyze stability of high and medium n ideal ballooning, peeling modes, and resistive ballooning modes, to understand the physics of edge fluctuations in the H-mode edge. For this analysis, accurate reconstruction of the equilibrium in the edge region is important. Since measurements of edge ion temperature and current density are not yet available, we study the effects of these parameters on stability by using various assumptions to model the edge profiles for EFIT reconstruction. Thus, for ion temperature we use profiles with the built in assumption that Ti=Te and profiles which have no change in the Ti gradient in the pedestal region relative to the core region. For edge current we use the profiles based only on neoclassical plasma resistivity and the measured loop voltage ("ohmic" profiles) and profiles with bootstrap current calculated with and without collisionality effects. Analysis of infinite n ideal ballooning mode stability carried out using BALOO code showed that the pedestal pressure gradient is generally unstable if no bootstrap current is included in analysis. However, taking into account the bootstrap current driven by the pressure gradient stabilizes the edge even when the current is reduced substantially by finite collisionality effects. It is interesting to note that two extreme assumptions for ion temperature gradient lead to very similar stability pictures. A reduced ion temperature gradient leads to a small reduction of total pressure gradient at the edge (less than 20% drop) and, since pressure gradients are normally about 1.5 - 2 times higher than first ideal stability limit, does not affect the overall results of the analysis. Jim Terry reported at the weekly Physics Meeting on his observations at the April meeting of the APS, which took place April 28 - May 2 in Washington D.C. This event is a general meeting of the APS. Jim presented an Alcator C-Mod overview poster entitled "Experiments in Plasma Physics and Fusion Science on the Alcator C-Mod Tokamak". It was one of six other fusion posters at the meeting. There was one invited talk from the fusion community on self-organized criticality in plasma transport. The participation at this meeting was an "outreach" by the fusion community to the rest of the physics community. Operations ---------- The C-Mod heater system was brought back online early this week in preparation for the 120C bake which began on Friday. The CRYO system for controlling liquid nitrogen to the magnets was also brought online early in the week. The magnets are now cooled down to -25C to protect them during the bake. The ECDC system was brought back into operation briefly to check the gas control system and the magnetron source. ECDC wall conditioning will begin early next week. The pulsed gas system will be baked out over the weekend and be ready for CAMAC cycle tests early next week. ICRF Systems ------------ We continue to upgrade and test phase detectors and fault reporting electronics for the transmitters. Delays are being measured on all fault signals to ensure the transmitters can be shut down in less than 10 us. Fast current monitors for the transmitters have been tested. S-Parameter measurements of the J-Port antenna indicate very little change from previous measurements. These measurements confirm our attempt to maintain a 50 Ohm transmission line while eliminating along field arc paths. Both the 78 and 70 MHz feedline and matching components have been fitup in the cell and tested successfully. The J-Port MKS pressure gauge cabling has been installed and the data acquisition tree prepared to read the new signal. The response time of the gauge and its proper operation will be tested during the CAMAC cycle and gas puff tests planned for early next week. All four FMIT transmitters have been tested into dummy loads and are ready for operation. Chris Brunkhorst (PPPL) has sent us his latest tuning models for the FMIT#3 and FMIT#4 transmitters. Lower Hybrid MIE Project ------------------------ Bids have been received by MIT for the Lower Hybrid site modifications, and they are in line with expectations. Work is expected to begin late summer following the next run period. We continue to work on the control program for the active controller. Parts of the program are being rewritten to improve performance. Testing of the new arc detection concept continues. DNB --- Conditioning of the DNB began this week with two days of arc only shots. Arc currents of 150 Amps were quickly achieved in an emission limited mode. This amount of arc current is required for full beam current operation. One day of beam shots achieved beam currents in the range of 4.5 A. Inner Divertor -------------- We expect all inner divertor tiles to be in-house by late next week. Vendors continue to work on the divertor C and backing plates. The inner divertor mockup is proceeding quickly. Power Systems ------------- All Alcator C-Mod power supplies are ready for operation. The Alternator will be brought up to speed next week and new instrumentation and sensors will be tested. Diagnostics ----------- The impurity injection laser has been replaced by Perry Phillips and Bill Rowan (UT-FRC). The replacement laser has been tested to 2.4 J. This new laser will allow us to continue laser ablation of impurities for cold pulse, impurity transport, and spectroscopic experiments. Travel and Visits ----------------- Martin Greenwald travelled to Washington DC to attend a meeting of the ESnet Steering Committee where he presented a talk on Office of Science programs and their network requirements to a group of DOE program managers. Eric Nelson-Melby traveled to the CRPP-EPFL in Lausanne, Switzerland and gave a seminar on C-Mod RF PCI results entitled "Phase Contrast Imaging of Ion Bernstein Waves in Alcator C-Mod." Ron Parker, Miklos Porkolab, Joe Snipes, and Spencer Pitcher attended the Burning Plasma Workshop II at General Atomics on May 1-3. Options for a future burning plasma experiment - Ignitor, FIRE, and ITER-FEAT, were discussed.