Alcator C-Mod Weekly Highlights August 30, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 14.3 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 2423 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 112 plasma discharges were produced with a startup reliability of 92%. Runs supported research in the Boundary Science, Transport, and Lower Hybrid and ICRF Physics topical areas. A fresh boronization was carried out on Wednesday night. Plasma operations are planned to continue this week. Planning -------- C-Mod has received authorization from OFES to extend our FY10 research operations by an additional two to three weeks beyond our original target of 18 weeks. Support for the additional operations comes from a combination of existing funds from the Cooperative Agreement at MIT and the C-Mod collaboration at PPPL through end of fiscal year reprioritizations. A meeting of the C-Mod Experimental Program Committee was held on Monday, August 23, to set research priorities for the remainder of the FY10 experimental campaign and to review new Miniproposals. Three new MP's were approved. Operations Details ------------------ Tuesday's run was devoted to MP#591 "Boundary Layer Heat Transport Experiments in H-mode Plasmas", which supports the 2010 SOL Heatflux Joint Research Target. The goal of this experiment was to examine the influence of field line length on heat flux footprint widths. Dynamic scans of the magnetic balance parameter SSEP were carried out in L- and H-mode plasmas for a range of target density and plasma current. In addition to the IR camera and target plate probes and thermocouple diagnostics for observing the heat flux profiles, the Gas Puff Imaging turbulence diagnostic operated routinely throughout the experiment, acquiring data on the variation of upstream turbulence as the equilibrium evolved from lower single null (LSN) to double null (DN) and finally to upper single null (USN). The heatflux data do not indicate a change in profile width as the configuration enters the DN phase, for which the effective field line length should be reduced by about a factor of two. During the course of the experiment we did observe a reduction in the L-H threshold in DN configuration, providing useful data for a separate MP from the transport/pedestal group. The run on Wednesday was dedicated to MP#468 "LH-driven fast electron diffusion time measurement", which comprises part of the thesis research of an MIT graduate student. Modifications to the Coupler Protection System parameters were successful in enabling use of a modulated waveform for the LH power as required for this experiment. Data were obtained with target parameters of 600kA and nebar~8e19/m^3 using 145 degree relative phasing (n-parallel~3.1). Good statistics were obtained for these conditions, and the data are being analyzed. Initial results were also obtained for a 1MA target plasma. Thursday's run was devoted to MP#568 for evaluating plasma performance with boron coated tiles. Recovery from boronization took ~5 discharges for D and E antennas and the J-port antenna recovered to 1.6 MW in about 10 discharges. Neon seeding was employed for high power operation. Molybdenum injections during high power ICRF were observed to originate, at least in part, from the J-port antenna. Neon seeding, which was attempted using valves connecting directly to the main chamber and through the lower divertor, reduced the level of Mo radiation and injections, and lowered the observed heat flux to the divertor, but was found not to be completely effective in eliminating the Mo injections. Friday's experiment supported MP#615 "Access to I-Mode Regime with Favorable Magnetic Drift". The purpose of this experiment is to document and extend the observed appearance of the I-mode enhanced confinement regime in a LSN equilibrium with extremely high lower triangularity, such that the x-point comes close to limiting on the inner divertor while the outer strike point intercepts the vessel floor at the corner of the EF1 coil pocket. A transition at low power to I-mode rather than H-mode was previously encountered fortuitously during variations about the so-called "JFT-2M" shape in a 2009 experiment on ELM behavior. On Friday, we successfully reproduced the effect in a series of discharges. In most cases the I-mode is a transient phase preceding an ELMy H-mode. A 'weakly coherent mode' is exhibited, and typically there is a gradual increase of density through the I-mode to the H-mode, indicative of a weak particle barrier. The I-H transition is therefore difficult to distinguish from the density evolution; however, the changes in fluctuations are very clear. We carried out a shape variation with an 800kA target discharge, and found that the shape in which the phenomenon was first encountered is close to optimum. As the X-point major radius is moved to the inner divertor "nose", the I-mode transition is lost and only a prompt L-H transition is observed, while at larger X-point major radii the transient I-mode persists but appears weaker, and subsequent ELM activity in the H-mode is enhanced. Changes in fluctuations similar to those characteristic of the I-mode were seen in the ohmic target plasma, prior to the start of RF heating. The LSN I-mode exists in a restricted power window. While we didn't establish a lower bound, the upper bound is set by the I-H mode transition, which as previously found, is quite low. At 800 kA, we could use up to 700 kW ICRF, and at 1 MA only 300 kW. Below this power threshold, I-mode can be extended to several hundred msec. ICRF Systems ------------ In preparation for the installation of the solid state Intermediate Power Amplifiers (IPA) in the FMIT#1 and #2 transmitters, the new AC power lines have been run between the transmitter and the appropriate power panels. The next step is to wire the breakers into the circuit. Initial test cuts in stainless steel have been completed to verify the manufacturing approach for the rotated four-strap antenna current straps. The actual straps will be fabricated in Inconel. Lower Hybrid System -------------------- The LH system was used one day last week in support of MP468 - LH-driven fast electron diffusion time measurement. This experiment required square-wave modulation of the LH power at high n|| (145 deg relative phasing), which had been difficult to obtain without spurious faults in previous attempts. The desired performance was achieved by adjusting the coupler protection system fault parameters. A damaged section of semi-rigid coaxial cable between klystron #4 and its vector modulator was identified and replaced, resulting in more reliable operation. The SOL reflectometer (X-mode) began taking data this week after overcoming data acquisition challenges. Initial data from the middle reflectometer is encouraging and could be inverted to provide a profile. The system still has some noise problems that may be related to grounding issues. The reflectometer is presntly working in differential mode. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly