Alcator C-Mod Weekly Highlights June 25, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 8.38 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. 3.25 run days were completed. A total of 69 plasma discharges were produced with a startup reliability of 72%. This week's experiments supported miniproposals in the Lower Hybrid and ICRF Physics and MHD areas. Overnight boronizations were performed prior to the runs on Thursday and Friday. The cryopump was employed to control the plasma density during the run on Friday. On Wednesday, June 20, our power utility made a change that increased the impedence of our primary 13.8 kV line, which affected several of our systems, including ICRF, lower hybrid, and some magnet power supplies. We are working on measures to address these problems. Plasma operation is planned to continue this week. Operation Details ------------------ The runs on Tuesday and Friday (under MP#473 and #490, respectively) were both aimed at increasing the launched lower hybrid power and operation with combined lower hybrid and ICRF heating. Some progress was made on both experiments, although technical difficulties, mainly related to the line modifications by our electric utility, prevented the run plans from being completed. For some discharges during Friday's run the cryopump was used for density control at nebar~1e20/m^3 during combined ICRF and LHCD operation in the upper null configuration, with promising results. These experiments contribute to C-Mod quarterly sub-milestone B.3 "Operate LHRF into L-mode and H-mode plasmas, and investigate combined LHRF and ICRF". Wednesday's run was devoted to an investigation of the neutral pressure "limit" effect observed in ICRF antenna operation. We completed a current scan from 0.6 to 1MA at 5.4 T. However, we were unable to reproduce the neutral pressure limit current dependence observed during operations previously during which large variations (0.1 - 0.6 mTorr) of the neutral pressure limit with plasma current were observerd. During this run the limit was fairly constant at about 0.6 mTorr with max at ~0.65 mTorr for 1 MA and 0.6 mTorr for 0.6 MA. We are analyzing these new results. Thursday was a scheduled six hour run devoted to MP#478 "Joint Experiments with JET on non-resonant n=2 magnetic braking". David Howell (UKAEA/Culham) served as Session Leader for this experiment, which was designed to measure the braking effect of an applied n=2 magnetic perturbation provided by the C-Mod A-coils on the intrinsic (no external torque) co-current rotation in ICRF-driven H-mode discharges. The goal was to compare these results with those from NBI-driven H-mode discharges on JET, using a very similar n=2 field perturbation, and to provide a test of the theory of neoclassical toroidal viscosity, which predicts a braking torque scaling with the non-resonant delta-B^squared. Surprisingly, in contrast to results on JET, no braking was observed in the C-Mod discharges, with rotation profiles measured by a number of diagnostics all showing essentially identical values with the maximum available n=2 perturbation as with the A-coils not energized. This result may provide important new information relative to the physics underlying the intrinsic rotation observed in C-Mod and other facilities. In the absence of any observable effect in the H-mode plasmas, several shots at the end of the day were devoted to investigating any possible effects of the non-resonant n=2 non-axisymmetric fields on the intrinsic locked mode threshold in low-density ohmic discharges. No obvious change in the density at which the locked mode occured was observed with and without the applied n=2 perturbation, although we did not succeed in obtaining as good a match in other parameters as in the H-mode cases, and some closer examination is required to confirm this result. The strong (resonant) rotation braking associated with error field penetration and growth of the locked island was clearly observed on the rotation diagnostics in this case. Physics -------- A broad range of MHD activity was observed in the current rise during lower hybrid operation between 0.02 and 0.1 sec. Low frequency (f < 10 kHz) large amplitude (> 30 G) n=1 modes were observed which exhibited two phase inversions with radius on the ECE temperature signals, indicated a double tearing mode, similar to what was seen a number of years ago with ICRH in the current rise. Such modes clearly show that LHCD is modifying the q profile with only 300 - 400 kW of input power. In addition, very early in the discharge, from 0.02 to 0.035 s, several bursts of high frequency MHD activity were found with frequencies increasing from about 200 kHz to about 700 kHz. The modes rotate in the electron diamagnetic direction and appear to have n numbers that increase with frequency in the range of n = 1 - 6. The peak frequencies are in the range of TAE frequencies for q=4, which is not an unreasonable minimum q value for this early in the discharge. Theoretically, fast electrons can drive Alfven cascades in the same way as fast ions so we tentatively label these modes as fast electron driven Alfven cascades. ICRF Systems ------------- The D- and E-port antennas (FMIT transmitters #1 and 2, respectively) operated reliably as needed for last week's runs, typically at the level of 1MW each. The J-port antenna, currently configured as a dipole, also operated at typical levels of 1MW. The control system for J-port was affected by the electrical utility changes which occurred mid-week, causing high-voltage trips during some of the operation. Lower Hybrid System -------------------- Low power testing of a new 4-way splitter prototype will begin this week. This splitter design could greatly simplify construction of the 2nd lower hybrid launcher. Long Pulse Diagnostic Neutral Beam System ------------------------------------------ The width of the diagnostic neutral beam in the toroidal direction was measured using the wide-view CXRS poloidal optical system. From the raw data, the full width of the beam at the half-density points is less than 0.12 m at a major radius of R~0.8m with nominal beam parameters. The current density profile of the beam at R~2m has a FWHM of ~0.06 m based on other recent measurements. The raw CXRS results may be revised pending more refined analysis. The DNB operated reliably during all of the C-Mod experiments last week. Travel and Visitors -------------------- Last week Stewart Zweben was at MIT to make additional measurements of edge turbulence with the new divertor gas puff imaging view. He also worked on a simple model for evaluating the parallel diffusion of blobs from the outer midplane to the divertor GPI view. Dennis Whyte served as a panelist for a workshop on basic research needs for "Materials Under Extreme Environments", sponsored by the DOE Office of Basic Energy Sciences held June 11-14 in the Washington D.C. area. Bruce Lipschultz attended the ITPA Coordinating meeting in Cadarache France June 18-19, 2007 as co-chair of the SOL/divertor group to present the progress in that topical group over the past year. Discussions were held on the future of ITPA and the current state of the ITER design review as well. Michael Hoffman and Dan Richman will be working with Darin Ernst and John Wright respectively for the summer under the DoE National Undergraduate Fellowship Program in Plasma Physics and Fusion Energy Sciences. David Howell (UKAEA/Culham) has begun a ten day visit to C-Mod. He served as Session Leader for the non-resonant magnetic braking experiment on Thursday, and will be working with Steve Wolfe and Bob Granetz on analysis of this experiment and of previous Joint Experiments between C-Mod and JET on the locked mode threshold. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly