Alcator C-Mod Weekly Highlights May 2, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 7.25 weeks Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed, following a fresh boronization on Monday night. A total of 94 plasma discharges were produced with a startup reliability of 78%. Plasma operations are planned to continue this week. The boronization on Monday used 120psi of deuterated diborane, with the ECDC resonance swept from R=0.52m to 1.03m, covering the divertor region and out to the outer wall. The previous boronization had covered only the radii near divertor. Tuesday's run, in support of MP#417 "Plasma performance, boronization and ICRF heating effectiveness with high-Z antenna guards and all-metal plasma facing components", aimed at evaluating the effectiveness of the boronization. Radiated power and core Molybdenum concentrations were both substantially reduced relative to the first boronization. Excellent plasma performance was obtained, with one shot reaching 250kJ of stored energy and a volume-averaged pressure of 1.8 atmospheres, which we believe to be the highest value ever obtained in any toroidal magnetic confinement device. This shot, which had a plasma current of 1.05MA at a field of 5.4T, had confinement enhancement H89=1.9 (equivalent to H98y2=1.0). The beta_N in this small ELM regime discharge reached 1.75, the ITER design value. The run on Wednesday was devoted to MP#415 "Rotation and H-mode power threshold for LSN and USN at different Ip and ne". The aim of this experiment is to document the relation between rotation and the L-H transition threshold for a wider range of plasma parameters than previously available. The power threshold in lower single null, 0.8MA discharges was found to be considerably higher in this experiment than was observed previously, making comparison with the earlier results difficult. The transition seemed to occur when core rotation passed through zero, but the initial counter-rotation was stronger and the zero-crossing occurred for higher auxiliary power than in the earlier studies. Upper null cases had higher threshold powers, but also exhibited the L-H transition near zero core rotation; in the upper null case the transitions were precipitated by a large (monster) sawtooth crash, which had not been the case in earlier studies. On Thursday we carried out the first run in support of the multi-day experiment described in MP#410 "Extension of pedestal scalings", which has the goal of documenting edge profile data over a range of plasma parameters in ELM-free and EDA H-modes, including the transient phase of initial H-mode development. Three values of ICRF power were employed on each shot, with sufficient time between RF pulses for the plasma to return to ohmic L-mode conditions so that the initial transient could be observed following the L-H transition at each power level. Target density was scaled proportional to plasma current as datasets were obtained at three toroidal fields (4.4, 5.4, and 6.2T) and three currents at each field. The run on Friday was devoted to MP#408a "Alfven cascade studies during current ramp-up with ICRF heating and improved n-number measurements". Scans of ICRF power were conducted at toroidal field values of 4.8, 5.0, 5.4, and 5.8T to determine the threshold power required to drive the cascade modes. At the lowest field only the downward chirping mode was observed at power up to 3MW; this result is consistent with earlier results and with theoretical predictions for the high field side instability. At the higher fields, threshold values for upward chirping cascades were found to be between 2.0 and 2.5MW at 5.0 and 5.4T. The 5.8T power scan showed no definite cascade activity, although many shots exhibited TAE transitions. Physics ------- GS-2 simulations of C-Mod ITB experiments carried out during a Summer Undergraduate Laboratory Internship with Jessica Baumgaertel has been recognized by Matha Redi (PPPL) being named the Department of Energy Office of Science, Undergraduate Research Programs "Outstanding Mentor" of 2004. The citation reads "In recognition of your dedication as a mentor, to share knowledge and to inspire and install confidence in the next generation of scientists and engineers by setting high expectations, seeking for creative solutions, and immersing inquisitive minds in the world of science." Quarterly Review ---------------- The Alcator C-Mod quarterly review for the second quarter of FY2005 was held via videoconference on Thursday, April 28. Presentations covered "Status of operations" (Jim Irby) , "Lower Hybrid status" (Ron Parker), "Progress on the JOULE target for evaluation of all-metal walls and antenna guards" (Yijun Lin), and "Long pulse diagnostic neutral beam" (Bob Granetz). Also attending the review at MIT were Earl Marmar, Randy Wilson (PPPL), Stefano Bernabei (PPPL), Miklos Porkolab, and Martin Greenwald; at PPPL, Gerd Schilling; and at OFES, Rostom Dagazian, Steve Eckstrand, and Ken Hill. The viewgraphs of the presentations can be found at: http://www.psfc.mit.edu/research/alcator/pubs/quarterlies.html Lower Hybrid System ------------------- Work continued on conditioning of the launcher grill, with significant increases in the power able to be coupled without arcing. Experments were carried out with an X-band arc detector, which is sensitive to second harmonic emission generated in an arc event, but is cut off at the fundamental 4.6GHz. Long Pulse Diagnostic Neutral Beam ---------------------------------- Working with the Budker DNB software engineer in Novosibirsk, we were able to trigger the beam this week using an external trigger supplied from MDSplus for the first time. It was confirmed that the previous software-required permissive is no longer necessary, which will make routine operation of the beam during runs much easier, but at least one minor software bug was discovered that is still to be resolved. Several issues with the torus gate valve were also discovered, but they should be resolved during the coming week. Travel and Visitors ------------------- Randy Wilson (PPPL) was at C-Mod last week participating in lower hybrid conditioning activities. Stefano Bernabei (PPPL) was also here on Thursday. Wilson and Bernabei also participated in the C-Mod quarterly review. Yves Peysson (Tore Supra) is visiting for two weeks to work with Joan Decker while Joan finishes his thesis. He is also assisting John Liptac in assessing the LH power required to form a tail in the electron distribution function sufficient for production of observable Bremmstrahlung. Professor Sir Christopher Llewellyn Smith, director of the Culham Laboratories, UKAEA, visited the PSFC last week. He engaged in discussions on a range of issues with scientists at the Center, and was given a tour of facilities, including C-Mod. In addition, he presented a joint Physics Department/PSFC seminar on "The Fast Track to Fusion". Dr. Bernard Bigot, High Commissioner of the French Atomic Energy Commission visited the PSFC last week. He discussed many topics of mutual interest with scientists at the center, was briefed on the C-Mod program, and had a chance to tour the facilities. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly