Alcator C-Mod Weekly Highlights Feb 2, 1998 Last week was a scheduled maintenance week at Alcator C-Mod. No runs were scheduled. Power systems tests were carried out in support of the modifications in the alternator excitation regulation circuitry described below. A Peer Review of the proposed Alcator C-Mod Fusion Research Program for 1999-2003 was held at M.I.T. January 27-28. The Reviewers heard presentations by C-Mod researchers from M.I.T. and collaborating institutions. This review was combined with the annual meeting of the C-Mod Physics Advisory Committee (PAC), which continued through Jan. 29. The voltage regulator that controls the alternator exciter current has been modified for better frequency response. This change will allow somewhat faster ramp-up rates for the TF field and less heat load on the TF magnet. In addition, changes have been made to the fault summation relay so that nearly all faults result in exciter supply inversion rather than pulling the exciter breaker. These changes were made in response to the review recommendations following the alternator fault last June. A series of gas puff tests were carried out to evaluate the pumping speed of the divertor cryopump for nitrogen, in the absence of plasma. With all other pumps closed off, N2 pumping speeds of about 800 l/sec were obtained. These tests used the J-bottom piezo valve. A new "duty-cycle" driver board has been installed on this valve, which uses a pulse-width modulated output rather than a proportional output, to allow a more linear valve response. During the maintenance week, a circuit was implemented in one of the ICRF transmitters (FMIT#1) to avoid grid overcurrent faults, which cause the transmitters to be shutdown during the pulse without a retry. A filter circuit was also added to the grid high voltage supply. These modifications will be evaluated during operations before being implemented on the second transmitter (FMIT#2). We continue to work on a pull back circuit because this circuit will solve most of the overcurrent situations and we still need to implement it for high voltage faults. We also started tuning FMIT#3 and #4 to higher frequency. They were originally tuned to 40 MHz, and we would like to have them tuned to as close to 80 MHz as possible. The criteria is for reliable 2 MW output from each transmitter. Once this frequency is known we can finalize the resonant loop design. Development of the DNB for C-Mod continued. Mechanical assembly and plumbing of the Mod/Reg cabinet continued. A technique was developed to apply a conducting coating to interior of the high capacitance section of the HV transmission line. The coating will facilitate a graded voltage scheme to prevent electrical breakdown inside the transmission line. A mini-review of the MSE optical system was held. Norton Bretz and Bob Parsells of PPPL presented an outline of the current design to members of the MIT and UT-FRC staffs. Dr. Steve Wukitch visited Tokamak de Varennes (Canadian Center for Magnetic Fusion) last week for two days. The purpose was to examine a working LHRF system and discuss their operational experience. On Jan. 29-30, Paul Bonoli travelled to PPPL to attend the PhD thesis defense of John Wright. Paul was one of the thesis readers and had suggested the problem which eventually became Wright's thesis topic - formulating the quasilinear diffusion coeffcient of Kennel and Engelmann to include poloidal mode coupling of fast ICRF waves in toroidal geometry. Dr. Bonoli also gave a seminar on Advanced Tokamak modelling in Alcator C-Mod. The talk emphasis was on the lower hybrid current profile control studies that had been discussed at the C-Mod Five Year Plan Review. Dr. Bonoli also continued working with Cynthia Phillips and Dan Clark (Princeton graduate student) on the development of an interface between the TORIC ICRF code and the FPPRF code, currently deployed in TRANSP. Roger Bengtson, UT-FRC, and David Winslow, a UT-FRC postdoc, spent the week completing the installation of the Texas Turbulence Probe on K-port. David will remain on-site to operate the probe for much of the remainder of the run period. Ron Bravenec, UT-FRC, visited to participate in the Program Review and to continue shakedown of the detectors, amplifiers, and data acquisition equipment that will form the bulk of the BES spectrometer. Josef Neuhauser from ASDEX-U visited the PSFC last week. He gave a seminar on "Results and Plans for Tokamak Improvement Research on ASDEX-U". He also spent the day having discussions with Alcator personnel on a number of subjects, including edge and core transport, atomic physics of the divertor and MARFE, pedestal physics, MDS-plus, and ICRF heating. Plasma operations are scheduled to resume this week. Four run days are planned.