Alcator C-Mod Weekly Highlights Feb 14, 2005 FY 2005 weeks of research operation -- planned: 17 weeks, completed: 0.2 weeks. Alcator C-Mod has begun its Startup and Conditioning operations. Five run days were scheduled and completed last week, including one day of research operation. Operations are planned to continue next week. Operations ---------- Alcator C-Mod operated Monday through Friday last week on a split shift schedule, with operations beginning at 12:00 noon and concluding at 7:00PM. A total of 65 successful plasma startups were obtained with an overall startup reliability of 54%. The primary goals of the runs last week were establishment of plasma operation using the new digital plasma control system (DPCS) and beginning the tokamak clean-up and conditioning process. Progress on startup reliability and disruption recovery so far seem typical for the first week of plasma operation. Dedicated tests of the digital control system on Thursday (MP#399) indicated that a standard discharge was well-controlled not only at the nominal (design) update rate of 10kHz but also for rates as low as 5kHz. When the cycle was slowed to 2.5 kHz the vertical position control was lost. The possibility of operating the DPCS at slower update rates could potentially allow more complicated calculations to be carried out in the real-time loop in a single processor, single cycle synchronous implementation. Friday's run was dedicated to MP#412, "Improved hydrogen-to-deuterium changeover using radiative terminations and disruptions". The purpose of these experiments is to reduce the operation time necessary to achieve the desired hydrogen-to-deuterium ratio in the plasma necessary for ICRH H-minority heating. Simultaneously, the experiments aim to provide uncover valuable information on wall fuelling sources and H isotope exchange. The underlying concept of this proposal is that disruptions can provide localized surface heating which would enhance diffusion of hydrogen in surface films, assisting the removal process. In Friday's experiments, co-led by Dennis Whyte (U. Wisc.) using a remote connection from Madison WI, intentional disruptions were induced by two techniques: ramping the toroidal field down to reduce q<2, and forcing vertical displacement events (VDE's) by turning off the fast vertical control feedback in elongated plasmas. In the case of the q<2 disruptions, ohmic H-modes were obtained prior to the disruption, increasing the plasma energy available for surface heating. Disruptions were produced in lower single null plasmas with different strike point locations, and also on inner-wall limited discharges and with small outer gaps. In some cases diverted H-mode discharges were moved rapidly to the inner wall as q approached 2 such that the H-mode energy was available to be coupled to the limiter. The disruptions were clearly effective in liberating gas and aiding hydrogen removal from the vessel surfaces, with the largest responses obtained in VDE disruptions terminating on the lower divertor structure. Rapid surface heating caused by the disruptions proved to be an efficient means of removing the hydrogenic species in the near-surface of the wall. Disrupting every shot did not cause unwanted side-effects to the overall operation of the device, with the exception of current rampup "fizzles" in succeeding discharges. The general goals of the experiment were met, with substantial H2 recovery and reduction of H/H+D. In addition we obtained net depletion of hydrogenic fuel from the Mo wall by consistently using planned terminations. an imporant demonstration of H/D/T control for a burning plasma experiment. Diagnostics ----------- Diagnostic systems are being brought back on-line. The Thomson scattering system is operational, now running with 2 lasers, increasing the data rate to 60 measurements per second. The TCI density interferometer system is also operational, and the feedback signal from TCI has been re-integrated into plasma control system. The ECE Michelson system and one of the grating polychromators are in service. Hydrogen/Deuterium isotope ratio measurements are being made, as in the past, using a high resolution visible spectrograph looking at balmer alpha radiation, filtered through a linear polarizer to accentuate the unshifted (pi) Zeeman components. The wavelength shift (about .18 nm) is used to distinguish the emission from the two isotopes. The single channel and high resolution imaging visible continuum diagnostics have been reinstalled and are operating normally. Four (out of six) of the High Resolution X-ray (HIREX) spectrometers used for ion temperature and rotation measurements are back on-line. The CXRS diagnostic was brought back into operation. The detector mounting was changed a few weeks ago to facilitate improved focussing of more channels. The full 45 channels of the detector were focused this week. A technique was developed to improve the focussing further although the present focussing is adequate. The dispersion and channel to channel sensitivity were calibrated. Data was taken during C-Mod discharges with the improved focusing. Although boron-containing tiles were removed from the machine during the last opening, there is still adequate boron to continue measurements with a boron line. ICRF Systems ------------ An apparent high voltage fault in FMIT transmitter #1 was diagnosed as being due to a faulty micro-switch position. This was corrected, and tuning of the transmitter resumed. Crowbar trigger tests, including "wire" tests and screen crowbar testing, have been completed. Tuning of these systems will be done this week. FMIT transmitter #2 (E-port dipole antenna) operated into plasma on Thursday, at power levels up to 0.5MW. Lower Hybrid System ------------------- Work to integrate the lower hybrid transmitter systems to the active controller made considerable progress this week. The CPS PLC controller was integrated into the data-highway-plus serial line. The CPS, which is now installed in the cell, is now connected in the PLC chain and can communicate with the operator computer. Installation of cabling from the lower hybrid launcher to the control and protection systems continued last week. Positioning of the waveguide runs from the klystrons to the launcher is being completed. The waveguide runs attach to the launcher with flexible sections of guide that are ready for installation. Travel and Visitors -------------------- Michiya Shimada, head of the ITER Physics Team, visited C-Mod on Thursday and Friday and held numerous discussions with C-Mod physicists. He also presented a PSFC Seminar "Opportunities of Reactor Plasma Development in ITER". Jim Irby participated in the TF Joint Operations Review for NSTX at PPPL via remote video connection from MIT. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly