Alcator C-Mod Weekly Highlights August 5, 2002 Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed, including experiments in the Transport topical area as well as RF conditioning and development. Work also continued on DNB and Lower Hybrid systems. Plasma operations are scheduled to continue this week. Operations ---------- Plasma operations were carried out Wednesday through Friday last week, with a total of 61 plasma discharges produced with a startup reliability of over 75%. The runs on Wednesday and Friday were primarily devoted to continued conditioning of the ICRF antennas. Good progress was made, particularly with respect to the four-strap J-port antenna, as described in the ICRF section of this report. Thursday's run was in support of MP#312, Toroidal Rotation Profiles in Ohmic H-mode plasmas. The new tangential HIREX (high resolution Xray) views were used to measure the rotation profiles of highly-stripped Argon ions in a series of ohmic H-mode plasmas. The subject plasmas were 1.05MA discharges with toroidal field ramped down to 3.6T, and then back up to 5.3T. While useful profile data were obtained, the discharges suffered from high disruptivity, some of which was associated with MHD activity observed when q_95 was about 2.7. During the runs on Thursday and Friday, both the DNB and lithium pellet injector "cigar" measurement were operating as piggyback experiments late in the shot. Nearly simultaneous MSE and pellet measurements of the poloidal field profiles were obtained, These data are being analyzed. ICRF System ----------- Two run days last week were devoted to RF conditioning. On Friday, the D- and E-port dipole antennas successfully ran into H-mode plasmas with 1.2 MW power each. With the four-strap J-port antenna, operating at 70MHz, we were able to heat plasma and produce H-modes at the 1.6 MW level, More conditioning on J antenna is needed to further increase the power and reduce faulting at L-H transitions for use in plasma experiments. The progress on conditioning the J-port antenna this week was facilitated by modification of the two transmitter modulator boards for longer turn on time on fault retry. The response of the two phase controller loop filter also increased by a factor of 10. This combination of changes allowed successful operation of J antenna with a soft startup. On Tuesday, we repaired the FMIT#2 lower driver screen bypass capacitor which had failed late last Friday. The damage appeared to be the result of water condensation on the capacitor. We also repaired the driver screen supply after the driver screen supply soft-start resistor failed. After the system was fixed, we did a few test shots before the crowbar system faulted, due to a malfunction of the interlock relay used to determine if the heat lamps for the ignitrons are turned on. The relay was replaced and the transmitter, which drives E-port antenna, was returned to service on Wednesday. Defective Kalmus (500W) amplifier parts in the unit formerly installed on transmitter #2 have been replaced. Once the amplifier bias is set this unit will serve as a spare. A tech request was entered to provide small closed-loop air conditioning systems on each of the transmitters. These will be designed to remove the moisture from the air in the cabinets, which may have contributed to the driver screen capacitor failure, and provide cooling and cleaner air for the driver screen tube filaments. Lower Hybrid MIE Project ------------------------ Progress was made in the installation and wiring of the 24VDC water flowmeter power supplies. The supplies were installed and partially wired. Modifications were made to the instrumentation and control cabinet in preparation for the power supply modifications. Specification of low-power microwave cable for the system was begun. Since there will be a lot of cable to be run, especially for the Coupler Protection System (CPS), the selection criteria include availability, cost, and ease of assembly. The preliminary selection is to use RG-402 semi-rigid coax. Another variant of this cable is flexible and uses the same solder-on connectors, at a higher cost. Tooling requirements to enable us to fabricate high quality cables in-house are being investigated. A cable assembly which incorporates a series diode has been fabricated. This cable 'patch' will go in-series with the output of the slow body-current sensors and prevents a high output voltage from the fast current sensor from feeding back and possibly damaging the slow sensor. DNB Systems ----------- The beam was conditioned on Monday and Tuesday, and operated into C-Mod for the remainder of the week. Beam parameters look good, with most shots lasting the full 50 ms at un-neutralized beam currents of near 5 A. Useful data were obtained on the MSE diagnostic during both L- and H-mode discharges. Variation in the signal to noise ratio as a function of plasma parameters is being analyzed. There seems to be a random delay from the beam hardware trigger to the beam firing time. Many shots are on time, but some shots are delayed by as much as 0.5 seconds. The hardware trigger into the beam camac has been verified to be accurate. The delay appears to be inside the Russian timing module. Restarting the Russian software to re-initialize the beam camac may have some positive effect. We are consulting with the BINP programmer to attempt to resolve this behavior. Travel and Visitors ------------------- David Mikkelsen (PPPL) spent the week at MIT examining the use of B_T sweeps to measure the electron temperature gradient scale length. This resulted in an experimental proposal to further explore the utility of the technique in a series of ohmic discharges with an Ip scan and a collisionality (density) scan. Due to an editing error, the following item was omitted from last week's report: Robert Granetz spent two days at APS headquarters in Maryland to help sort and schedule the sessions for the upcoming DPP conference in Orlando.