Alcator C-Mod Weekly Highlights Mar 15, 2004 Research operations continued at Alcator C-Mod last week. Two and a half run days were completed, with experiments focused on internal transport barrier physics. Progress continued on the Lower Hybrid project. Plasma operations are scheduled to continue next week. Operations ---------- Two and a half run days were completed last week, out of four originally scheduled. 54 plasma discharges were produced with a startup reliability of over 80%. A fresh boronization was performed on Monday evening. FMIT#3 and 4, which are connected to the J-port four-strap antenna, had been tuned to 70MHz prior to the start of operations on Tuesday. After about two hours of boronization recovery and RF conditioning, Tuesday's run was devoted to MP#341, which is concerned with heating inside the ITB formed by off-axis ICRF heating. ITB discharges were produced by ICRF at 80MHz, resonant on the high-field side of plasmas with a central field of 4.5T. Additional ICRF at 70MHz, resonant near the magnetic axis, was applied after the barrier formed. In these experiments, we succeeded in applying up to 1.5MW of on-axis power without destroying the barrier. Central pressures up to 4 atmospheres were obtained. We were unable to determine the upper limit of on-axis power compatible with maintaining the ITB because of a fault in the final power amplifier tube (FPA) in the FMIT#4. Additional time will be scheduled for this MP. Wednesday's run was in support of MP#371, aimed at determining the effect of the plasma current (or q) on the location of the barrier in ITB's formed with off-axis ICRF. These experiments were carried out at 6.4 tesla with 80MHz ICRF, placing the resonance on the low-field side. These experiments complement earlier current scans at different toroidal field. In this run, ITB's were obtained at 0.85, 1.1, 1.2, and 1.3MA, corresponding to a range of 3.7 < q95 < 5.5. The location of the ITB "foot" clearly moved to larger radii in the lower q plasmas. Detailed analysis and comparison with results of previous scans is in progress. The run on Thursday (MP#300) was a search for hysteresis effects in ITB formation as a function of magnetic field (resonance location). Previously, we have observed a sharp threshold in BT for formation of ITBs produced by off-axis RF heating. The threshold is interpreted as an effect of deposition radius, with barriers formed when the ICRF resonance location is greater than about |r/a| = 0.45. The threshold is seen for deposition on both the low and high field side and is quite sharp - amounting to about 0.1 T or less than 5-10% in r/a. The point of this MP is to probe this threshold boundary dynamically to look for hysteresis which could help categorize the transition and shed light on the underlying physics. Field ramps about the nominal (high-field side) threshold field of 4.55T were carried out. Initial results did not indicate any hysteresis in the field, with ITB's formed at the nominal threshold and terminated as the field was raised above that value. This run was terminated early due to problems with the data acquisition server; additional runtime will be required in order to make a definitive statement about the existence of any hysteresis in the ITB formation/destruction dynamics. Friday was given over to maintenance, primarily to begin work on replacing the FMIT#4 FPA tube, as described below. Additional work was also carried out on the data servers. In addition, a series of beam-into-gas (without fields) pulses was taken in order to provide additional data for MSE calibration. Physics ------- Temperature fluctuations have been measured by the FRCECE radiometer. These newly discovered fluctuations were found in the lower density regions of the same ITB discharges from which density fluctations were reported earlier. The ECE diagnostic can detect density fluctations in those circumstances in which refractive effects are important. The temperature fluctuation have the same frequency (~80kHz) as the density fluctuations and are detected over most of the plasma but with a phase change of pi/2 inside the ITB. The temperature fluctuations were measured before the ITB formed only for radii outside the q=1 region. These fluctuations were very well correlated with the QC mode from the beginning of the EDA through the ITB. The link between the QC mode and these interior temperature and density fluctuations is not yet known. ICRF ---- Late in the run on Tuesday, the FPA in the FMIT#4 transmitter lost vacuum and required replacement. Though three high power tubes have been on order for several weeks, we do not currently have a high power replacement tube. To maintain full power from J-Port, and optimize the physics program, a high power tube was removed from FMIT#1 and installed in FMIT#4. A lower rated power tube was then installed in FMIT#1 (D-Port antenna). Both FMIT#4 and FMIT#1 installations were completed over the weekend, and both transmitters were tested successfully into dummy loads, ready for operation on Monday morning. Lower Hybrid System ------------------- The forward wave guide was baked on the vacuum test stand three days last week at temperatures similar to those it will experience during bakes on C-Mod. It was successful leak checked before the bake, and while at temperature. However, a small leak developed during cooldown that will be tracked down early this week. After finding and fixing the leak, the FWG will be removed from the vacuum test stand and then prepared to be mated with the rear wave guide. High power tests of the combined components will then be carried out. We have prepared several 3-window prototypes to exactly match the coupler design. Braze tests of these components will be completed this week. A 24-window prototype is also being prepared and will be brazed following analysis of the 3-window samples. Testing of the circulators to full power with 0.5 s pulses continued last week. We have now successfully qualified five of the twelve circulators for operation. A sixth circulator has higher reflected power than the others and will be re-evaluated following completion of the tests on the remaining six klystrons. Travel and Visitors ------------------- Ron Bravenec (UTx) was at C-Mod this week, working with the BES diagnostic.