Alcator C-Mod Weekly Highlights Sept. 3, 2002 Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. Experiments in the Transport and Advanced Tokamak areas were conducted. Good progress was also made on the Lower Hybrid fabrication project. Plasma operation is scheduled to continue next week. Physics ------- At the C-Mod Staff meeting on August 19, Ken Gentle (U. Texas) reported on his analyis of C-Mod sawtooth data from ECE measurements. The complete electron temperature profiles with high space and time resolution from ECE open the possibility of time-dependent transport analysis, for example of sawteeth. By averaging over many sawteeth, waveforms of sufficiently low noise can be obtained. The analysis shows the expected large outward fluxes inside the mixing radius at the crash that flatten the profile. Surprisingly, the fluxes become negative at larger radii at the crash. The negative fluxes are an indication of missing energy, in this case interpreted as the dissipation associated with reconnection. The amount is small ~ 1 kJ, consistent with a decrease in internal inductance at the crash of 1-2 percent. Excluding the brief interval about the crash, the remainder of the cycle can be analyzed for transport, presented as thermal flux versus temperature gradient. Outside midradius, these curves are very steep (stiff?). Inside the mixing radius, they become almost flat. The results are preliminary and must be refined with better density and power deposition profiles and analysis of ion coupling, but they illustrate the potential of the approach. Operations ---------- Plasma runs were scheduled and completed on Tuesday through Friday last week. A total of 68 plasma discharges were produced, with a startup reliability of about 60%. The third boronization of the campaign was carried out on Wednesday evening, resulting in reduced reliability on the following runs. Experiments on Tuesday and Wednesday (MP#301 and MP#305) were aimed at investigation of the Internal Transport Barriers observed with off-axis ICRF heating. Difficulties in obtaining satisfactory ITB performance, and the observation of increased Molybdenum concentration and H/(H+D) ratios during these experiments prompted us to carry out a fresh boronization following Wednesday's run. A total of 140psi of diborane were applied overnight using the ECDC technique, corresponding to an average Boron layer thickness of approximately 2000 Angstroms. Following the boronization, about an hour of additional ECDC in helium was performed. He ECDC was followed by a half hour of ECDC in D2 at the beginning of the Thursday run. Most of the run on Thursday was devoted to recovery from the boronization process, and re-conditioning of the ICRF antennas. The target shot for this procedure was an 800kA fiducial shot selected from the preceding run which featured a pre-programmed gas puff with no density feedback, to allow us to assess changes in the wall source/sink rate following the boronization. The resulting density was about 20% higher on the initial shots, and decreased to pre-boronization level by the end of the run. H/D levels were observed to be around 2% before and during ICRF operation, and the Mo concentration was reduced by more than a factor of four. The poor reproducibility of the startup resulted in too few discharges for the ICRF antennas to be fully re-conditioned on Thursday. Friday's run began with a series of shots devoted to MP#310, a study of thermal transport in ohmic plasmas using the sawtooth heat pulse propagation and employing small programmed ramps in the toroidal field to enable local electron temperature gradients to be determined from the variation in ECE emission on individual channels of the FRCECE radiometer. The experiment, which comprises part of the doctoral research of a U. Texas graduate student, included a scan of the (line-averaged) density from 4e19 to 1.5e20/m^3, in order to evaluate the effects of collisionality and electron-ion coupling. This MiniProposal was successfully completed. The remainder of the run continued the process of re-conditioning the ICRF antennas. The C-Mod Experimental Program Committee met on Monday, August 26, to consider new MiniProposals submitted for experiments in the upcoming weeks. Six new MP's were approved for scheduling, with two others to be re-considered following suggested revisions. The approved experiments include proposals from the Transport and MHD areas and the AT and Burning Plasma Support task forces. The present plan is to continue experiments with the J-port ICRF antenna operating at its present frequency of 70MHz at least through the end of September, rather than changing to 78MHz as originally proposed. This schedule change will permit additional experimental time for studies using 70MHz and two-frequency heating to be scheduled. ICRF System ----------- FMIT#4 has had several tube arcs in the past week. These arcs have come before and during applied RF. After baking the transmitter, high voltage was successfully applied. A series of pulses into the dummy load indicated that the transmitter is performing fine. We attribute the arcing to the wet/moist conditions we have been having. Diagnostic Neutral Beam Systems ------------------------------- The beam ran well during the first three experimental days. There was an enhanced water concentration at the beginning of the week, but the component mix appeared to improve toward the end of the week. A leak developed during the week which caused us to stop beam operation on Friday. The leak was found and repaired in a copper gasket associated with the cryo system. The cryo system will be restarted on Tuesday evening with the expectation that we will be back in operation within a couple of days. BES was used to measure the beam width as the beam current was varied to check the perveance and support interpretation of the diagnostics. The beam diameter is approximately 8.4 cm at a beam current of 5A and increases as the current decreases. The dependence of MSE signal strength on DNB current has been analysed with a newly-developed software tool; the analysis confirms the original observation that the number of MSE photons varies faster than linearly with DNB current. The cause of this behavior is being investigated. Lower Hybrid MIE Project ------------------------ Issues related to the preliminary acceptance of the high voltage power supply are being addressed with the vendor. Operation of the HVPS will begin after these are resolved. An updated delivery schedule for the first circulator unit is under discussion with the vendor. Design of a mechanical waveguide support to be added to the carts to support the heavier RF output pieces, including the circulators and dummy load, is in progress. The LH Low Power rack i-q detector back panel assembly was started. Connectors were mounted, i-q detectors were mounted and the panel was labeled. Cable fabrication is underway. Special tooling has been developed to trim the jacket and insulator off the semi-rigid cable in order to install the SMA connectors. A prototype tool has been built and is currently being evaluated. Sub-assembly of the LHCD waveguide structures is proceeding on schedule at PPPL. The first of 4 couplers has been brazed successfully (ceramic window vacuum tight). All other vacuum components have been delivered and are being assembled. A reconfigured 3 dB splitter has been designed and is being incorporated into drawings for fabrication and testing. The procurement process of the rear waveguide is expected to begin shortly.