Alcator C-Mod Weekly Highlights September 7, 2001 The partial disassembly of Alcator C-Mod required for inspection of the TF joints, OH coaxes, coax extensions, and bus connections is nearly complete. Work continues on ICRF, LH, diagnostic, and inner divertor systems. Physics ------- Milestones 76 and 77 have been completed on schedule. A description of each milestone, and the work completed is discussed below. **** 76. Investigation of ITB control with multiple frequency ICRF. (Baseline target AUG 2001) Plasmas with Internal Transport Barriers (ITBs) show promise as the goal for future advanced tokamak steady state reactor operation. Plasmas with dual edge and internal, energy and particle barriers have been formed in Alcator C-Mod with auxiliary radio wave heating, in the absence of the usual neutral beam particle and momentum sources (which will be unavailable in future reactors). The prescription for achieving these ITBs is to lower the magnetic field, which causes the radio waves (at 80 MHz) to be concentrated in the inner portion of the plasma, off-axis. Further heating power at 70 MHz will be concentrated at the core of the plasma to increase the temperature of plasma center, inside of the ITB, to hold the density profile steady and to arrest impurity accumulation. The fundamental role of plasma rotation in the ITB formation will also be investigated. Milestone 76 was completed in August, 2001. The ITBs, formed in conjunction with a reversal of the co-current central toroidal rotation velocity, are apparent for both particles and energy, and are confirmed by a dramatic reduction in the inferred core thermal diffusivity from TRANSP modelling. Among the unique features of the C-Mod ITBs are the presence of sawtooth oscillations, the monotonic q profile (with the ITB foot location near q=1.5) and that they form in the absence of external momentum input. For ITBs formed with single frequency ICRF waves, the particle and impurity densities continue to increase at the plasma axis until the central radiation exceeds the central input power (which is lower with the off-axis heating used to create the ITB) and the barrier collapses. Additional central heating power from a second frequency ICRF antenna has been found to arrest the particle and impurity build-up at the same time as increasing the core temperature, and double transport barrier plasmas have been held in steady state for as long as six energy confinement times. Simultaneously with the arrest of the core density and impurity buildup, the toroidal rotation reappears in the co-current direction. Steady state double barrier plasmas have been achieved both at 4.5 T (with 80 MHz ICRF on the high field side to form the ITB and 70 MHz ICRF for the core heating) and 5.4 T (with 70 MHz ICRF on the low field side to form the barrier and 80 MHz ICRF for the core heating). This final result shows that the triggering of the barrier formation is not explicitly related to high-field side ICRF heating, but rather the transition can be triggered by strong off-axis heating on either the high- or low-field side. This, in turn, rules out the primacy of icrf-induced ion orbit effects which formed the basis of some of the theories proposed to explain the phenomenon. Further details of these results will be elucidated by Steve Wukitch in an invited talk at the upcoming APS-DPP meeting, and in its accompanying manuscript. **** 77. Evaluate operation of the modified J-Port 4 strap antenna. (Baseline target AUG 2001) Operate the modified 4-strap antenna at 78 MHz with improved arc detection and additional diagnostics up to the maximum power that can reasonably be achieved. Using heating phase, evaluate the heating efficiency, power handling, reliability, and impurity generation of the 4-strap antenna. Milestone 77 was completed in August, 2001. The operation of the modified J-Port antenna (see description below) was quite successful in the past campaign, concluded at the beginning of August, 2001. The modified strip lines were in excellent condition (no indication of arcing on the strip lines or vacuum vessel) after the campaign. Furthermore, arc damage was predicted to be located at the ground bridge between the bridge and the strap because E||B and estimated to be ~15 kV/cm when the maximum voltage on the transmission line was 25 kV @78 MHz (~50% higher than during the last campaign) and 30 kV @70 MHz (consistent with expected frequency scaling). Arc damage was found at this location particularly on straps #2 and #3. The modified J-Port antenna was operated at 70 and 78 MHz and at power levels up to 3.0 MW without significant RF-plasma edge interaction at the antenna corners in H-mode plasmas. From camera data, damage was expected on the BN tile fasteners and this damage was found to be generic to all antennas where a BN-metal interface was exposed to the plasma. This result suggests further modifications to be implemented before the FY2002 run campaign, based upon the empirical observations of limiting the E||B field to <15 kV/cm and removing the plasma facing BN-metal interfaces. The overall heating efficiency of the J-Port antenna is similar to that of the D and E antennas. A phase scan showed that the nominal [0,pi,0,pi] was the most effective heating phase and had little or no negative edge interaction. An outer gap scan was also completed and suggested a gap of 1-1.5 cm was better than larger outer gaps. Antenna performance was insensitive to toroidal field from 5-5.6T. The modified J-Port antenna differed from the previous version in the design of the strip line components, front tiles, and back plate. Due to arc damage found after the previous (winter 2000) campaign, the radial strip lines elements were aligned with the magnetic field to eliminate as much as possible E parallel to B arc paths. In addition, the electrode spacing was increased from 1 cm to 1.5 cm. S-parameter measurements of this new configuration indicated that the antenna was not significantly modified with respect to RF electrical characteristics. These measurements confirm our attempt to maintain a 50 Ohm transmission line while eliminating E parallel to B-field arc paths. During the winter 2000 campaign, a strong RF plasma edge interaction limited the injected power to ~2.5 MW into H-mode and the interaction appeared to follow field lines. The BN tiles were aligned and all metal surfaces except the Faraday screen were covered or removed. Measurement of the antenna tile position on all antennas confirmed that the J-Port antenna is at the same radial location as the D and E-Port antennas. In order to interrupt long field lines across the antenna, an insulating septum was installed. The back plane feedthrus were also modified to reduce the E-field parallel to the B-field. In addition, four optical arc monitors, six B-dot probes, and an MKS pressure gauge were installed. These modifications have all contributed to the successful operation and understanding of this antenna. The optical arc monitor signals had recorded large transient signals that correlated with reflected-to-forward power arc detection during some vacuum conditioning shots. In plasmas operation, the optical arc monitor signals did not correlate with most arc detect faults. This result suggests that faults were occurring outside their field-of-view. In addition, the optical monitors detected signal when the D and E-Port antennas were active. Comparisons with the voltage data indicate the induced voltage from D and E-Port antennas are low when the light signal is detected and high when no light is observed. This result suggests that the low induced voltage is sufficient to initiate multipactoring. This low power multipactoring does not impact plasma operation, nor does it affect the subsequent operation of the J-Port antenna. The new B-dot probes indicated that the current in strap 4 was equal in both the top and bottom half of the antenna strap. They also proved to be the most sensitive to arcs in the antenna. The new 200 kHz and 1 MHz digitizers allowed direct monitoring of the arc protection system. During a particular experimental day, the fast data indicated that some arcs survived for 30-80 micro-sec. Up to 100 J could be available to dissipate in these arcs; therefore, we reduced the reflected-to-forward power ratio necessary to generate a trip by 25% for all future experiments. This change successfully limited the arcs to ~15 micro-sec or ~15 J per MW injected. Operations ---------- On Thursday of this week the 65,000 lb top dome that together with the cylinder retains the TF magnets in position during a discharge, was removed from C-Mod. On Friday, the 44,000 lb cylinder was removed, approximately one week ahead of schedule. A picture of this operation may be seen at http://www.psfc.mit.edu/cmod/operations/EngImages/Inspection_2001-2002/Disassembly_Assembly/P0002038.JPG The TF finger joints are now exposed, and we will begin a careful inspection and measurement process early next week. Lower Hybrid MIE Project ------------------------ Work continues on the TPS PC board with a large portion of the work now complete. The TPS front panel wiring design has been completed and released for construction. A vendor has been selected and the PO released for the waveguide switches that will allow the klystrons to be switched between the launcher and the dummy load. The front panels for the transmitter equipment racks have been received and installed. The HV junction box design is nearing completion. Work continues on the PLC programs needed to control the HVPS. The supply will be shipped in the next few days and is expected in Cambridge early next month. The fabrication of the dummy load needed to test the supply continues in our shop. The contractor continues to run the klystron cooling water piping. The run between the power room and cell is now being fabricated. ICRF Systems ------------ We continue to refine the antenna simulation and use it to model changes we plan to make to the J-Port antenna to increase its voltage handling performance. We are also continuing to develop models of the transmitter transmission and coupler systems. On Friday of this week a meeting was held to discuss modifications to the the boron nitride tiles required to suppress plasma interaction with the tile support hardware. We expect to finalize and review these changes by the end of this month. Work continues on the new phase demodulators. Results from the prototype indicates a very good linear response of the demodulator except within a few degree of zero relative phase. We expect a faster ECL XOR chip will improve the response, at which point we will be ready to go into production of the demodulator boards. Work continued on documentation and verification of all fault system electronics including fault indicator, phase balance detector, and voltage limit detector boards and communication links. Inner Divertor -------------- All 12 inconel rear girdle plates are now inhouse and along with the c-plates are being prepared for invessel fitup. We hope to have all inner divertor inconel components installed invessel next week for this pre-installation fitup. Work has also continued on layout and fitups of the divertor probes and preparation of the inner wall magnetics for the divertor installation. Diagnostics ----------- The high resolution ECE system has been moved back to UT for repairs and upgrading. Two of the IF amplifiers are being tested before being returned for repairs. All the other IF amplifier are also being tested to check their gain. Work is starting on a temperature control system and rf shielding is being improved. The source of BES background light has been identified. It has been verified via simulation that a small set of impurity lines will produce the observed interference. The DNB penetration and diagnostic signals (CXRS, MSE, BES) expected from several sets of beam parameters were generated as a basis for improving the diagnostics for the next run campaign.