Alcator C-Mod Weekly Highlights Mar 15, 1999 Last week began a scheduled maintenance interval at Alcator C-Mod. No runs were scheduled. The maintenance period continues this week. Preparations for a quick clean vent, scheduled for this week, continue. We will be installing a detector array designed to measure deuterium Lyman alpha emission with high spatial resolution. It will have a tangential view, slightly below the midplane with 20 channels, spanning 6 cm, with a nominal radial resolution of 3mm. This will help in diagnosing neutral density and ionization profiles near the separatrix. We plan also to verify alignment of the Omegatron, located on a vertical port, with modifications to its support to allow for small rotations about its supporting axis. Additional work includes inspection of invessel components, such as the new RF antenna and limiters, with a boroscope. Physics Analysis: ----------------- New high spatial resolution magnetic fluctuation measurements of Type III ELM precursors have verified that the toroidal and poloidal mode numbers are typically n = 10, m >~ 16 with oscillation frequencies of 160 kHz. The modes are strongly ballooning in the sense that they are visible only on the outboard coils and not on the inboard coils even though the rational surface is closer to the inboard coils than the outboard coils. It is now possible to also compare outboard coils that are beneath molybdenum tiles just like the inboard coils and the ballooning of the ELMs is still present, so the lack of observable oscillations on the inboard coils is not an effect of attenuation at high frequency due to the molybdenum tiles. The tangentially viewing divertor camera system is installed and operating. The new camera system contains two cameras viewing the divertor tangentially, each filtered for a different deuterium Balmer spectral line (i.e., D_alpha and D_gamma). Images from recent shots have been analyzed and compared to the previous campaign. Most inversions of images show features very similar to those from the last campaign, although the spatial resolution from the recent inversions is now approximately 5mm, a factor of about 2 better than the resolution in previous inversions. The typical features are: strong emission on the inner divertor ``nose,'' and strong emission along the outer divertor leg when the outer divertor is detached. However, images recorded during Ohmic L-mode shots imply a plasma in the private flux region of sufficient temperature and density as to dominate the deuterium emission. This observation is confirmed by the divertor Langmuir probes showing a higher plasma density below the inner strikepoint than nearly on it. Analysis of the mechanism generating the private flux plasma is continuing. RF Systems: ----------- J-port ICRF system: FMIT Transmitter #4 has now produced 2.2 MW at 78MHz for 1 sec into a dummy load. The tuning was done with the essential help of Chris Brunkhorst and Gary D'Amico from PPPL. Work began on retuning FMIT#3 to reduce the reflected power levels between the driver and final amplifiers. An ignitron was replaced in FMIT#3's crowbar cabinet. The 78 MHz resonant loops have been installed along with the decoupler loop. The stub dimensions have been determined and the stub is being fabricated. The resonant and decoupler loop dimensions for 60 MHz have also been determined. The decoupling at 60 MHz is more sensitive than 78 MHz and will require more investigation. The first operation into plasma is planned to be at 78MHz. Remote operation of FMIT#3 and #4 was re-established after considerable effort. The remaining critical path items for this week are retuning FMIT#3 and installing the 9" coax in the cell. D- and E-port ICRF system: An inspection of the 9" coax revealed that a number of elbows had poor contact between the anchor bullet finger assembly and center conductor due to oversized teflon supports. This poor contact resulted in arcing at an elbow that was near a high current location. An inspection of the D-port phase shifter revealed that it was in good condition. Several factors could have contributed to the observed arcing in the D-port stub tuner. First the push/pull rod hair pin assembly had been counter bored leaving a sharp edge that would enhance the local electric field. Second the gap between the rod and copper conductor was occupied by a dielectric having a lower index of refraction than the rod, resulting in a locally enhanced electric field. Finally the loss tangent of the rod may have resulted in signficant local heating during vacuum conditioning. The proposed solution is to install a corona ring in the counter bore that should reduce field enhancement due to sharp edges and remove the gap between the rod and conductor. The rep rate of vacuum conditioning will also be reduced. The tuner repair is awaiting the soldering of a new corona ring. The problem with E-port's PLC has been rectified. A more robust design will be installed in the near future to make the system more reliable. Diagnostic Neutral Beam: ----------------------- Progress, as measured by the target schedule, continues to be satisfactory. The beam pulsed high voltage supply has been tested and operated at full performance and is ready for operation. Testing of high voltage crowbar controls has been completed. Cleaning, replacement, and testing of dielectric oil for the arc notcher transformer has been completed. Problems were encountered with the PC board routing software, and the layout of the tube regulator monitor circuit boards has been delayed. Travel and Visitors: -------------------- Chris Brunkhorst and Gary D'Amico from PPPL were onsite last week working on tuning of the ICRF transmitters. Dave Mikkelsen (PPPL) spent the week at MIT becoming familiar with the MDSplus tools for generating and displaying TRANSP runs based on C-Mod data. These will be used as the starting point for comparisons with various transport models. Paul Bonoli attended a workshop at General Atomics last week (March 9-11, 1999) on the Physics Requirements for Advanced Tokamaks. Paul was one of the meeting organizers, along with Ed Synakowski (PPPL) and Alan Turnbull (GA), and gave a talk on "Lower Hybrid Current Profile Control Studies in Alcator C-Mod". Miklos Porkolab also attended the "Physics Requirements for Advanced Tokamaks" workshop on March 9-11 at General Atomics, where he made the following presentations: 1. Overview of the C-MOD AT Program 2. ITB formation Experiments in C-Mod: PEP Mode During Current Ramp-Up and ITB Induced by H/L Transitions. He also led the Discussion Session:"What Issues Remain for MHD Stability ?" Miklos also had discussions with Ed Synakowski(PPPL) and Keith Burrell(GA) about the role of sheared electric fields and flows in the termination phase of past PEP experiments with ICRF and pellet injection. A new interpretation of the collapse of the PEP has emerged from these discussions, which may be tested in future experiments on C-Mod and DIII-D. If the interpretation is correct, it may be possible to extend the PEP duration by spinning the plasma in the counter current direction with either beams (DIII-D) or mode converted IBW (C-Mod). Ian Hutchinson participated in the Fusion Facilities Coordinating Committee (FFCC) meeting in San Diego, and also attended some sessions of the AT Workshop.