Enabling Technology on Alcator C-Mod

1. Enabling Technology on Alcator C-Mod Jim Irby MIT-PSFC

2. Outline • ICRF • Antenna development and protection electronics • Fast automatic and load tolerant tuning • Lower Hybrid • Coupler/Launcher • Status • Modelling • First Wall • Development of disruption/heat resistant hardware • Tungsten brush prototype • Cryopump, upper divertor, and lower outer divertor modifications • High pressure puff • Technology Collaborations

3. ICRF 4-Strap Antenna • Development of reliable, efficient antennas is of prime importance to the C-Mod Program and fusion research generally • Large range of technological issues must be addressed and well documented • Plasma coupling • Real time/passive tuning • Transmission line design • Rf breakdown • Fault protection/detection • Metal/dielectric interfaces • Impurity generation • Mechanical integrity 2-Strap Antennas

4. Fast Tuning System fft transmitter antenna SWR< 1.1 Passive SWR<1.5 • Isolate transmitter from antenna using ferrite tuners/stubs and combine with passive load tolerant components • Expect approx 1 ms response time to changing plasma conditions (4 ms full range tuning) • Tuning is currently a shot-to-shot process, so we expect to greatly reduce number of discharges required to prepare for physics runs • Will reduce manpower requirements on the rf physics staff • Protection systems even more critical since the tuner is capable of matching to low impedance arcs • Improved phase/amplitude/fault detection • Improved diagnostics

5. Rf Breakdown Experimental Program • Rf breakdown in antenna and transmission line structures, and at metal-dielectric interfaces limit the performance of ICRF systems • A program to experimentally parameterize rf breakdown in geometries found in ICRF systems is being pursued • E||B --- we have found 15 kV/cm empirical limit --- why? • E^B • Electric fields from 10 to 30 kV/cm • Model systems in detail using simulation software • Develop understanding of the physics involved

6. New 4-Strap Antenna • Installation of second LH launcher in FY2005 will displace important diagnostics • We will install a new 4-Strap antenna to replace our two 2-Strap antennas (free up one horizontal port) • Total available ICRF source power maintained • The antenna will incorporate the experience we have gained with our 2- and 4-strap antennas

7. Lower Hybrid Project • In collaboration with PPPL, 3 MW of lower hybrid source power at 4.6 GHz is being installed on C-Mod • Twelve 250 kW, CW, klystrons will be used for the first phase of operation; 16 klystrons for 4 MW of source power during second phase • 2nd launcher will be added during phase II • Development of couplers, antennas, vacuum windows, phase and amplitude control systems, and high voltage power supplies and related protection systems is required

8. Launcher Installation on C-Mod • PPPL launcher scans radially • Power splitters built into launcher

9. Antenna • 24 waveguides X 4 modules • Each waveguide is vacuum sealed with Al2O3 ceramic brick windows • Windows are coated and then brazed into the waveguide • Modules are vacuum sealed to the rest of the launcher with a 0.03” gold seal

10. Lower Hybrid Status • All klystrons have been tested • 12 available with spares • Phase II will require repairs and purchase of spare tube • HVPS (50 kV, 208 A) is near the end of the commissioning phase (passed wire tests) • Prototype launcher coupler being tested at PPPL • Procedure for fabricating waveguide windows developed • Phase/Amplitude control system well into development • Klystron control and protection system nearly operational • Expect first full power tests using HVPS in the next few weeks

11. Lower Hybrid 3db LH coupler Commercial software development has reached the point where major LH and ICRF components can be modelled in detail Antenna/Plasma interface? ICRF Antenna Surface Currents

12. First Wall • Design of first wall components capable of withstanding large disruption forces and heat loads is required • Increased triangularity and plasma shape control • System designed for 9 T, 2.5 MA operation, 11 MJ / pulse • Extensive diagnostic set for comparison with code results • Rogowski: eddy/halo • Rangefinder: deflection • Thermocouples

13. Tungsten Brush Divertor Module • Development of first wall components capable of very high heat loads are clearly needed for fusion reactors and will be tested on C-Mod • Tungsten rod PFC with copper heat sink being developed for burning plasma experiments • Prototype has been tested at 20-25 MW/m2 • Collaboration with Sandia National Lab Install vertical column of tiles in C-Mod outer divertor

14. New Cryopump and Divertor • Cryopump • Liquid helium core with LN2 heat shield • Approx 20k l/s pumping speed for D2 • Upper divertor tiles extended to protect pump and enhance upper null operation • Lower Outer Divertor • Simple, easy to align cylindrical shape • Edge effects minimized • Reliable operation during long pulse, high heat load operation High pressure Gas puffing

15. Technology Collaborations We welcome expert participation and collaboration in ongoing C-Mod technology developments: • Design of fast ferrite and passive/load tolerant tuning systems • Fault detection and related diagnostics for both ICRF and lower hybrid systems • Modelling of antenna/plasma interface • Antenna design • RF breakdown experiments/theory • Cryopump design • Divertor design and materials And scoping studies on possible future initiatives: • Compact high-field side fueling • MHD feedback/wall stabilization • Advanced walls and liquid divertors

16. 21 Weeks • Alcator C-Mod has run for 21 weeks (1995) • Fewer engineering systems • More people • We are in the process of increasing personnel levels in several areas in part to aid in increased operation • New rf engineer • New mechanical engineer • New rf physicist • New technicians • New post docs

17. During the machine inspection period we have made many improvements to increase machine reliability • Cryostat seals improved • Vacuum seals now protected from LN2 • Cooling lines replaced and re-routed • Heater/Cooling instrumentation improved • Improvements made to the OH-Coax connections • TF inspection completed successfully • RF diagnostic/fault electronics greatly improved • Data acquisition upgrades are being implemented • Power systems instrumentation upgraded (as part of long pulse operation)

18. New more robust inner divertor installed • 8T/2MA • Larger triangularity • More shape control • Upgrades to all ICRF antennas • Mods to metal/dielectric interfaces • Improved shielding of hardware • Increased spacing at high voltage locations • Extensive list of spare parts for the power systems has been developed

19. Machine Status • Pumpdown 5/20 • First plasma 6/07 • Tokamak Workshop 6/10-6/12 (no operation) • Now conditioning machine for physics operation • Developing double null high triangularity discharges in preparation for FIRE related mini-proposal and to aid in vessel cleanup --- exploits new divertor geometry • New RFX DNB online with much improved diagnostics • Rf systems coming back online • New divertor diagnostics, fueling lines, pressure gauges are being brought into operation