Managed by UT-Battelle for the Department of Energy The Use of a 28 GHz Gyrotron for EBW Startup Experiments on MAST John Caughman,Tim Bigelow, Steffi Diem, Martin Peng, and Dave Rasmussen Oak Ridge National Laboratory Vladimir Shevchenko, Julian Hawes, and Brian Lloyd UKAEA, Culham Science Centre US-Japan RF Theory Workshop March 8, 2010 Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Dept. of Energy under contract DE-AC05- 00OR22725

2Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Agenda  Motivation and approach for studying startup physics on MAST  Previous EBW startup results –Short pulse 28 GHz gyrotron: 100 kW for 90 ms –Up to 33 kA plasma current without solenoid flux –55 kA plasma current with limited solenoid assist  High power ORNL gyrotron on loan to MAST –200 kW cw gyrotron tube at 28 GHz –Capable of 300 kW for 0.5 second pulses –Recently installed and commissioned on MAST  Initial results with upgraded gyrotron –160 kW, 200 ms pulses into calorimeter dummy load –100 kW, 200 ms pulses into MAST –Up to 24 kA plasma current without solenoid flux –Limited by waveguide/window box arcing and machine impurities  Summary and Future Work

3Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 EBW Assisted Startup on MAST  One of the objects of the MAST program is to explore the potential of the ST concept as a basis for a fusion Component Test Facility  Startup techniques that use little, if any, solenoid flux are required  Electron Bernstein wave (EBWs) may be used to assist in plasma startup because EBW absorption and current drive efficiency is high  The EBW startup technique demonstrated on MAST creates a plasma by ECH pre-ionization at the fundamental resonance and then excites EBWs via a mode conversion process  Initial results look promising: need to determine power scaling

4Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Previous Capability  28 GHz 100 kW, 90 ms pulsed gyrotron  Long 40 mm waveguide run with several radius bends  Mode converting launcher –Outside O-mode –Polarization rotating reflector on center stack reflects X mode

5Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 EBW Startup on MAST  An O-mode beam is converted to X-mode from a grooved tile on the central rod  Ray tracing has been used to model the EBW CD initiation

6Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 EBW hardware on MAST  Reflector on central rod and mirrors to guide the 28 GHz beam

7Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Previous experimental results –Shevchenko & Saveliev reported on EBW assisted plasma startup on MAST –28 GHz 100 kW, 90 ms power –Achieved 33 kA plasma current with no solenoid flux and 55 kA with limited solenoid flux –Optimized vertical plasma shift to suppress negative pressure driven currents

8Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 EBW assisted non-solenoid startup  Formation of closed flux surfaces and plasma current of up to 33 kA without solenoid flux  Vertical shift of plasma to change proportion of co/counter EBW CD

9Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 EBW startup with limited solenoid assist  Plasma current of up to 55 kA  Electron temperature of ~700 eV

10Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Next step: install higher power 28 GHz CW gyrotrons  Several 28 GHz 200 kW CW gyrotrons were used on EBT, RFTF, ATF and other applications at ORNL in past years  Vacuum conditions are acceptable but need some re- conditioning  The short pulse (~0.5 sec) power capability is estimated to be kW

11Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 CW gyrotron testing at ORNL  CW testing of several gyrotrons has been performed on a low power test stand –-45 kV, 1 A beam –Typically 10 kW cw output –Can run for hours to restore vacuum conditions  High power test stand will be ready soon for full power long pulse testing  One tube was conditioned at 8 kW for 6 hours, high-potted at 80 kV, and then shipped to MAST in May/June of 2009

12Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 MAST 28 GHz Installation  Gyrotron was installed at MAST in July, 2009  Checkout of improved high-voltage controls through early September –New magnet power supplies –All controls are isolated by analog fiber optic links  Plasma operations started in mid-September

13Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Gyrotron commissioning at MAST  The gyrotron was operated into a calorimeter dummy load for ~2 weeks  Short repetitive pulses at reduced pulse length and beam current  Arcing in the gun area was a problem if parameters were pushed too fast  Eventually, fairly reliable operation at ~160 kW for 200 ms was achieved and plasma experiments commenced  International collaborators included Gary Taylor (PPPL), Yuichi Takase (U. Tokyo), and Hitoshi Tanaka (Kyoto U.)

14Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 EBW startup with ORNL gyrotron  RF pulses of up to 200 ms from 100 to 160 kW were directed to the MAST vessel in September, 2009  Closed flux surfaces were observed and plasma currents of up to 24 kA were achieved  A new scenario was developed at lower vertical and radial fields  Plasma currents were generally smaller than in previous experiments –Arcing in waveguide caused numerous power dropouts –Electron temperatures were lower –Poor vacuum vessel conditioning and impurity generation is suspected

15Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Plasma images show formation of closed flux surfaces  EC resonance and banana orbits of fast electrons are observed early in the pulse  Closed flux surfaces are observed later in the pulse

16Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Recent progress in arc-free operation  A portable dummy load has been used in the waveguide transmission system to try to isolate the location of the arcing  Arc-free conditions have been observed through the TE01 waveguide and the TE11-HE11 converter ( kW for 200 ms)  Light emission (arcing) has been observed in the final mirror optics box 180 kW, 105 ms 160 kW, 200 ms

17Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Near-term plans  Concentrate on solving the arcing problems in the microwave transmission system –Light emission (measured with a fiber optic) in the final mirror optics box indicates arcing is taking place inside the box –Need to examine the box assembly to locate and fix problem –Note: light emission is seen at lower power levels (100 kW) and shorter pulses (10 ms), but it is not enough to cause waveguide breakdown  Raise gyrotron power to highest achievable with reasonable reliability  Operate during more pristine MAST vacuum conditions  Investigate long-pulse regimes where current and density profiles stabilize

18Managed by UT-Battelle for the Department of Energy Caughman, November 18, 2008 Summary  An EBW assisted startup technique has been demonstrated on MAST using a 28 GHz gyrotron –Limited to 100 kW at 90 ms –Achieved 33 kA with no solenoid and 55 kA with limited solenoid assist –Closed flux surfaces have been observed  A 200 kW cw gyrotron from ORNL has been commissioned on MAST –Initially conditioned up to 160 kW for 200 ms into a dummy load –Expected to eventually operate at kW for up to 500 ms  Initial experiments with the ORNL tube on MAST have taken place –Power levels of up to 160 kW directed to the MAST vessel –Achieved up to 24 kA of plasma current –Currently limited by arcing in transmission system and vacuum vessel conditions  Near term work will concentrate on solving arcing issues and further conditioning to increase power  Possible upgrades to the experiment include additional 28 GHz gyrotrons and/or a higher power 18 GHz gyrotron