Magnetic Field Generation and Sustainment in the SSPX SpheromakL
Author: Harry Mclean
Requested Type: Consider for Invited
Submitted: 2006-12-18 21:27:32
Co-authors: R.D. Wood, E.B. Hooper, B.I. Cohen, B. Hudson, J. Jayakumar, L.L. LoDestro, J.M. Moller, L.D. Pearlstein, C.A. Romero-Talamás
Lawrence Livermore National Laboratory
PO Box 808
Livermore, CA 94550
Experiments in SSPX include exploring techniques for efficient field buildup, an essential part of advancing the spheromak concept. We have recently demonstrated several scenarios where the maximum edge magnetic field obtained for a given discharge current (B/I) exceeds the limiting value previously observed on SSPX (0.9 T/MA vs. old value of 0.65 T/MA). Toroidal magnetic field (approaching 1 Tesla on the magnetic axis) and toroidal current (up to 0.8 MA) have increased to record values on SSPX. Magnetic energy confined within the separatrix has doubled. Improved efficiency has helped make longer discharges possible, leading to exploration of energy confinement in greater detail, and enabled the development of operating regimes suitable for future neutral beam injection experiments. Expanded capabilities have contributed to studies of methods to increase current amplification, often cited as necessary for a practical spheromak fusion reactor.
Recent progress is due in large part to the commissioning of a new modular capacitor bank. The 1.5 MJ programmable solid-state modular capacitor bank with 30 independent 50 kJ modules provides greater flexibility in controlling the discharge current and its time evolution. This allows us to increase the pulse length (>10 ms), increase peak injector current (>0.6 MA), and apply multiple current pulses to build the magnetic field to higher values. Conditions that build magnetic field (“reflux mode”) usually result in fluctuations large enough to limit energy confinement, but good energy confinement can be maintained when not building magnetic field (“coasting mode”). Repetitive-pulse build-up opens up the possibility of applying periodic reflux pulses to a high-confinement coasting mode to produce time-averaged quasi-steady-state operation with good confinement. This operating mode may still require significant current on open flux lines outside the separatrix for stability so we are also exploring the possibility of decreasing discharge current and gun flux in tandem to maintain stability while reducing edge dissipation (“active bias reduction” or “ABR” mode).
NIMROD modeling of field generation calculates a limiting B/I that agrees well with the limiting value observed in the experiment. Modeling of confinement shows the expected temperature cycling behavior from flux surfaces opening and closing during repetitive pulsing. Additional NIMROD runs are exploring stability and confinement properties during ABR and neutral beam heating.
* Work performed under the auspices of the US DOE by University of California Lawrence Livermore National Laboratory under contract W–7405–ENG–48.
Please place posters in the following order by first author:
Wood, McLean, Hudson, Mezonlin, Romero-Talamas, LoDestro, Hooper