Energy confinement and magnetic field generation in the SSPX spheromak Page: 5 of 31
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plasma. Previous work at the CTX spheromak at Los Alamos [6] resulted in temperatures of
>150 eV for driven spheromaks and toroidal currents of >250 kA in discharges up to 2 ms in
duration. Decaying spheromaks on CTX heated to 400eV[7]. The Sustained Spheromak Physics
Experiment [8,9] (SSPX) at Lawrence Livermore National Laboratory, built upon these
achievements and has attained electron temperatures of >500 eV, Itor > 1 MA, and discharges of
9 ms. [10]
We report on magnetic field generation [11] and energy confinement in the SSPX
spheromak. Four operating modes are examined: 1) "extended formation" which we note leads to
higher magnetic fields and record electron temperatures [10], 2) "long decay" which maintains
the spheromak in stable decay, with duration recently extended from 3.5 ms to over 8 ms, 3)
"extended drive" with a flat-top current just slightly above the threshold for building magnetic
field [19] that is compared to NIMROD simulations, and 4) "multi-pulse" or "pulsed-reflux," a
repetitively pulsed mode [12] that demonstrates quasi-steady-state sustainment of the magnetic
field with partial recovery of plasma energy between field-building pulses. These are compared
to "Standard" discharges [11] that feature a shorter higher-current formation pulse followed by a
lower-current flat-top sustainment pulse.
Several more terms need to be defined in addition to those described above. The
efficiency of field build-up and current drive can be described in terms of Flux Amplification
and Current Amplification. Flux Amplification = FA = vpoi/Vgun where Vpoi is the confined
poloidal flux and Vg. is the magnetic flux generated by the inner electrode solenoid and bias
coils (Fig. 1). Current Amplification = CA = Itor/Igun where Itor = toroidal plasma current inside the
mean-field separatrix and Ig. is the discharge current flowing into the gun from the capacitor
banks. Since Wpot and Itor are not directly measured but obtained through equilibrium3 of 29
V.11
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Hudson, B.; McLean, H. S.; Wood, R. D.; Hooper, E. B.; Hill, D. N.; Jayakumar, J. et al. Energy confinement and magnetic field generation in the SSPX spheromak, article, February 11, 2008; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc897283/m1/5/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.