Redmond Plasma Physics Lab
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Star Thrust Experiment (STX)

K. E. Miller, John Slough, and Alan Hoffman
University of Washington

Abstract submitted to Space Technology & Applications International Forum, (STAIF-98), January 25-29, 1998, Albuquerque, New Mexico.

An updated version of the paper presented at this conference is available in PDF format.
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It is recognized that nuclear fusion could be an ideal source for space power or propulsion due to the high specific energy of its fuel, and the high specific impulse which is inherent in its exhaust products. However, most fusion confinement concepts are unsuited for space power production due to their large size and complexity, and are non-ideal for propulsion due to the use of D-T fuel which releases most of its energy in the form of high energy neutrons. A notable exception to these restraints is provided by the Field Reversed Configuration (FRC) which is a simple elongated current ring confined in a modest field solenoidal magnet, as sketched in the Figure. FRCs lack any significant toroidal field, which results in a compact high b plasma that is suitable for burning advanced aneutronic fuels. (Synchrotron radiation would limit ignition of high temperature aneutronic fuels in the low b environment of most confinement geometries.) The linear geometry and magnetic separatrix are a natural attribute for propulsive applications. The stability of FRCs is uncertain due to the lack of shear, but kinetic effects have been demonstrated to stabilize FRCs up to at least modest sizes in the recently completed LSX experiments [1], and recent theory points the way to maintaining stability at reactor relevant sizes [2].

 

STX Drawing

 

The near ideal reactor attributes of FRCs has led the DOE to fund an extension of the LSX program, TCS (Translation Confinement & Sustainment), designed to reach ever larger sizes for investigating confinement and stability limits. An important aspect of that program is the application of a rotating magnetic field (RMF) to both enlarge the plasma and sustain it in steady state. The RMF is a uniform rotating field that is always transverse to the axis of symmetry. It has been demonstrated to form and sustain small cold FRCs in Australian Rotomak experiments [3], but has never been applied at the power levels needed to overcome initially high plasma energy loss rates. In TCS this start-up problem will be circumvented by using the standard Field Reversed Theta Pinch (FRTP) technique for making large, hot FRCs.

FRTP startup technology is well developed, but is too bulky and heavy for space applications. In recognition of this, NASA is supporting a very high power, but short duration, RMF startup experiment called STX (Star Thrust Experiment). STX will study the RMF formation and sustainment of hot (100s eV) mid-sized FRCs, where the classical skin depth of the RMF is much less then the radius of the FRC. Penetration of the RMF has been demonstrated under such circumstances due to the collisionless Hall effect. In the laboratory frame of reference, the RMF works by pulling the electrons around azimuthally while leaving the ions behind, whereas in the electron frame, the RMF appears at rest due to synchronous rotation. This is accomplished by selecting the RMF frequency and amplitude such that the electrons are magnetized with respect to the rotating field (wRMF<<wce) and the ions are not (wci<wRMF). It is also necessary that the electrons be highly collisionless (nei<<wce) in order to experience synchronous rotation, and thus allow RMF penetration.

The STX vacuum chamber consists of a 40 cm diameter by 3 m long quartz tube. Two high Q (400) tank circuits will produce the required .01T .5MHz RMF at the tens of megawatts level for .2 msec. In addition to an ion Alfven heater and axial discharge, these high power tank circuits will be responsible for ionizing and heating the plasma past the radiation barrier, and thus will have the capability of briefly attaining power outputs in the hundreds of megawatts, levels common to FRTPs. Additional IGBT circuits (solid state) will sustain the RMF for a longer duration at lower power levels. STX is presently under construction. The experimental design, power supply performance and preliminary ionization and heating data, along with FRC RMF theory, will be presented.

 

[1] A. L. Hoffman et al., "The Large S Field Reversed Configuration Experiment", Fusion Technology 23, 185 (1993).

[2] L. C. Steinhauer, A. Ishida, R. Kanno, "Ideal Stability of a Torroidal Confinement System Without a Torroidal Magnetic Field", Phys. Plasmas 1, 1523 (1994).

[3] A. J. Knight, I. R. Jones, "A Quantitative Investigation of Rotating Magnetic Field Current Drive in a Field Reversed Configuration", Plasma Physics and Controlled Fusion 32, 575 (1990).


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