Magneto-Inertial Fusion...Enlisting Magnetic Fields for Improved Laser-Fusion Implosions

August 2007

Figure 1: MIFEDS device in the diagnostic TIM (ten-inch manipulator) facility.

While most of the research at Laboratory for Laser Energetics (LLE) is devoted to the priorities of the National Ignition Campaign and future direct-drive experiments on the NIF, a small group of scientists at LLE has been looking (since 2006) at alternative, potentially more productive ways to achieve ignition and burn of laser-fusion capsules. One of these alternative concepts involves the use of strong magnetic fields, something normally associated with tokamaks and other devices for magnetic-fusion (MF) research. The main idea is to use such strong fields not as the main confining factor (as is typical for MF) but as an assistive element providing thermal insulation of the forming hot spot during the stagnation phase of an Inertial Confinement Fusion (ICF) implosion. Thermal losses during fuel assembly remain a detrimental factor for successful heating of the ICF target hot spot. A conventional way to combat these factors has been to keep the shell implosion velocity Vi sufficiently high, providing for fast implosions. This is normally accompanied by increased fuel entropy, ultimately reducing the compressibility of the fuel assembly and, therefore, lowering the burn-up fraction (and, as a result, the energy gain), which directly depends on the resultant fuel areal density.

Figure 2: D1-filled shell placed between the MIFEDS coils and imaged with the OMEGA Target Viewing System. The proton backlighter is visible in the lower right. The inset depicts a time-integrated x-ray self-emission image showing the enhanced emission from the compressed hot core.

In the proposed magneto-inertial fusion (MIF) approach, a seed-field generator called MIFEDS (Magneto-Inertial Fusion Electrical Discharge System) has been designed and constructed to provide magnetic pulses with significant magnitude (0.08 to 0.1 MGauss) during an OMEGA implosion. The seed field within the capsule is expected to undergo amplification, which in the ideal case of full flux conservation, Φ=Φ0, (no resistive dissipation of magnetic-field energy through the converging shell) scales as the square of the implosion convergence ratio (R0/Rmin)2.

Initial experimental work has concentrated on the MIFEDS seed-field generator and on some benchmark experiments to help design the main field diagnostic, which, for these high-density plasmas, is based on proton deflectrometry. Significant challenges exist to field such a compact device (Fig 1) that is capable of delivering gigawatt electric pulses to the coils, generating 10 Tesla fields. The geometry, size, and placement of the coils are not trivial due to the small and restricted OMEGA target interaction volume. Obscuration of OMEGA beams and the view of the numerous diagnostics must be avoided. For this reason, Helmholtz-type single-turn coils, mounted at the tip of a short, tapered stripline are used. The whole assembly has sub-1-Ohm impedance at all times. The resulting current pulses in the coils are <400-ns wide with a peak current of 75 to 120 kA and rise time of 150 ns.

Figure 3: Density map of protons imaging the target near peak compression.  The protons passing through the core are slowed down below the detection threshold.

In the initial experiments, 40 OMEGA beams were radially incident on a cylindrical target, while the remaining 20 were used to generate 14.7-MeV probe protons in a separate D3He-filled glass shell implosion. The technique is an extension of the work of Li and collegues1. Figure 2 shows the configuration with the cylindrical target mounted between the MIFEDS coils as imaged with the OMEGA target viewing system (TVS). The inset shows a time-integrated, x-ray self-emission image of the imploded target. The time-integrated emission of keV x rays can be seen from the hot compressed core. The glass microballoon, used as a proton backlighter, is visible in the lower right corner. The darker areas in Fig. 3 are with higher proton density. One can see the deficiency of protons in the area of the compressed core. This is undesirable since these are the particles to be deflected by the compressed fields. Monte Carlo simulations based on the experimental data were used to identify the optimal filter thickness for the next experiment. The goal for those experiments will be to match the CR-39 detector surface with a specific portion of the energy loss versus depth curve (near the Bragg peak) of the particles that traverse the compressed core. As a result, these specific particles will be centered in the limited readout energy band with a maximum signal-to-noise ratio.

1 C. K. Li, F. H. Séguin, J. A. Frenje, J. R. Rygg, R. D. Petrasso, R. P. J. Town, P. A. Amendt, S. P. Hatchett, O. L. Landen, A. J. Mackinnon, P. K. Patel, V. Smalyuk, J. P. Knauer, T. C. Sangster, and C. Stoeckl, Rev. Sci. Instrum. 77, 10E725 (2006).