Magnetized Target Fusion Experiments at LANL


 

 

 

 

 

 

 

 

 

 

 

 

 

 

We operated the Field Reversed Configuration (FRC ) plasma injector at LANL, named FRX-L, for FRC physics and diagnostics development. The FRC plasma machine at the Air Force Research Laboratory in Albuquerqe, named FRCHX (Field Reversed Compression and Heating experiment), works in tandem with the Shiva Star Pulsed Power Facility, to demonstrate actual magneto inertial fusion implosions of FRC plasmas. In June 2012, FRX-L was shutdown, to be converted to the Merging Shock Experiment (MSX). Magnetized target fusion work continued at AFRL on FRCHX for another 18 months, until ~ Nov. 2013. Institutions working on the FRCHX collaboration include AFRL, LANL, UNM, UNR, SAIC, NumerEx LLC, and Varitech Services.

First plasma operation on FRX-L was achieved in March 2001.

As of Jan 2006, high density FRC's operating at 3 Tesla magnetic field strength, have temperature and densities suitable for our targeted parameters, while the lifetime is about a factor of 2 lower than eventually desired. There are four different electrical pulsed power systems (bias bank, pre-ionization bank, main theta-coil bank, and main crowbar circuit). We tried passive mirror coils (that extend the theta coil length by 14 cm, and reduce the coil diameter by 6 mm), but they failed due to error fields introduced near the theta pinch.

Below: 8-chord He-Ne interferometer, axial view through 10-cm diameter quartz chamber, side view with top-1/2 of theta coils removed, showing 4 flux loops

In Nov. 2006, we took the experiment down for a major upgrade. This construction will be finished by Jan. 2008, and the purpose of the upgrade is to enable translation and trapping of the FRC in a liner region. Conical theta coils are now used. The bias bank was quadrupled in energy, a fast cusp bank and coil were added, and a translation and mirror bank and coils built. The control systems were completely rebuilt, due to more things being controlled, and the vacuum control system was upgraded and hardened. On Nov. 8, 2007, vacuum in the new vessel was pulled, and checked at 3x10-8 Torr base pressure.

The photo below shows a neon glow discharge in the machine. The trapping section is to the right.

The new FRX-L vessel, with top half of conical theta coils removed, Nov. 8, 2007.*

 


MTF Diagnostic Issues

Consider a beta ~1 plasma, at 5 Megagauss magnetic field, ~10^19cm^-3 density, ~10 keV temperature, extremely limited side-on access,  with ~1000 Gigawatts of auxiliary heating (adiabatic compression in ~ 1 microsecond)! .....oh, and the first ~1 meter diameter around the device is vaporized after each shot. In short, that is the diagnostic challenge.

GENERAL DESCRIPTION

Controlled thermonuclear fusion remains one of today’s outstanding challenges in fundamental science and applied technology. An approach to creating fusion conditions in the laboratory which has the potential for much lower cost than traditional approaches is magnetized target fusion (MTF), in which a magnetized, wall-confined plasma is compressed by a magnetically driven imploding liner to fusion conditions. If a target plasma is created with sufficient initial temperature, density, and magnetization (which provides thermal insulation), it may be compressed to thermonuclear burn conditions with relatively low-velocity (e.g., 1 cm/m sec) magnetically driven liner implosions that have already been achieved using existing electrical pulsed power drivers. Plasma thermal, particle, and radiation transport must be small enough during the formation and implosion process to allow near-adiabatic heating. Liner and plasma stability during implosion are also critical issues.

A relatively unexplored approach to fusion which has emerged in the last few years is known as Magnetized Target Fusion (MTF). MTF is intermediate between magnetic confinement and inertial confinement fusion (ICF) in time and density scales. In contrast to direct, hydrodynamic compression of initially ambient-temperature fuel (e.g., ICF), MTF involves two steps: (1) formation of a warm (e.g., 100 eV), magnetized (e.g., 100 kG), wall-confined "target" plasma prior to implosion; (2) subsequent quasi-adiabatic compression by an imploding pusher, such as a magnetically driven imploding liner. In many ways, MTF can be considered a marriage between the traditional magnetic and inertial confinement approaches, which potentially eliminates some of the pitfalls of either. In particular, MTF requires simpler, smaller, and considerably less expensive systems than either magnetic confinement or inertial confinement ("laser") fusion. The instabilities which plague traditional approaches to fusion are potentially mitigated in MTF due to wall confinement, shockless acceleration and relatively low velocity (e.g 1 cm/m sec) of the pusher, and low required convergence ratios (e.g., 10:1). Similar to inertial confinement fusion (ICF), MTF relies on an implosion to compress a DT fuel to ignition conditions. Yet, also similar to magnetic fusion energy (MFE), MTF relies on a magnetic field to reduce the thermal diffusion of energy to the walls of a chamber. The marriage of these two aspects has several interesting advantages:

Requirement/Feature MFE MTF ICF
Starting density 1014 cm-3 1017cm-3 1021cm-3
Starting temperature 20 keV 200 eV cyrogenic
Pulsed? 1000 seconds or longer Yes, a few microseconds Yes, a few nanoseconds
Driver characteristics >150 MW, 25 MA, (ITER) 10 MJ, 50 MA pulsed power 1.8 MJ laser (NIF class)
Cost of driver $10 Billion $50 Million $ 1.2 Billion
Fusion Yield ~0.5-1.5 GW ~ 20 MJ ~ 5 MJ
Magnetic field required? Yes, superconducting yes no
Plasma wall interactions? Yes, wall erosion is a problem Mix of metal and plasma is bad Rayleigh-Taylor limits convergence
Plasma Beta < 1 ~1 to >> 1 (in some scenarios, the plasma leans on wall) ----
By using a magnetic field to reduce the energy losses to the wall, the time-scale for "inertial fusion" can be relaxed, so that more readily available electrical pulsed power technology (rather than an expensive, energy- inefficient laser) can be used as a driver. The electrical pulsed power is used to implode a metal liner (can) around the pre-formed target plasma. This reduces the convergence ratio requirement to only 10-20x for MTF, and such ratios have already been demonstrated in fast liner hydrodynamic experiments.
  For MTF, the issues are: formation of a long-lived (~ 10 microsecond) target plasma, compatibility of the formation scheme with an implosion geometry, symmetry of the implosion, prevention of metal-plasma "mix", and actual plasma transport losses to the nearby moving-metal-wall. Diagnosing the small, imploding plasma will also be extraordinarily difficult. Initial tests are proposed on a non-imploded target plasma, to insure proper understanding and reliability (of generation) of the pre-formed plasma conditions.

 Los Alamos, which has long been a leader in the study of fusion physics for nuclear weapons, inertial confinement fusion, and magnetic fusion, has a unique combination of resources to be the world leader in magnetized target fusion. Theoretical work at Los Alamos has stimulated worldwide interest in MTF. Along with its theoretical and computational modeling capabilities and high energy diagnostics expertise, Los Alamos has existing pulsed power facilities which are well suited to this research. MTF involves complex liner-on-plasma experiments which exercise nearly all the theoretical and experimental skills necessary for preservation of nuclear weapons design and testing capabilities, in a scientifically exciting and challenging endeavor.

The FRX-L plasma is formed in a 10.5 cm diameter quartz tube, with limited side-on views (through narrow slots in the theta pinch coils), and clear end views. Later, during actual compression shots, the only realistic view will be from end-on, once the plasma is translated into the aluminum metal liner, which is roughly the size of a large tomato juice can. As the compression proceeds, the plasma density will climb to ~10^19 cm-13, the plasma temperature to ~5-10 keV, and the magnetic field to ~5 MegaGauss. This is a pretty exotic plasma, and will require some innovative plasma diagnostics (due both to the conditions and the access constraints). At the end of the burn phase, as the metal continues to collapse to the axis, a jet of hot molten metal will probably squirt axially in both directions out the ends of the experiment, making survivability of end-on diagnostics also questionable.

Present diagnostics include external B-dot probes, excluded flux loops, H-alpha monitors, gated-intensified visible spectroscopy (0.25 m spectrometer with PAR-1256 ISIT array), 3.39 micron laser interferometry, end-on fast visible framing camera (up to ~15 frames at 1 million fps on polaroid film). Future diagnostics will include a 20-Joule ruby laser system for Thomson scattering. All of these apply to the pre-compression target plasma, and will be adapted for the plasma translation tests into a "fake metal liner", where interactions with the metal wall may also be observed through tracer spectroscopy. The diagnostics during the implosion phase will be much more limited, but intrinsically more interesting since this kind of performance/conditions have never been available to be studied before. An example diagnostic that could work, and provide interesting information (n, T, B?) would be far-forward laser scattering, in the microsecond before the end effects close off the view of the plasma. Of course, neutron diagnostics will be key for the burn phase, and don't require optical access. Soft x-ray measurements may also be more interesting than visible ranges, since the Stark/Zeeman/Paschen broadening is less at short wavelengths.

An artist's conception of a Field Reversed Configuration "target plasma", with integrated liner assembly, is shown in the figure below:

The LANL Magnetic Fusion Energy Program Office has a webpage with other MTF details.

A Bibliography of MTF or MTF-related papers is included here.

A Bibliography of FRC (Field Reversed Configuration) papers can be found here.


Last updated: 09/12/13

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wurden@lanl.gov