High Density Magnetic Fusion

The regime between conventional magnetic fusion (n ~ 10¹⁴ cm^-3) and inertial fusion (n ~ 10²⁶ cm^-3) offers a vast, comparatively unexplored area for development of alternative approaches to fusion. This regime is characterized by plasma pressures in excess of the strength of materials, so that operation is typically pulsed and confinement may ultimately be limited by the inertia of a tamper. At densities below the inertial fusion limit, a magnetic field is required to provide heat insulation and, in some cases, play a role in force balance. Given the wide parameter space, this is the primary remaining area with enough "wiggle room" to allow truly radical inventions in thermonuclear fusion. In particular, it offers intriguing possibilities for small, cheap breakeven/ignition experiments, and the potential for a step change in our concept of the fusion reactor product.

There are several advantages to fusion at high density. The high plasma density offers the potential of high power density and a compact (and likely cheaper) energy source. Confinement requirements are typically relaxed in comparison to standard magnetic fusion, i.e., Bohm confinement is often adequate. The techniques tend to be compatible with liquid walls as has been suggested for inertial fusion; certainly the prospects for employment of liquid walls are more credible that for conventional, low density magnetic confinement reactors. Therefore, reactors with lifetime components and very low activation may be possible. Drive power -- although not necessarily drive energy -- is usually somewhat less than that required for inertial fusion which may permit lower cost drivers (e.g. capacitor banks cost <=1$/J whereas high power lasers are ~$100/J).

Given these concepts and potential advantages, one may ask why concepts in high density magnetic fusion have not been pursued more vigorously. The answer lies in difficulties encountered in early attempts: magnetohydrodynamic (MHD) instability and the interaction of the plasma with the wall. The static equilibrium Z-pinch was found to disrupt, in some regimes, due to MHD instability. Liner fusion concepts were prone to mixing of wall material with the fusion fuel that leads to excessive radiative losses.

Since the early attempts, a number of ideas have been advanced to overcome these difficulties. High density magnetic fusion concepts which are either presently under study or probably deserving of further research include:

* Magnetic fast ignition

* Magnetized target fusion

* High yield systems (batch burn, propagating burn, various ignitors)

* Conventional, static equilibrium Z-pinch

* Staged Z-pinch

* Assisted pinches (notch, laser....)

* Continuous flow-stabilized pinch

* Liner-confined, magnetically insulated concepts

Note that these include operation of pinches in non-MHD regimes, addition of sheared axial flow to suppress MHD modes, and more complex field topologies such as the compact torus with improved stability properties. Dynamic pinch concepts that operate on the MHD time scale have also been suggested such as the staged pinch, ignition of burn waves with the axisymmetric "sausage" instability, and conversion of pinch kinetic energy to soft x-rays for indirect drive of inertial fusion capsules. For liner concepts, the Russian MAGO plasma formation device has come to light and there has been recent work on small, low energy liners. Improved field topologies may also reduce liner-fuel mixing. Most of these ideas have received only a modest level of effort so the likelihood of success remains unknown.

Two exciting developments have arisen during the past few years in the inertial fusion and weapons physics communities, paving the way for a renaissance in high density fusion research. First is the advent of 2D (and now 3D) radiation MHD codes capable of integrated modeling of these complex, dynamical systems. These codes are being successfully applied to modeling of imploding pinches for radiation production. The codes give us the ability to design and assess the prospects for success of a given concept, including much of the relevant physics. The second is the development of high resolution time, space and spectrally-resolved diagnostics that provide a far more detailed description of plasma properties and constituents than in early experiments. The availability of good diagnostics is of particular importance for understanding and controlling the wall-plasma interaction. Existing experimental facilities capable of testing critical physics issues for high density fusion can be found at the national laboratories and universities.

New ideas continue to be suggested. For instance, we have examined a concept combining the magnetic "sausage" ignition mentioned above with laser-driven inertial fusion implosions. The result is a predicted significant increase in gain when the process is modeled with a 2D radiation MHD code. The high density plasma community is now poised to explore both new ideas and old ideas that have languished for lack of the tools to assess their potential. All that is needed is the allocation of sufficient resources to do so. These required resources are modest in relation to the mainline program.