Direct conversion

Direct conversion converts a charged particle's kinetic energy into a voltage. It is a scheme for power extraction from nuclear fusion.

A basic direct converter

History and theoretical underpinnings

In the middle of the 1960s direct conversion was proposed as a method for capturing the energy from the exhaust gas in a fusion reactor. This would generate a direct current of electricity. Richard F. Post at the Lawrence Livermore National Laboratory was an early proponent of the idea.^[1] Post reasoned that capturing the energy would require five steps:^[2] (1) Ordering the charged particles into linear beam. (2) Separation of positives and negatives. (3) Separating the ions into groups, by their energy. (4) Gathering these ions as they touch collectors. (5) Using these collectors as the positive side in a circuit.

Post argued that the efficiency was theoretical determined by the number of collectors. Designs in the early 1970s by William Barr and Ralph Moir used metal ribbons at an angle to collect these ions. This was called the venetian-blind design, because the ribbons look like window blinds.^[3]

Experiments

William Barr ran a group which did a series of direct conversion experiments through the late 1970s and early 1980s.^[4] Machines were divided into beam direct converters, which used a beam of positives and negatives as fuel, and plasma direct converters where the input was more random. The former were commonly tested using a beam of 100 keV ions. These machines demonstrated energy capture at a peak efficiency of 65 percent and a minimum efficiency of 50 percent.^[5]

The plasma direct converter was tested on the Tandem Mirror Experiment ("TMX"), an operating magnetic mirror fusion reactor. In the experiment, the plasma moved along diverging field lines, spreading it out and converting it into a forward moving beam with a Debye length of a few centimeters.^[6] Suppressor grids then reflect the electrons, and collector anodes recovered the ion energy by slowing them down and collecting them at high-potential plates. This machine demonstrated a energy capture efficiency of 48 percent.^[7]

Technical challenges

Marshall Rosenbluth argued that keeping the plasma's neutral charge over the very short Debye length distance will be very challenging in practice, though he said that this problem does not occur in every version of this technology.^[8]

Other examples

In the late nineties research was undertaken by Sandia National Laboratories, Los Alamos National Laboratory, The University of Florida, Texas A&M University and General Atomics to use direct conversion to extract energy from fission reactions. Essentially, attempting to extract energy from the linear motion of charged particles coming off a fission reaction.^[9]

References