Direct energy conversion

Direct energy conversion (DEC) or simply direct conversion converts a charged particle's kinetic energy into a voltage. It is a scheme for power extraction from nuclear fusion. Direct energy conversion (DEC) or simply direct conversion converts a charged particle's kinetic energy into a voltage. It is a scheme for power extraction from nuclear fusion. In the middle of the 1960s direct energy 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. Post reasoned that capturing the energy would require five steps: (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 theoretically 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. Those metal ribbon-like surfaces are more transparent to ions going forward than to ions going backward. Ions pass through surfaces of successively increasing potential until they turn and start back, along a parabolic trajectory. They then see opaque surfaces and are caught. Thus ions are sorted by energy with high-energy ions being caught on high-potential electrodes. William Barr and Ralph Moir then ran a group which did a series of direct energy conversion experiments through the late 1970s and early 1980s. The first experiments used beams of positives and negatives as fuel, and demonstrated energy capture at a peak efficiency of 65 percent and a minimum efficiency of 50 percent. The following experiments involved a true plasma direct converter that 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. Suppressor grids then reflect the electrons, and collector anodes recovered the ion energy byslowing them down and collecting them at high-potential plates. This machine demonstrated an energy capture efficiency of 48 percent. However, Marshall Rosenbluth argued that keeping the plasma's neutral charge over the very short Debye length distance would be very challenging in practice, though he said that this problem would not occur in every version of this technology. The Venetian Blind converter can operate with 100 to 150 keV D-T plasma, with an efficiency of about 60% under conditions compatible with economics, and an upper technical conversion efficiency up to 70% ignoring economic limitations. A second type of electrostatic converter initially proposed by Post, then developed by Barr and Moir, is the Periodic Electrostatic Focusing concept. Like the Venetian Blind concept, it is also a direct collector, but the collector plates are disposed in many stages along the longitudinal axis of an electrostatic focusing channel. As each ion is decelerated along the channel toward zero energy, the particle becomes 'over-focused' and is deflected sideways from the beam, then collected. The Periodic Electrostatic Focusing converter typically operates with a 600 keV D-T plasma (as low as 400 keV and up to 800 keV) with efficiency of about 60% under conditions compatible with economics, and an upper technical conversion efficiency up to 90% ignoring economic limitations. From the 1960s through the 1970s, methods have been developed to extract electrical energy directly from a hot gas (a plasma) in motion within a channel fitted with electromagnets (producing a transverse magnetic field), and electrodes (connected to load resistors). Charge carriers (free electrons and ions) incoming with the flow are then separated by the Lorentz force and an electric potential difference can be retrieved from pairs of connected electrodes. Shock tubes used as pulsed MHD generators were for example able to produce several megawatts of electricity in channels the size of a beverage can. In addition to converters using electrodes, pure inductive magnetic converters have also been proposed by Lev Artsimovich in 1963, then Alan Frederic Haught and his team from United Aircraft Research Laboratories in 1970, and Ralph Moir in 1977. The magnetic compression-expansion direct energy converter is analogous to the internal combustion engine. As the hot plasma expands against a magnetic field, in a manner similar to hot gases expanding against a piston, part of the energy of the internal plasma is inductively converted to an electromagnetic coil, as an EMF (voltage) in the conductor.