Editing Fusor (section)

==Fusion in fusors==

=== Basic fusion ===

[[File:Fusion rxnrate.svg|thumb|upright=1.35|The cross-sections of different fusion reactions]]

[[Nuclear fusion]] refers to reactions in which lighter [[atomic nucleus|nuclei]] are combined to become heavier nuclei. This process changes [[Mass–energy equivalence|mass into energy]] which in turn may be captured to provide [[fusion power]]. Many types of atoms can be fused. The easiest to fuse are [[deuterium]] and [[tritium]]. For fusion to occur the ions must be at a temperature of at least 4&nbsp;keV ([[kiloelectronvolt]]s), or about 45&nbsp;million [[kelvin]]s. The second easiest reaction is fusing [[deuterium]] with itself. Because this gas is cheaper, it is the fuel commonly used by amateurs. The ease of doing a fusion reaction is measured by its [[cross section (physics)|cross section]].<ref>John Lindl, "Development of the indirect drive approach to inertial confinement fusion and the target physics basis for ignition and gain", Physics of Plasma, 1995.</ref>

=== Net power ===

At such conditions, the atoms are ionized and make a [[plasma (physics)|plasma]]. The energy generated by fusion, inside a hot plasma cloud can be found with the following equation.<ref name = "Lawson">John Lawson, "Some Criteria for a Power producing thermonuclear reactor", Atomic Energy Research Establishment, Hanvell, Berks, 2 November 1956.</ref>

: <math>P_\text{fusion} = n_A n_B \langle \sigma v_{A,B} \rangle E_\text{fusion},</math>
where
: <math>P_\text{fusion}</math> is the fusion power density (energy per time per volume),
: ''n'' is the number density of species A or B (particles per volume),
: <math>\langle \sigma v_{A,B} \rangle</math> is the product of the collision cross-section ''σ'' (which depends on the relative velocity) and the relative velocity ''v'' of the two species, averaged over all the particle velocities in the system,
: <math>E_\text{fusion}</math> is the energy released by a single fusion reaction.

This equation shows that energy varies with the temperature, density, speed of collision, and fuel used. To reach net power, fusion reactions have to occur fast enough to make up for energy losses. Any power plant using fusion will hold in this hot cloud. Plasma clouds lose energy through [[Thermal conduction|conduction]] and [[radiation]].<ref name = "Lawson"/>  Conduction is when [[ion]]s, [[electron]]s or [[neutral particle|neutrals]] touch a surface and leak out. Energy is lost with the particle. Radiation is when energy leaves the cloud as light. Radiation increases as the temperature rises. To get net power from fusion it's necessary to overcome these losses. This leads to an equation for power output.

: <math>P_\text{out} = \eta_\text{capture} (P_\text{fusion} - P_\text{conduction} - P_\text{radiation}).</math>
where:
: ''η'' is the efficiency,
: <math>P_\text{conduction}</math> is the power of conduction losses as energy-laden mass leaves,
: <math>P_\text{radiation}</math> is the power of radiation losses as energy leaves as light,
: <math>P_\text{out}</math> is the net power from fusion.

John Lawson used this equation to estimate some conditions for net power<ref name = "Lawson"/> based on a [[Maxwell–Boltzmann distribution|Maxwellian]] cloud.<ref name = "Lawson"/>  This became the [[Lawson criterion]]. Fusors typically suffer from [[Thermal conduction|conduction]] losses due to the wire cage being in the path of the recirculating plasma.

=== In fusors ===
In the original fusor design, several small [[particle accelerator]]s, essentially TV tubes with the ends removed, inject ions at a relatively low voltage into a [[vacuum]] chamber. In the Hirsch version of the fusor, the ions are produced by ionizing a dilute gas in the chamber. In either version there are two concentric spherical [[electrode]]s, the inner one being charged negatively with respect to the outer one (to about 80 kV). Once the ions enter the region between the electrodes, they are accelerated towards the center.

In the fusor, the ions are accelerated to several keV by the electrodes, so heating as such is not necessary (as long as the ions fuse before losing their energy by any process). Whereas 45 megakelvins is a very high temperature by any standard, the corresponding voltage is only 4 kV, a level commonly found in such devices as [[neon sign]]s and CRT televisions. To the extent that the ions remain at their initial energy, the energy can be tuned to take advantage of the peak of the reaction [[cross section (physics)|cross section]] or to avoid disadvantageous (for example neutron-producing) reactions that might occur at higher energies.

Various attempts have been made at increasing deuterium ionization rate, including heaters within "ion-guns", (similar to the "electron gun" which forms the basis for old-style television display tubes), as well as [[magnetron]] type devices, (which are the power sources for microwave ovens), which can enhance ion formation using high-voltage electromagnetic fields.  Any method which increases ion density (within limits which preserve ion mean-free path), or ion energy, can be expected to enhance the fusion yield, typically measured in the number of neutrons produced per second.
 
The ease with which the ion energy can be increased appears to be particularly useful when [[aneutronic fusion|"high temperature" fusion reactions]] are considered, such as [[proton-boron fusion]], which has plentiful fuel, requires no radioactive [[tritium]], and produces no neutrons in the primary reaction.