Editing Fusor (section)

== Mechanism ==
===Underlying physics===
Fusion takes place when [[Atomic nucleus|nuclei]] approach to a distance where the [[nuclear force]] can pull them together into a single larger nucleus. Opposing this close approach are the positive charges in the nuclei, which force them apart due to the [[electrostatic force]]. In order to produce fusion events, the nuclei must have initial energy great enough to allow them to overcome this [[Coulomb barrier]]. As the nuclear force is increased with the number of nucleons, protons and neutrons, and the electromagnetic force is increased with the number of protons only, the easiest atoms to fuse are [[isotope]]s of hydrogen, [[deuterium]] with one neutron, and [[tritium]] with two. With hydrogen fuels, about 3 to 10&nbsp;keV is needed to allow the reaction to take place.<ref name=overcome>{{cite web | url = http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/coubar.html |title= Coulomb Barrier for Fusion |website=Hyperphysics}}</ref>

Traditional approaches to [[fusion power]] have generally attempted to heat the fuel to temperatures where the [[Maxwell-Boltzmann distribution]] of their resulting energies is high enough that some of the particles in the long tail have the required energy.<ref name=overcome/> High enough in this case is such that the rate of the fusion reactions produces enough energy to offset energy losses to the environment and thus heat the surrounding fuel to the same temperatures and produce a self-sustaining reaction known as ''ignition''. Calculations show this takes place at about 50&nbsp;million&nbsp;[[kelvin]] (K), although higher numbers on the order of 100&nbsp;million&nbsp;K are desirable in practical machines. Due to the extremely high temperatures, fusion reactions are also referred to as ''thermo''nuclear.

When atoms are heated to temperatures corresponding to thousands of degrees, the electrons become increasingly free of their nucleus. This leads to a gas-like state of matter known as a [[plasma (physics)|plasma]], consisting of free nuclei known as ions, and their former electrons. As a plasma consists of free-moving charges, it can be controlled using magnetic and electrical fields. Fusion devices use this capability to retain the fuel at millions of degrees.

===Fusor concept===
The fusor is part of a broader class of devices that attempts to give the fuel fusion-relevant energies by directly accelerating the ions toward each other. In the case of the fusor, this is accomplished with electrostatic forces.  For every [[volt]] that an ion of ±1 charge is accelerated across it gains 1 [[electronvolt]] in energy. To reach the required ~10&nbsp;keV, a voltage of 10&nbsp;kV is required, applied to both particles. For comparison, the [[electron gun]] in a typical television [[cathode-ray tube]] is on the order of 3 to 6&nbsp;kV, so the complexity of such a device is fairly limited. For a variety of reasons, energies on the order of 15&nbsp;keV are used. This corresponds to the average kinetic energy at a temperature of approximately 174&nbsp;million&nbsp;Kelvin, a typical [[magnetic confinement fusion]] plasma temperature.

The problem with this [[colliding beam fusion]] approach, in general, is that the ions will most likely never hit each other no matter how precisely aimed. Even the most minor misalignment will cause the particles to [[Scattering|scatter]] and thus fail to fuse. It is simple to demonstrate that the scattering chance is many orders of magnitude higher than the fusion rate, meaning that the vast majority of the energy supplied to the ions will go to waste and those fusion reactions that do occur cannot make up for these losses. To be energy positive, a fusion device must recycle these ions back into the fuel mass so that they have thousands or millions of such chances to fuse, and their energy must be retained as much as possible during this period.

The fusor attempts to meet this requirement through the spherical arrangement of its accelerator grid system. Ions that fail to fuse pass through the center of the device and back into the accelerator on the far side, where they are accelerated back into the center again. There is no energy lost in this action, and in theory, assuming infinitely thin grid wires, the ions can circulate forever with no additional energy needed. Even those that scatter will simply take on a new trajectory, exit the grid at some new point, and accelerate back into the center again, providing the circulation that is required for a fusion event to eventually take place.<ref name=MM/>

[[File:Fusor Mechanism.png|thumb|center|upright=2|Basic mechanism of fusion in fusors. (1) The fusor contains two concentric wire cages: the cathode is inside the anode. (2) Positive ions are attracted to the inner cathode, they fall down the voltage drop and gain energy. (3) The ions miss the inner cage and enter the neutral reaction area. (4) The ions may collide in the center and may fuse.<ref name="Tim Thorson 1996">Tim Thorson, "Ion flow and fusion reactivity characterization of a spherically convergent ion focus", Thesis work, December 1996, The University of Wisconsin–Madison.</ref>]]

===Losses===
It is important to consider the actual startup sequence of a fusor to understand the resulting operation. Normally the system is pumped down to a vacuum and then a small amount of gas is placed inside the vacuum chamber. This gas will spread out to fill the volume. When voltage is applied to the electrodes, the atoms between them will experience a field that will cause them to ionize and begin accelerating inward. As the atoms are randomly distributed to begin, the amount of energy they will gain differs; atoms initially near the anode will gain some large portion of the applied voltage, say 15&nbsp;keV. Those initially near the cathode will gain much less energy, possibly far too low to undergo fusion with their counterparts on the far side of the central reaction area.<ref name=MM>{{cite book |first1= George |last1=Miley |first2=S. Krupakar |last2=Murali |title= Inertial Electrostatic Confinement (IEC) Fusion: Fundamentals and Applications |date= 2013 |publisher=Springer |isbn=9781461493389 |url=https://books.google.com/books?id=f-S5BAAAQBAJ}}</ref>

The fuel atoms inside the inner area during the startup period are not ionized. The accelerated ions scatter with these and lose their energy, while ionizing the formerly cold atom. This process, and the scatterings off other ions, causes the ion energies to become randomly distributed and the fuel rapidly takes on a non-thermal distribution. For this reason, the energy needed in a fusor system is higher than one where the fuel is heated by some other method, as some will be "lost" during startup.<ref name=MM/>

Real electrodes are not infinitely thin, and the potential for scattering off the wires or even capture of the ions within the electrodes is a significant issue that causes high [[Electrical resistivity and conductivity|conduction]] losses. These losses can be at least five orders of magnitude higher than the energy released from the fusion reaction, even when the fusor is in star mode, which minimizes these reactions.<ref>J. Hedditch, "Fusion in a Magnetically Shielded grid interial electrostatic fusion device", Physics of Plasmas, 2015.</ref>

There are numerous other loss mechanisms as well. These include charge exchange between high-energy ions and low-energy neutral particles, which causes the ion to capture the electron, become electrically neutral, and then leave the fusor as it is no longer accelerated back into the chamber. This leaves behind a newly ionized atom of lower energy and thus cools the plasma. Scatterings may also increase the energy of an ion which allows it to move past the anode and escape, in this example anything above 15&nbsp;keV.<ref name=MM/>

Additionally, the scatterings of both the ions, and especially impurities left in the chamber, lead to significant [[Bremsstrahlung]], creating [[X-ray]]s that carries energy out of the fuel.<ref name=MM/> This effect grows with particle energy, meaning the problem becomes more pronounced as the system approaches fusion-relevant operating conditions.<ref name=rider>{{cite tech report |url=https://dspace.mit.edu/handle/1721.1/11412 |title= Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium |first=Todd |last=Rider |publisher=MIT |date=1995}}</ref>

As a result of these loss mechanisms, no fusor has ever come close to [[Fusion energy gain factor|break-even energy]] output and it appears it is unable to ever do so.<ref name=MM/><ref name=rider/>

The common sources of the high voltage are [[Switched-mode power supply#Quasi-resonant zero-current/zero-voltage switch|ZVS]] [[Flyback converter|flyback]] [[High voltage|HV]] sources and [[neon-sign transformer]]s. It can also be called an [[Particle accelerator#Electrostatic particle accelerators|electrostatic particle accelerator]].