Editing Inertial electrostatic confinement (section)

===Penning trap===
[[File:Penning Trap.svg|thumb|300px|Penning trap cross-section. Axis is vertical. Electrons orbit the center under DC electrostatic (blue) and DC magnetic (red) confinement. In this diagram the confined particles are positive; to confine electrons, the electrodes' polarities must be swapped.]]
A [[Penning trap]] uses both an electric and a magnetic field to trap particles, a magnetic field to confine particles radially and a quadrupole electric field to confine the particles axially.<ref>{{cite web |title=Penning Traps |url=https://web2.ph.utexas.edu/~iheds/IntroductionPlasmaPhysics/375%20P%207%20(Penning).pdf |access-date=2014-01-07 |url-status=live |archive-url=https://web.archive.org/web/20130120015434/http://www.ph.utexas.edu/~iheds/IntroductionPlasmaPhysics/375%20P%207%20(Penning).pdf |archive-date=2013-01-20}}</ref>

In a Penning trap fusion reactor, first the magnetic and electric fields are turned on. Then, electrons are emitted into the trap, caught and measured. The electrons form a virtual electrode similar to that in a polywell, described above. These electrons are intended to then attract ions, accelerating them to fusion conditions.<ref name="barnes">{{cite journal |last1=Barnes |first1=D. C. |last2=Nebel |first2=R. A. |last3=Turner |first3=Leaf |title=Production and application of dense Penning trap plasmas |journal=Physics of Fluids B: Plasma Physics |publisher=AIP Publishing |volume=5 |issue=10 |year=1993 |issn=0899-8221 |pages=3651–3660 |bibcode=1993PhFlB...5.3651B |doi=10.1063/1.860837}}</ref>

In the 1990s, researchers at [[LANL]] built a Penning trap to do fusion experiments. Their device (PFX) was a small (millimeters) and low power (one fifth of a [[tesla (unit)|tesla]], less than ten thousand volts) machine.<ref name="ReferenceB"/>