Injection, compression and confinement of electrons in a magnetic mirror

A Helmholtz coil configuration has been constructed where the magnetic field can be increased to about 10 kGauss in 20 μsec. Electrons are injected from a hot tantalum filament between two plates across which a potential of about 5 keV is applied. The electric field E is perpendicular to the magnetic field B so that the direction of the E × B drift is radial into the magnetic mirror. About 10
¹⁴
electrons were injected and about 10
¹³
electrons were trapped. The initial electron energy was about 5 keV and after compression 500 keV X-rays were observed. The confinement time is very sensitive to vacuum. Confinement times of milliseconds and good compression were observed at vacuum of 5.10
^-5
torr or less. Above 5.10
^-5
torr there was no trapping or compression. After a compressed ring of electrons was formed, it was released by a pulse applied to one of the Helmholtz coils that reduced the field. Ejection of the electron ring was observed by X-ray measurements.