A hot-filament discharge with very low electron temperature

Very low electron temperatures have been obtained in a hot-filament discharge device by having a set of conditions designed to minimize heating of the confined electrons. Heating is reduced by having the energy of primaries from the filaments sufficiently large (80 eV) that they do not become confined after ionizing collisions and that they lose little energy through Coulomb collisions. The primaries create secondary electrons at the wall with several eVs of energy which heat the confined electrons through collisions. This heating is minimized by having a liner for the vacuum system coated with colloidal graphite to reduce the emission of secondary electrons and to create a uniform boundary potential. Argon plasmas are created with plasma potentials typically less than 0.2 V. Secondary electrons from the wall are not confined by the plasma potential and have a very low density of order 105 cm-3. The confined electrons have densities of order 108 cm-3 and electron temperatures as low as 0.031 eV (360 K).