Possible application of carbon nanotubes to the field emission electron source for portable betatrons

In order to study the feasibility of a carbon nanotube-based electron gun for small and portable betatrons, the characteristics of electron emission from carbon nanotubes were measured at around ≈1×10−4 Pa, a typical pressure in the vacuum chamber of such betatrons. The sample cathodes were made from powders of single-walled nanotube (SWNT) and multi-walled nanotube (MWNT) that were fixed on the stainless steel plate with gold paste. We found that the emission was weaker at the lower vacuum for both MWNT and SWNT. However, when the vacuum level was restored to ≈1×10−4 Pa after being kept at 4.0×10−4 Pa for 15 minutes, the emission-current recovered to some extent but did not fully come back to the original value. The observed current densities from SWNT and MWNT are 550 and 540 μA/mm2, respectively, at the high-voltage we tried: 2 kV for SWNT and 1.1 kV for MWNT across a 300 μm gap. It is likely that the carbon nanotube, under the pulsed operation, has advantages over the thermal cathode for the electron emitters of portable betatrons.