Field emission at 10Vcm−1 with surface emission cathodes on negative-electron-affinity insulators

Abstract

Surface emission cathodes reported here consist of two electrodes separated by ∼10μm on a negative-electron-affinity glass, Cs2Si4O9. The electrodes consist of a W film suspended over the insulator by a gap of 0–70 nm. When electron emission is initiated with a bias of 0–300 V, between the electrodes, the cathodes continue to emit after the bias is removed and for anode voltages as low as 20 V, electric fields <10Vcm−1. The emission is modeled by the electrons tunneling from the electrode onto the glass surface and from there they are emitted into vacuum. Emission without bias is the result of positive charge in the insulator, which replaces the need for a bias voltage.

Initial Plasma Testing of the Ion Proportional Surface Emission Cathode

“…To understand the cathode physics between the electrode and the glass surface, its properties are summarized; emission requires undercut electrodes and a Cs-glass layer, emission increases with substrate bias, 0 V bias emission occurs at exceptionally low anode fields, but saturates at higher fields. Geis 6 argues that Fowler-Nordheim field emission theory cannot explain these results, and offers an alternative explanation in which electrons tunnel from the cathode onto the glass surface and from there into vacuum. The undercut electrode catalyzes the tunneling of electrons out of the metal onto the glass.…”

“…A gate to cathode bias voltage of about 200 V is typically needed to initiate emission, which will continue even if the gate is allowed to float, as long as few 10's of Volts of anode to cathode potential is maintained. When the gate and anode currents are both monitored, the ratio of anode to gate current is called the gain, and gains greater than 10 3 have been observed 6 . To understand the cathode physics between the electrode and the glass surface, its properties are summarized; emission requires undercut electrodes and a Cs-glass layer, emission increases with substrate bias, 0 V bias emission occurs at exceptionally low anode fields, but saturates at higher fields.…”

“…al. 6 Surface emission cathodes, 7 ' 8 ' 910 first reported by Dittmer in 1972 7 , consist of two electrodes placed on an insulating substrate. When sufficient bias voltage is placed across these electrodes, electrons are emitted into vacuum.…”

“…By replacing the Pyrex glass used by Dittmer with a negative-electron-affinity (NEA) glass, Cs 2 Si 4 0 9 ," and using a new electrode geometry, seen in Figure 2 to float on the surface of the insulator without being absorbed into the bulk. 6 A typical SEC configuration involves two symmetric inter-digitated planar comb-like arrays of fingers perhaps 10 microns wide with a few micros separation between the alternating array fingers. One array functions as a gate and is biased positive with respect to the cathode array which is in turn biased negative with respect to an anode element held above the assembly.…”

“…Experiments were conducted in the Jumbo space simulation chamber at Hanscom AFB. The chamber has an ultimate pressure in the low 10" 7 Torr range, with the gas flow from the cathodes the experiments were conducted at high 10" 6 Torr. The chamber can use both a turbomolecular pump and a cryopump to achieve high-vacuum conditions, with the turbopump utilized in these experiments.…”