Carbon‐film field‐emission cathodes in a compact orbitron‐type ionization vacuum sensor

Abstract

A possibility to using large-area field-emission cathodes as electron sources in ionic vacuum sensors was investigated. In the specially developed electron-optic scheme, electrons are extracted from a box-shaped cathode by the field of two anode strings. For a large part of the cathode, trajectories of emitted electrons are infinite (in theory), thus high efficiency of gas ionization and overall gauge sensitivity can be achieved. Experimental testing of a sensor prototype has demonstrated general viability of the proposed principle. For a residual gas pressure of about 10 −5 Torr and a cathode emission current 10 µA, the current of collected ions had value of about 1 nA. PROJECT BACKGROUND AND OBJECTIVEThe principle of operation of the most popular highvacuum sensors involves the measurement of ionic current originating from interaction of residual gas with a flow of electrons. Subclasses of ionic vacuum gauges 1 differ in the type of electron source used. Traditionally, the choice was between thermionic cathodes and forms of self-maintained discharge (magnetron, Penning, etc.). Rapid progress in technologies of nanostructured carbon materials 2 -4 suggests new promising solutions for these devices. Carbon-film distributed-field electron emitters combine advantageous features, such as low energy consumption, durability to poor vacuum, zero start-up time, simple design, small weight and no need for magnets. These features potentially make them more competitive in comparison with thermionic and discharge electron sources. The principal objective of the work reported here was the development and testing of ion vacuum sensor designs adapted for operation with carbonfilm field-emitters, in such a way as to realize their inherent advantages. GAUGE ELECTRON-OPTICAL SYSTEM DESIGNThe electron-optical system (EOS) of the developed vacuum sensor (Fig. 1) and their acceleration, emission current control and electron flow confinement and direction of the generated ions to the collector. The anode consists of two thin (50-100 µm) tungsten wires fixed through spring holders to ceramic insulators. The cathode has a 6 ð 6 mm cross-section and a length of 50 mm, and consists of two components: the foil emitter with an active carbon layer and the mesh electrode separating the diode from the ion collector volume. For these dimensions, the macroscopic electric field strength needed at the cathode for field electron emission (E D 0.3-3 V/µm) is achieved for applied voltages U D 1-10 kV.Electrode shapes and positions were optimized by digital simulation of the electron motion to achieve maximum trajectory length for particles emitted from different parts of the cathode. As shown in Fig. 2, for approximately 20% of the cathode area in the optimized EOS configuration, the particles are emitted into a 2D trap and can leave the system only in the axial direction under the influence of the comparatively weak axial electric field component. This enables efficient utilization of the electron energy for the purpose of ionizing residual gas m...