# Anomalous Beam Transport through Gabor (Plasma) Lens Prototype
## 2 The Gabor Lens  
A schematic of the prototype Gabor lens is shown in Figure 1. The total length of the lens, from end flange to end flange, was 540 mm. The central anode was formed of a copper cylinder with an inner diameter of 85.7 mm and a length of 444 mm. The copper cylinder was 1.6 mm thick and had four rows of four 10 mm diameter holes forming lines along the axis of the cylinder spaced by 90\({}^{\circ}\) so that the volume inside the anode would be evacuated efficiently. Two ceramic isolating spacers were used to maintain the position of the central high-voltage electrode and to electrically isolate it from the vacuum tube. A 15 mm copper high-voltage (HV) connector was soldered to the central electrode to provide a socket for the high-voltage feed-through designed for voltages up to 60-65 kV.  
The two end electrodes were formed of two copper cylinders with a length of 34 mm, an inner diameter of 66.7 mm, and a wall thickness of 1.6 mm. The ends of the cylinders were manufactured with rounded edges to reduce the likelihood of sparking, with a minimum gap of 16.8 mm to the high-voltage anode. The end electrodes were press fitted into the mild steed end flanges. The outer tube and end flanges were connected to ground.  
The configuration of the pancake coils is shown in Figure 1. The input and output tails of the coils exited through the 50 mm gap in the outer tube of the lens. A water cooled copper conductor was used with a square cross-sectional area of 10.87 mm\({}^{2}\). The base configuration of the coil included four windings. The number of windings was locally increased to seven at specific positions to generate a more uniform magnetic field. A maximum magnetic field of approximately 55 mT was achieved at 45 A.  
A power supply (Glassman LP 60-46) was used to regulate the current that flows through the coils. Typical values for the current were in the range of 14 A to 30 A. The volt  
Figure 1: Internal structure of the IC Gabor lens viewed in longitudinal cross-section. The main components are: 1-central anode, 2-end electrodes, 3-end flanges, 4-vacuum tube, 5-pancake coils, 6-outer tube, 7-high-voltage feed-through.  
age for the central electrode was provided by a high-voltage supply of the Glassman Series FR type with typical values of between 8 kV and 20 kV.  
The pumping system was comprised of a roughing pump (Edwards 5) and a turbo molecular pump (Leybold Turbovac 151) with a pressure gauge (Leybold Penninvac PTR 90 N) and a pressure-gauge monitor (Leybold Graphix One). The lowest pressure achieved was 3 \(\times\) 10\({}^{-7}\) mbar, with pressures of up to 3 \(\times\) 10\({}^{-5}\) mbar when a non-neutral plasma was established inside the lens. Settings of HV and current that produced a stable plasma could be distinguished clearly from those which gave rise to an unstable plasma at low pressure (\(\sim\)10\({}^{-7}\) mbar). For higher pressures, this distinction was more difficult to observe.  
Plasma in the lens was produced by increasing the high voltage applied to the anode and the current in the magnetic coils. A significant increase in pressure was observed when a stable plasma was first established in the lens. Simulation of the plasma discharge within the lens indicated that a high electron density, \(\sim\)5 \(\times\) 10\({}^{-7}\) cm\({}^{-3}\), was produced.