The Gridded Electrostatic Lens

The principle of the device is like an Einzel Lens but with grids placed at each end of the device. This strongly increases the strength of the lens and allows it to focus or defocus the beam (Einzel lenses only ocus the beam).

The Einzel lens and the electric field lines.

The GEL lens and the electric field lines.

The real GEL drawing is shown below (but with poor resolution).

The layout of the real GEL.

The beam would pass from left to right at the level of the diameter marking (60mm).

The dynamics of a GEL are shown below. An almost correct layout of the geometry of a real set of electrodes are shown. The resulting equipotentials are shown in light blue. The ions (which have been accelerated with a potential five times higher than the lens potential) travel from left to right and are marked in red. The units on the x and y axis correspond to 0.5 mm per step. The lens electrodes are typically operated at half the beam energy to provide a very strong focusing, which can counteract the space-charge.

The layout of the real GEL.

The GEL has several critical parts leading to several problems in operation.

The grids are formed of thin wires (typically 100 um in diameter) and this leads to a very high gradient on the surface, which may lead to breakdown. It has been found that 50um tungstun wire coated with gold, start to give breakdowns after a few days operation. Furthermore when the beam strikes the wires electrons are released which may cause a discharge to the main lens electrode. Thinner wires were found to suffer more in this case, but changing to 100um diameter and using CuBe wire at present leads to a more reliable system.

The voltage holding of the system can also be improved by pulsing the electrodes only for a few hundred micro-seconds around the ion beam, but pulsing of the electrodes to 30 kV is not trivial and at present a simpler system is used to switch on DC high voltage supplies at the time of the beam (which is at present only 1 shot per 30 seconds). Finally the system must withstand 1Hz operation, with a very low breakdown rate.

It is also possible that the wires can be erroded by the beam, but models for this do not show a high risk and in practice only one grid has ever broken, which was more likely due to mechanical shock during mounting.