The essentials of high-voltage breakdown and electrical discharges

High-voltage design & technology

This section covers some of the design principles required to successfully design and build high-voltage systems. It also discusses the commercial high-voltage technology required to bring complete ion source systems together.

Electrodes

Electrode design

Electrodes are used to shape the electric field or as a source/sink for current. Field shaping electrodes are used in accelerating, deflecting and focusing beams. Current source/sink electrodes are used in plasma generation and beam dumps.

Some electrodes such as extraction electrodes can be both field shaping and current sinking.

Electrode design obviously depends on application. For field shaping electrodes, the shape of the electrode is critical. For current source/sink electrodes, the sputtering resistance and current-carrying capability is critical.

Using modern finite element techniques, the electric fields produced by electrodes can be accurately calculated. This allows designs to be tested and optimized before manufacture. Extraction and beam acceleration electrodes can be modeled and simulated beams tracked through them.

Corona shields

Corona shields are a type of field shaping electrode used to shield sharp points that would otherwise suffer from corona. The sharp points are usually unavoidable components like bolts or other fixtures and fittings.

Figure 1 shows two examples of how to shield a point with two different types of corona shield.

Sputtering

Sputtering is where ions bombard electrode surfaces, knocking atoms off the surface. This process causes erosion of the electrode surface. Electrodes in plasmas suffer the most from sputtering.

Sputtering eventually leads to failure of the plasma electrode system. The process also slowly deforms extraction electrodes.

Electrode materials

For cathodes in high-sputter environments, molybdenum or tungsten are often used. For field shaping electrodes, any conductive material can be used, so material choice is application-dependent.

For operation at very high field strengths in vacuum >10 MV m−1 d.c., titanium or its alloys give lower spark rates, especially when used as the cathode electrode material.

Titanium–aluminium–molybdenum alloy and pure titanium produce approximately equal spark rates. Titanium–vanadium or titanium–manganese alloys have the lowest spark rates of all.