The Pelletron Charging System is the heart of the accelerator system. The Pelletron charging system excels above all others when it comes to durability and stability due largely in party by the unique Pelletron charging chain.
The Pelletron charging chain used in NEC accelerators was developed in the mid 1960s as an improvement over the older Van de Graaff charging belts. These belts suffered from a number of operational difficulties including terminal voltage instability and susceptibility to spark damage. They also generated belt dust, which necessitated frequent cleaning inside the accelerator tank. The chain rapidly proved to be more durable than the old belts, while producing a greater terminal stability than had been possible before. It eliminated the belt dust problem as well. The chain does not limit ultimate terminal potential, and it is in use in electrostatic accelerators up to and above 25 MV.
In addition to being far superior to the belt-driven Van de Graaff, Pelletron charging systems also offer significant advantages over solid-state charging systems. These systems have fragile electronics in the high-voltage column that are prone to problems that are both difficult to diagnose and expensive to repair. Because they are susceptible to spark damage, solid-state systems require a long time to condition up to rated voltage.
Most Pelletrons, as shown in the charging system diagram above, employ “down-charging” as well as “up-charging.” Down-charging works identically to up-charging, except the inductor/suppressor polarities are reversed, and it effectively doubles the charging current capacity of the chain. For the configuration shown here, small, slightly-conductive “pickoff pulleys” provide the voltages for the terminal suppressor/inductor electrodes by drawing a tiny amount of charge from the chain, each pulley thus biasing the opposing electrode. This technique allows down-charging without the use of HV supplies in the terminal.
Depending on the particular design options, the system delivers charging currents of 100 – 200 µA or more per chain to the high voltage terminal. The drive pulleys, typically 30 cm to 60 cm in diameter, and motors are supported on movable platforms that are counterweighted, automatically providing proper chain tension.
There are well over 250 Pelletron charging systems in use in 24 countries as part of electrostatic ion beam accelerator systems. Approximately 30 of those charging systems are replacement charging systems for belt-based charging systems in HVEC Models ESTU, MP, FN, and EN. Please contact NEC if you are interested in replacing a belt-based system with a Pelletron charging system.