Plasma commissioning in a high power external RF-coil volume-type H^- ion source

Abstract

A high power, high duty cycle, negative hydrogen ion source is in development at ISIS. It will operate in pure volume-production mode and is driven by a 52-turn RF-coil mounted external to the plasma chamber. A solid-state amplifier with a maximum output of 100 kW in 50 Hz, 1 ms pulses delivers RF power to the coil via an impedance-matching network. The amplifier has a relatively wide bandwidth, able to deliver full power from 1.8-4.0 MHz. This flexibility allowed straightforward commissioning of the matching network into an inductively-coupled plasma. Striking of the pulsed plasma is facilitated by a compact microwave ignition gun, requiring only 10 W of power at 2.45 GHz to deliver 1 mA seed pulses of electrons. Experiments have shown that it is vital to encapsulate the RF-coil properly to mitigate high voltage sparking. In addition, the location of the coil relative to the ion source’s permanent magnets has a critical effect on the ease of plasma ignition. The result of commissioning work is that a full duty-factor (50 Hz, 800 ^s) plasma has been achieved at nominal operating power (30 kW) and detailed optical studies have begun.

RF efficiency measurements of inductively-coupled plasma H− ion sources at accelerator facilities

“…The ISIS and SNS external coil sources have been described elsewhere in detail [3,[9][10][11][12][13]. They have similar designs which include an aluminium nitride ceramic plasma chamber, a plastic cooling jacket and a solenoidal RF-coil, as shown in figure 1.…”

First H- beam extracted from the non-caesiated external RF-coil ion source at ISIS

“…The RF ion source design has been described previously in detail [17,18]. To summarise, a watercooled solenoid RF-coil is wound in air around a plain cylindrical-tube ceramic plasma chamber.…”

“…Figure 1 shows the latest design of the ion source. An RF efficiency measurement campaign [19] on the previous design iteration [18] indicated significant coupling between the RF-coil and the nearby stainless steel main housing. This resulted in heating which had a risk of demagnetising the low Curie temperature filter magnets.…”