The new RF photo injector cavity was a successful design project, but now the time has come to integrate the cavity into the CLARA accelerator.
The RF cavity has now has been extensively tested with milli-Watts of power, and fitted with a state-of-the-art cathode exchange vacuum system that allows different cathodes to be tested. The cavity is now on the secondary CLARA accelerator beam line at Daresbury Laboratory and must be conditioned to operate at high power: up to 8 Mega-Watts pulsing 100 times a second. This level of power will produce 100 Mega-Volts/metre electric fields in the cavity. Currently, the cavity cannot sustain fields at this level without electrical arcing taking place, which would damage the cavity surface. The power must be increased gradually, conditioning the surface by removing contaminants as well as nano-metre scale imperfections that “grow" from within the copper cavity surface due to the high electric field.
This conditioning process, when performed on previous, lower field RF cavities, was laborious and time-consuming, involving trained RF staff to manually increase the power, whilst observing the pressure within the cavity. Large electrical arcs cause the normally low vacuum pressure within the cavity to rise, so the RF was increased up until the pressure spiked, and then the operator would wait for the pressure to decrease before resuming ramping up the power. The process takes weeks.
Recent research from CERN and KEK in Japan has shown that for high gradient cavities like this one, what is important is to avoid too many breakdowns during conditioning. A better strategy is, therefore, to detect the breakdowns very quickly and turn off the RF power before more can occur. The fastest way to detect a breakdown is through the RF signals from the cavity itself. If the power reflected from the cavity increases suddenly, it means there is an arc, but because the power is pulsed 100 times per second it is impossible for a human operator to detect this and switch the RF off.
Scientists at Daresbury lab have designed and written a computer program in python that reads in the reflected RF signals and analyses them before the next pulse. The program then turns off the RF immediately, the whole process happening in 0.01 seconds. This prevents the arcs from growing too large and damaging the cavity. The program also automatically ramps the power up, so trained staff are no longer required to spend weeks of time manually conditioning the cavity. Early versions of the program were tested on other CLARA RF structures.
This is all part of a larger drive to automate more of the repetitive processes on the CLARA accelerator. Automation means that tasks are repeatable, user-friendly and that important data is regularly saved.