LS2 Report: Accelerated beams in SPS

New solid-state amplifier system for the SPS accelerating cavities
The SPS’s new RF system.
(Image: CERN)

The Super Proton Synchrotron (SPS) is now sending accelerated beams to the North Area (NA), where the physics season has started. As well as being a key element of the LHC’s acceleration chain, the SPS provides beams to the fixed-target experiments located in the North Area, as well as to the AWAKE experiment and the HiRadMat facility.

First acceleration

On 12 April 2021, the first beam was injected from the Proton Synchrotron (PS) into the SPS and, three weeks later, on 4 May, the first acceleration took place in the SPS, using the accelerator’s brand-new radiofrequency (RF) acceleration system. “We first accelerated what we call ‘LHC beams’, which are different from the beams needed for the North Area,” explains Verena Kain, head of SPS operation. “LHC beams are made of up to 288 high-intensity bunches with 25-ns spacing, while the NA beams fill almost the entire circumference of the SPS with many more bunches of lower intensity with a spacing of 5 ns.”

Indeed, because of the production scheme in the SPS injectors, the fixed-target beams are injected into the SPS at a lower energy than the LHC beams. Because of this lower injection energy, the fixed-target beams have to be accelerated through what is called “transition energy”. “The radial beam position needs to be controlled by feedback while accelerating through transition; the beams easily become unstable around transition,” explains Verena Kain. “Adjusting the relevant parameters for crossing transition with the new RF controls took a considerable amount of time until good beam transmission could be achieved.”

A brand-new RF system

The SPS’s new RF system uses solid-state amplifier technology. It is revolutionary, to say the least! It’s the result of five years of research and development in collaboration with the French firm Thales,” explains Éric Montesinos, deputy group leader of SY-RF. The system has been in operation since November 2020 and reached the peak power of 1.6 megawatts – a world first – earlier this year.

“This is a huge success for CERN, which has been possible thanks to the close cooperation between our group and Thales experts,” continues Éric Montesinos. “It has not been an easy path – we developed as many as 28 prototypes! But we are now fully operational, with a failure rate below the calculated one and an availability rate of almost 100% for the SPS.”

The SPS teams are now preparing the intensity ramp-up, with the eventual goal of achieving HL-LHC parameters. The beams extracted from the SPS are “brighter” now – in other words, there are more particles in a given volume, which will increase the number of collisions in the LHC. It will, however, still take several years for the designed HL-LHC parameters to be achieved in the SPS.

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