DAE-STFC Technology and Skills Programme; Acccelerators

WP1: SRF Thin-Film Cavity Development - Particle accelerators consist of a complex suite of 'energy hungry' systems. Cavities are fed with Radio Frequency (RF) power to enable electro-magnetic fields to boost beam bunches (i.e. electrons, protons, ions, etc.) to higher energies. The cavity is typically normal conducting (i.e. copper, Cu) or superconducting (i.e. niobium, Nb).

Superconducting RF (SRF) provides a large beam energy increase, in a smaller footprint compared to normal-conducting systems. SRF requires a complex, expensive and low-efficiency cryogenic system to minimise the intrinsic RF losses. The applied RF-power interfaces into the surface by a skin-depth of ~2 um for 1.3 GHz operation.

If a superconducting thin-film (TF) material of a few 10's um's is deposited onto the surface of a cheaper substrate, i.e. oxygen free Cu, the costs for manufacture could be substantially reduced. If the SRF TF is a material with a higher critical temperature than Nb, then it is possible to achieve high performance whilst operating at a higher cryogenic temperature (i.e. >4 K), substantially reducing system complexity, its capital and operational costs.

This programme will maximise the complimentary use of capabilities, infrastructure and expertise to expedite High Power Impulse Magnetron Sputtering (HiPIMS) coating of SRF TF onto Cu cavity substrates, to achieve high accelerating gradients as well as high-power transfer efficiency at higher cryogenic temperatures.

WP2: Space Charge Compensation (SCC) of High Current Hydrogen Ion Beams - SCC is crucial to overcome the transport limits of pulsed high intensity ion beams in the low-energy beam transport section of particle accelerators. SCC occurs when the secondary particles, created by the ionisation of the background gas by the beam, are trapped by the beam potential, leading to a decrease of the local charge density and therefore the electric field inside the beam.

The compensating particles are either electrons (for positive beams) or ions (for negative beams). The degree to which the space charge is compensated in the steady state depends on the loss mechanism of the secondary particles from the beam potential well. The SCC time is the time required for the SCC process to reach a steady state and is inversely proportional to the residual gas pressure.

Before equilibrium, the SCC degree is not constant, which affects the beam transport properties and leads to beam losses and damage to downstream accelerating structures such as Radio Frequency Quadrupole (RFQ).

WP3: Permanent Magnet Systems for Intense Low-Emittance Ion Beams - Microwave ion sources operating at 2.45 GHz frequency are used for production of intense proton beams e.g. at the European Spallation Source and International Fusion Materials Irradiation Facility. Microwave ion sources require a magnetic field for resonant coupling of the microwave power to the plasma electrons.

The required field strength is approximately 90 mT for 2.45 GHz frequency. This field is usually created by solenoid coils surrounding the plasma chamber of the ion source. Unfortunately, the solenoid field increases the emittance of the extracted ion beams, which complicates matching of the beam into the accelerator.

The minimum emittance of the beam is defined by the ion temperature of the plasma and the magnetic field strength at the extraction aperture. The ion temperature in microwave ion sources is low, which means that the contribution of the magnetic field on the emittance is significant. The plasma generation and sustenance in a microwave ion source depends on: 1) the microwave power coupling from the magnetron to the plasma chamber via power coupling schemes; 2) The energy transfer between the electromagnetic fields in the resonant cavity and the plasma via electron cyclotron resonance mechanism using magnetic field configurations like solenoid and multicusp and 3) the plasma confinement schemes (axial, radial or combined).