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Crab cavities are special electromagnetic cavities that use electric and magnetic fields to rotate the bunch prior to collision in a particle collider so that the beams collide head-on rather than at an angle. This is one of the most critical elements of the LHC luminosity upgrade. While the concept has been proven on electron beams at KEK, no one had ever attempted to put a crab cavity on a proton beam. In addition, the space available in LHC required special compact crab cavities much smaller than conventional systems. Last month saw the first test of these compact crab cavities on a proton beam. The Cockcroft Institute staff working on the HL-LHC-UK project played a key role in this landmark experiment, and we profile the key roles played by CI researchers and PhD students.
Nick Shipman (Postdoc at Lancaster University)
I performed cold testing of the crab cavities at CERNs cryogenic facility SM18. Cold testing is a step which must be performed before a cavity can be used in an accelerator to make sure it satisfies the required specifications. It involves cooling down the cavities to 2K or approximately -271 degrees Celsius, to do this the cavity is placed inside a large tank called a cryostat which is then filled with liquid Helium. At this temperature the Niobium from which the cavities are made becomes superconducting which means that a steadily flowing electrical current sees no resistance. Once the cavities are cold radio frequency power is injected into the cavity which causes electromagnetic fields and energy to build up inside it. We are then able to test, among other things, whether the cavities can support sufficiently large electromagnetic fields without “quenching” which is where the walls of the cavity heat up and they stop being superconducting at which point all the energy inside the cavity is lost.
Emi Yamakawa (Postdoc at Lancaster University)
I studied luminosity reduction caused by phase modulation at the HL-LHC crab cavities. A new RF control scheme in RF acceleration cavity saves Klystron RF power, however, the phase of acceleration cavity is not constant over the whole turn. Crab cavity cannot follow the phase modulation, i.e. the bunch centre arrives at the crab cavity early or late. This results in an asymmetric transverse kick and bunch distortion at the interaction point. This bunch distortion of colliding beam reduces peak luminosity. I analysed the reduction of peak luminosity caused by phase modulation analytically and numerically. It is concluded that the impact on peak luminosity is acceptable for HL-LHC physics run. I have also calculated beam loading in the HL-LHC crab cavity caused by long-range beam-beam interaction and injection oscillation. As for the transient beam loading due to the injection beam oscillation, the maximum peak generator power required for full correction of beam loading is superior to the power limit of klystron in the nominal operational scenario. However, with a fast transverse damping system, the orbit displacement in the crab cavity can be damped to less than 1 mm within 10 turns. In this case, the maximum peak demanded power is within the limit (40 kW) and acceptable for the RF power system. Currently we try to measure the beam loading in SPS crab cavities with transverse offset of bunches.
Lee Carver (Postdoc at The University of Liverpool )
Two crab cavities have been built and installed into the SPS (the second largest accelerator in the CERN facility – behind the LHC) at CERN. The plan is to try and verify the fundamental behaviour of crab cavities with protons – something that has never been observed before. My job is to carry out the measurements of the SPS beam properties that will show what effect this behaviour has on the beam. I have to plan the beam measurements to measure the beam degradation and crabbing and prepare the machine to provide the required beam in the days before the dedicated tests to ensure that we are ready. Then I carry out the beam measurements from the CERN Control Center, where the beam is carefully manipulated within the crab cavities to provide information for how crab cavities will perform for HL-LHC.
Andri Alekou (Postdoc at The University of Manchester)
The measurements we make are very important to understand how the crab cavity reacts to the beam and how the beam reacts to the crab cavity. My job is to make careful calculations and simulations of the these effects, making sure all the physics of the crab cavity is understood. These simulations have predict what we will see in the control room, help us understand what is going on and see how the conclusions apply to the LHC. I also help with the measurements of the crab cavity in the control room.
James Mitchell (3rd Year PhD student Lancaster University)
I work on superconducting RF devices called HOM couplers which are used to extract unwanted power from the crab cavities that the beam induces in higher order modes (HOMs). The power is a result of the charged particle beam exciting resonant modes in the cavity. If the power is not extracted, high heat-loads or beam instabilities can result and hence it is crucial that these structures are optimised to provide maximum power extraction at the frequencies of the most detrimental modes. My job is to measure the power produced by all these HOMs in the SPS under different operational scenarios and to use this to estimate the effect in LHC to ensure that the crab cavities don’t cause the LHC beam to break up or cause the crab cavities to heat up.
Thomas Jones (STFC Mechanical engineer and part-time PhD student at Lancaster University)
I am Technical Team Leader for the build of one of the crab cavity cryomodule (Radio frequency dipole, RFD) at the Daresbury Laboratory. I am responsible for providing technical solutions, for example the supporting system for the cavities within the cryomodule, as well as the development of tooling, processes and infrastructure for the module assembly. I have been involved in the project since 2011 and since 2015 I have been working on a Part-time PhD with Lancaster University taking some of the technical aspects into increased focus such as fluid analysis of the chemical etching of the complex cavity geometries
Edward Jordan (STFC Mechanical Engineer)
I am a mechanical design engineer working on the overall assembly procedure for the Radio Frequency Dipole (RFD) Cryomodule. I work very closely with my colleagues in the UK and abroad to ensure that every minutia of the assembly process is realised. This ranges from installing heavy duty lifting infrastructure that lifts the assembled ‘cavity string’ and enables a wide range of technicians to install components, to designing very precise mechanical tooling to handle the delicate cavities in such a way that it won’t damage them during assembly. I have been working on the Crab Cavity project since 2016 and have been focusing on the assembly of the second prototype Cryomodule of the series – the RFD Cryomodule; this Cryomodule is to be assembled in the United Kingdom at Daresbury Laboratory.
Nik Templeton (STFC Mechanical Engineer)
I was responsible for the cryomodule magnetic shielding and thermal screen for the SPS cryomodule. The double layer magnetic shielding consists of two cryogenic ‘Cold’ shields, located inside the cavity helium tank, as well as a global ‘Warm’ shield, which lines the vacuum vessel. I worked with magnet simulation engineers at CERN and STFC to optimise the performance of the shields and liaised with a leading supplier to optimise the designs for manufacture and assembly. I performed thermo-mechanical and tolerance analysis then arranged material testing to validate the shield designs. I led the procurement, quality assurance and incoming acceptance of the magnetic shields at CERN. The cryomodule thermal screen is cooled with recycled gaseous helium at 50 kelvin and consist of OFHC copper panels and a brazed copper cooling circuit. The screen is used to thermally intercept critical components to minimise the conductive heat leak to the cavities at 2 kelvin. The thermal screen is also dressed in Multi-Layer Insulation to minimise radiative heat leak to the cold mass. As part of the collaboration I undertook a one month placement at CERN to learn their CAD systems and work more closely with the design team.
This video shows the design and function principle of these new cavities: