WP4: Crab Cavities & RF


Task 4.1. Coordination and Communication

  • Coordination and scheduling of the WP tasks

  • Monitoring the work, informing the project management and participants within the JRA 

  • Follow up the WP budget and use of resources

Task 4.2. Support studies

  • Tunnel preparation SPS and LHC 

  • Local IR layout and spatial integration 

  • Effect of phase noise, LLRF system conceptual design 

  • RF power system specification 

  • Operational aspects (how to commission/make invisible) 

  • Interlocks and fast Feedback

Task 4.3. Compact Crab Cavity Design

  • Complete cavity and cryomodule specifications 

  • Design optimisation for novel schemes 

  • Conceptual design of SOM, HOM and LOM couplers 

  • Conceptual design of helium tank and cryostat 

  • Multipacting simulations on cavity & couplers 

  • FEM simulations: mechanical & thermal aspects 

  • Initial down-selection of the CC options 

  • Completion of a full technical design on the initial down-selected options, with mechanical drawings and specification. 

  • Design of tooling, dies and cavity fabrication equipment

Task 4.4. Elliptical Crab Cavity Technical Design

  • Coupler development and testing 

  • Tuner design and mock up on copper models 

  • Study of mechanical effects: resonances, microphonics. 

  • Cavity performance with couplers and horizontal cryostat 

  • Performance difference between 2 K & 4 K 

  • Cryostat and He Tank Design 

  • Complete the full technical design

Task 4.5. Compact Crab Cavity Validation Prototyping

  • Procurement / fabrication of tooling, dies and equipment 

  • Construction of models (in copper initially) to refine manufacturing techniques and tooling. 

  • Fabrication of prototype niobium cavity 

  • Cleaning and electro-polishing on the bare niobium cavity. (i.e. no couplers, antennas or other accessories), including cavity surface inspection 

  • Development and procurement of all test equipment and instrumentation 

  • Low power tests and measurements on the bare cavity in a test cryostat to test for compliance with design gradient and other cavity performance specs. 

  • Make the final CC design down-selection

Description of work

WP4 is part of a larger “Crab Cavity Project”, which aims at obtaining a significant luminosity increase by installing local Crab Cavities in IR5 and IR1 as part of the luminosity upgrade planned for around 2021. To fully exploit the inner triplet upgrade, these crab cavities are needed. They are the instrument of choice both for compensation of the crossing angle and for luminosity levelling, allowing for optimum integrated luminosity during the collision run without the need of excessive peak intensities. This project will require four superconducting Compact Cavities for each high luminosity IR, two per beam on either side of IP. At present, there are a number of proposals of possible concepts of Compact Crab Cavities; their detailed study will be part of WP4. To mitigate the risk of the not yet established technology of the Compact Crab Cavities, more conventional Elliptical Crab Cavities are another important element of the project. The primary objective of Work Package 4 is to prepare for the construction phase of Compact Crab Cavities which should start around 2015. WP4 equally includes the Technical Design of Elliptical Crab Cavities. Prototyping and tests have been included in this design study only to the absolute minimum necessary to validate design choices.

Task 4.1. Coordination and Communication

The activities of this task are to oversee and co-ordinate the work of all the other tasks of the work package concerned, to ensure the consistency of the WP work according to the project plan and to coordinate the WP technical and scientific tasks with the tasks carried out by the other WPs when it is relevant. The task will be executed jointly by CERN, ULANC and STFC. The coordination duties also include the organization of WP internal steering meetings, the setting up of proper reviewing, the reporting to the project management and the distribution of the information within the WP as well as to the other work packages running in parallel.

The task also covers the organization of and support to the annual meetings dedicated to the WP activity review and possible activity workshops or specialized working sessions, implying the attendance of invited participants from inside and outside the consortium. 
WP4 requires coordination with other WPs on the following subjects: 
• With WP2 (Accelerator Physics and Performance) on: 
o Impedance and growth rate estimates for specific HOM’s to specify exact damping needs and feedback measures, 
o Effect of non-zero dispersion and stable working points, 
o Crab consistent optics, 
o Local doglegs & feedback to control beam transverse position at the location of the Crab Cavity, 
o Beam-beam simulations to investigate instabilities, noise issues and DA. 
• With WP3 (Magnet Design) on: 
o The possible need of doglegs if Compact Crab Cavities are impossible (see task 3.3). 
• With WP5 (IR Collimation) on: 
o Tracking simulations for loss maps for fast failure modes.

Task 4.2. Support studies

The studies will look at integration of the Crab Cavities in the accelerator tunnels, at the study of the preparation of the tunnels of both SPS (for tests) and LHC (BNL, CERN). In addition, RF studies common to Elliptic and Compact Crab Cavities will be carried out (KEK, LBNL and SLAC). This includes the study of the influence of phase noise and its reduction, along with the conceptual design of the low-level RF system. Another item is the study of the high power RF system. The conceptual study of typical necessary operational scenarios (CERN, CNRS, and ULANC) such as: how can the Crab Cavity RF system be commissioned? How can the Crab Cavity system be made “invisible” to the beam if it is not needed? How can cavity or amplifier trips be dealt with, what are the necessary interlocks and feedbacks?

Task 4.3. Compact Crab Cavity Design

For each of the proposed compact cavity topologies, the conceptual design including the power coupler as well as the wrong-order-mode couplers will be performed (STFC, ULANC, BNL, FNAL, KEK, ODU, SLAC). Coordinated by CERN, the design study will include multipacting simulations as well as mechanical and thermal stress analyses that will allow detailed definition of the structure of the cavity. Common performance specifications will keep the alternative designs directly comparable. The goal of the design study is to determine performance limitations of the various options; comparison will allow an initial down-selection, leaving at least two CC options. Complete full engineering designs follow, including drawings and specifications of the selected contenders, along with the design of necessary tooling and dies in preparation for next stage, the compact cavity prototyping (Task 4.5). A conceptual design of the helium tank and the cryostat will equally be performed (CNRS, CEA, KEK). This study will feed into task 3.3 to determine whether a dogleg (magnetic chicane) is needed or not.

Task 4.4. Elliptical Crab Cavity Technical Design

A conceptual design of an Elliptical Crab Cavity exists. Building on this, the development and testing of power couplers, design of the tuner and mock up on copper models are subject of Task 4.4 (BNL, CEA, CNRS, KEK). Also mechanical effects (resonances, microphonics) will be studied. A comparison of the performance at 2 K and 4.4 K cooling will be studied qualitatively and quantitatively in order to allow the optimum choice of the cryogenic system (CERN). The technical design of cryostat and He tank at the optimum operation temperature shall subsequently be done (CERN, CNRS). The given constraints of the LHC IR4 have to be taken into consideration (studied in Task 4.2).

Task 4.5. Compact Crab Cavity Validation Prototyping

Actual prototyping and tests of prototypes are not included in HiLumi LHC; some initial prototyping as necessary for validation of design choices is however included (CERN, KEK and ODU). This includes procurement/development of tooling, dies and other equipment needed for cavity manufacturing. If necessary, Cu models may have to be built before fabricating in niobium. Initial measurements on the cavity models and checks against the simulations and design specifications. Evaluation of measurement results will be done jointly with ULANC and STFC. Preparation of RF tests in cryogenic conditions (“Vertical Tests”) to test for compliance with design gradient and other cavity performance specifications; the actual tests will not be part of the design study.