Crab Cavity RF Design, Couplers & Components

Exotic Concepts for Compact Cavities
Elliptical Cavity Design
Impedance & HOM Damping
Dual Beam-line Common Cryostat & Integration
Mechanical Design & Fabrication

Two Cell Elliptical Prototype

A coupled two-cell cavity at 800 MHz is being considered as a fundamental unit in the π mode to impart a total kick of ~2.5-3.0 MV per module. The beam to beam separation in the LHC is 42 cm in the IR4 region and only 19 cm else where in the ring. Therefore, only a global scheme where the cavities are placed in IR4 is feasible using elliptical cavities of 800 MHz. Higher frequencies are not possible due to the long bunch length of the protons (7.55 cm). Initial design of the cavity body originated from a 400 MHz design and later converged to 800 MHz.
Fig. 1: Schematic of two Semi-Optimized 2 Cell 800 MHz Cavities
Cavity Beta, β =v/c 1.0 1.0 1.0 1.0
Parameter Crab Cavity
Frequency [MHz] 400 800 800 800
Iris Radius, Riris [cm] 14 7 7 7
Beam Pipe Radius [cm] 7 7 9
Wall Angle, α [deg] 10 6 0 -
Equatorial Ellipse, R=B/A 1.0 0.8 1.0 1.0
Iris Ellipse, r=b/a 1.5 2.0 0.8 1.0
Cav. wall to iris plane, d [cm] 1.5 1.0 3.375 -
1/2 Cell Length, L=λ β /4 [cm] 18.75 9.375 9.375 9.375
Equator Height, D [cm] 46.5 23.8 23.2 23.1
Mafia File: 800 MHz CI/DL (or use this)
Mafia File: 400 MHz
Mafia File: 800 MHz BNL (use this)
Mafia File: 800 MHz SLAC (or use this)

Prototype Cavity-Coupler Concepts

Three proposals for the cavity-coupler system has been proposed which adhere to the transverse constraints posed by the LHC and the required beam pipe apertures (see optics section). They are required to provide very strong damping (Qext ~ 200) for the lower order mode, same order mode and higher order modes. See below for more details on damping requirements (impedance estimates).
LARP Design UK Design KEK Design
MWS Format
Mode Freq [GHz] R/Q Qext
TM010 0.5396 35.17 < 100
  0.6974 194.52 < 100
TM110 0.8000 117.26 106
  0.8129 0.46 -
TM110 (SOM) 0.8896 93.40 < 100
  0.9090 6.79 < 100
SAT Format (baseline) SAT Format (Single Cell)
SAT Format (Alt Design)
MWS Format
Mode Freq [GHz] R/Q Qext
TM010 ?
TM110 0.8000
SAT Format
STL Format
MWS Format
Mode Freq [GHz] R/Q Qext
TM010 ?
TM110 0.800
SAT File
STL File

Parametric Scans
An extensive scan of the cavity geometric parameters was performed to obtain the optimum RF characteristics for the inner and outer half-cell of the two-cell cavity. The choice of the final parameters will be based on further fine tuning (higher order mode (HOM) damping, peak field specifications, and mechanical constraints) after the beam-pipe radius for efficient HOM damping is determined. The figure below shows an updated scan performed for the 800 MHz, Version 2.2 (unlike the ver 1 & 2 design which was scaled from 400 MHz cavity scan).

Cavity Parametric Scan
Fig 2: Cavity geometrical parameters as a function of relevant RF parameters leading to an input for cavity optimization

Impedance & HOM Damping

Dual Beam-line Common Cryostat & Integration

Mechanical Design & Fabrication

Exotic Concepts for Compact Cavities

Due to the very tight transverse size constraint posed by the LHC several compact RF structures have to proposed to lower the frequency while significantly reduce the transverse size of the cavity body. Some of the them shown in figure below.

  • A SLAC design aiming a 1/2-wave structure (typically referred to as 1/4-wave structure for the TM$_{010}$ mode). A similar design is under fabrication for use in RHIC to improve the losses at transition and collision energy operated at TM$_{010}$ mode. It maybe possible to drive this structure when installed in the deflecting mode to probe several issues related to hadron colliders.
  • A spoke structure operated at the deflecting mode. This structure although mechanically stable has strong multipacting issues and kick gradients are typically smaller than the elliptical counter parts.
  • A FNAL mushroom type cavity which uses the typical concept of the elliptical cavities but with dramatic bends to reduce the transverse size. This structure is also prone to heavy multipacting near the bend regions which need detailed study and a similar struture is under testing but at higher frequencies.
  • A UK design of the original JLAB type double rod structures. The original design consisted of cylindrical rods which were sensitive to mechanical resonances, so conical cross sections for the rods are employed to improve mechanical stability.
  • A BNL proposal to use TM$_{010}$ mode in the conventional pill-box structure but with offset beam pipes close to the cavity equator to utlize the kick from magnetic field of this mode. Although the concept is conceptually simple and HOM damping relatively simpler compared to the other designs, the large offsets in the cavity may lead to higher order cavity modes to couple to the beam very strongly which is not desired. Additionally, the non-zero longitudinal electric field needs to be compensated. Multipacting needs careful to be evaluated in such a configuration.
  • A KEK proposal to use a similar pill-box type structure but with beam-pipes mounted transversely to the cavity as opposed to the nominal pill-box. In this configuration the transverse electric field is use to deflect the bunch and special nose cones are required to shield the magnetic field.
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Topic revision: r26 - 2010-06-02 - Rcalaga
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