Compact RF Cavity Development at the Cockcroft

Institute

Dr G. Burt

Lancaster University & Cockcroft Institute

CI SAC meeting 29 - 31 October 2012

Hi-Lumi LHC crab cavity

CI SAC Meeting 4th – 6th November 2013 A Wheelhouse5th November 2013

Crab crossing

4 Rod Crab cavities

IR

Why do we require compact cavities?

400 MHz elliptical

800 MHz elliptical

400 MHz elliptical

Using 800 MHz RF causes a S-shaped bunch which reduces luminosity hence a 400 MHz compact cavity is desired

There is limited space for the crab cavities due to the opposing beamline. The cavity must within a 143 mm radius.

4R crab cavity – Cockcroft - Jlab

• To make suitable we increased the separation between the rods and made the rods wider.

• This meant they were close to the outer can.

Cavity manufacture

CI SAC Meeting 4th – 6th November 2013 A Wheelhouse5th November 2013

• Lancaster and ASTeC are developing a comapct SRF crab cavity for the LHC upgrade

• The structure, manufactured by Niowave was the first compact crab cavity to be tested at high field and has achieved a gradient of 3.5 MV

First TestsThe first test were performed March 2013.Cavity was not etched after bakeout hence had some surface contamination.

FIRST EVER HIGH GRADIENT TEST OF A COMPACT SRF

CRAB CAVITY!

CI SAC Meeting 4th – 6th November 2013 A Wheelhouse5th November 2013

After a quick buffered chemical

polish and ultrapure water high pressure

rinse the cavity meets the design

gradient.

2nd Test results

Stainless Steel FPC

Copper

Q0= 7.189E9

Rs = 8.3 nOhms

Stainless Steel Q0=2.737E9

Rs = 22.0 nOhms

Recent tests with new couplers confirm this

We recently discovered that the couplers had accidentally been made from stainless steel instead of copper. To test the effect we simulated a cavity with no losses other than the couplers in frequency domain using the same methods as in the vertical test.

Cavity welds

A mistake was made in machining the rods. To fix this Niowave cut it off and welded a new one on.

The magnetic field has a longitudinal component on the weld which is not ideal.

CryomoduleASTeC have been developing a flexible cryostat for the LHC crab cavity. The cryostat has to include a beampipe for the opposing beamline and fit in the limited space within LHC. The cryostat is also side loading to allow better access during testing and commissioning

A prototype cryomodule is currently being developed to allow cavity testing in the SPS. This will be the worlds first test of a crab cavity on a hadron beam.

CI SAC Meeting 4th – 6th November 2013 A Wheelhouse5th November 2013

CLIC and CTF3

Prototypes

The 1st CLIC crab cavity prototype has been manufactured by Shakespeare Engineering in the UK. Tolerance and surface roughness on single parts have been measured.

Structure is planned to be tested at SLAC in the near future.

The structure being built for high gradient test at CERN has only a single feed as it will not see beam.

Cavity is being machined at VDL along with main linac structure to allow comparison of gradients.

Cavity Field Measurements

CI-SAC Dec 2011

Surface Fields

Property Value

Energy stored, J 1

QCu 6395

Rt/Q, Ohm 54.65

vgr, % -2.92

Esurf/Et 3.43

Hsurf/Et 0.0114

Sc (W/m2) 3.32

Esurf

Hsurf

Ssurf=(ExH)surf • Peak electric and magnetic fields of the dipole mode are located 90 degrees from each other on the iris

• Surface Poynting flux Ssurf is however at 45 deg to both E and H

• Location of the breakdown on the iris provides critical information about the role of magnetic field in breakdown.

•The cavity has a large Sc but relatively low E and H fields at the surface so this also provides an independent verification of new theories.

Compact Accelerators for Cargo scanning applications

Cargo Screening Accelerators

Luggage Scanning requires a few tens to hundreds keV. This can be delivered by traditional X-ray tubes up to 450 keV.

Truck or shipping cargo is larger requires ~6 MeV. Industrial linacs can provide this.

Aircraft ULD or pallets are too large for baggage scanners and too small for cargo scanners. Currently searched by hand.

Ideal energy is around 1-2 MeV but no current source available.

Why X-band?

• For a mobile linac mounted on a robotic arm the weight of the linac is critical.

• While the linac isn’t very big or heavy the shielding is.

• X-band means that the shielding diameter is much less.

• Area of shielding is given by

• (2rcavtshield + tshield2)

• Availability of 9.3 GHz magnetrons

CI-SAC Dec 2011

1 MeV X-ray linacDC Electron Gun e2V collaboration

Buncher and Accelerating Structure (1 MeV)

Magnetron

e2V collaboration

(8-12 GHz, 1-2 MW, 100-200 Hz) Dynamic switching of amplitude and

phase pulse-to-pulse)

Automated Control System(Energy, rep-rate, dose)

Proprietary Rapiscan Imaging and Data Analysis

CI Proposal ScopePhase-I

X-ray Target

1 MeV Linac DesignParameter Value

Energy 1 MeV

Frequency 9.3 GHz

Length 130 mm

Rsh max 116 M/m

Pin 433 kW

Pulse Length 4 s

Pulse Rate 250 Hz

Peak Beam Current 70 mA

Average Beam Power

70 W

Cavity Tuning

Structure was found to have poor matching and field flatness.

Low beta cells were further off frequency than could be tuned.

Ideal profile

Linac TestingSo far the linac has produced a 750 keV, 1 mA beam as measured on the spectrometer and Faraday cup/ICT.

This is limited by the cavity having the incorrect e-m field pattern.

Modified structure

New490 MHz wide

Old60 MHz wide

We have developed a new X-band structure with much greater cell-to-cell coupling to increase tolerances.Simple structure design with no slots to help tolerances (low fields and low voltage make this acceptable)

Cavity measurements• Preliminary

measurements of the new structure show good agreement with simulations.

Simulation

Measurement

Conclusion

• The 4R Crab cavity for LHC was designed that met all specifications, unfortunately it was not selected for LHC.

• CI is heavily involved in X-band structures and CLIC and are currently undertaking high gradient tests of X-band crab cavities for CLIC.

• Compact Linac work is nearing commercialisation and the new structure from Comeb is performing well in low power measurements.