# Presentation: 390907

## Super-LHC Crab AmbitionsH. Padamsee, LARP MeetingOctober 3, 4, 2005

## Intro Remarks

- Derived from experts:
- Ohmi, Zimmerman, and Ruggiero

## Several Possible Paths to Super-LHC, All Challenging

- Goals: increase luminosity to ≈ 10 X nominal (1x1034)
- In several stages, with different approaches
- Beam-beam compensation with wires
- Reduce beta*
- Increase bunch charge
- Reduce emittance
- Increase number of bunches
- Reduce bunch length
- Large angle crossing with crab compensation to allow head-on collisions
- Upgrade time scale 2012 - 2015
- Several different paths arranged into phases 0, 1, 2..

## Attractive Features of Crab Crossing Option with Large crossing angle (0.3 - 8 mrad) 

- Avoids parasitic collisions
- Avoids geometric loss of luminosity for crossing angles larger than beam aspect ratio
- Aspect ratio of LHC beam is 0.17m/7.5 cm = 0.23 x10-3.
- Two beams can pass through separate magnetic channels, without sharing a quadrupole aperture.
- The crab cavities lead to a simplified IR design.
- Strong-strong beam-beam simulations predict an  increase in the KEK-B beam-beam tune shift limit  by a factor 2-3 for head-on collision compared with  the present crossing angle.

## Slide 5

## Kick Voltage

## Slide 8

## The bunch is synchronized to pass through the center of the cavity at the zero crossing of the magnetic field. The head and tail of the bunch are deflected in opposite directions. The bunch oscillates in the magnetic field of the insertion quads and reach their peak amplitude at the interaction point.  

- The bunch is synchronized to pass through the center of the cavity at the zero crossing of the magnetic field. The head and tail of the bunch are deflected in opposite directions. The bunch oscillates in the magnetic field of the insertion quads and reach their peak amplitude at the interaction point.

## Crab Voltage

- To realize head-on collision effectively, the beam deflection angle ** should be equal to the half crossing angle ***c*.
- Voltage V required for a deflection angle is

-  Super-LHC needs
- 1/2 Crossing angle max: 0.15 mrad - 4 mrad,
- Frequency: 400 to 1200 MHz,
- E.g. Vc (400 MHz) = 28 MV/mrad => 112 MV for max angle

- LHC : R12 = 30 m

## Crab Crossing Requirements

- Rotate the bunch without deflecting the bunch
- Jitter of RF phase of crab cavity induces a random transverse offset at the collision point, which causes diffusion due to beam-beam interaction.
- Jitters of RF phase of main accelerating cavity causes a deviation of timing of beam arrival at the crab cavity.
- Since there is no damping mechanism due to the absence of synchrotron radiation, a slow diffusion can be serious for hadron colliders.
- Emittance can grow

## Noise Issues

- amplitude noise introduces small crossing angle
-    tolerance ~0.1% jitter from emittance growth
- phase noise causes beam-beam offset;
- 	tolerance on LHC IP offset random variation
- 	xmax~10 nm
- → tight tolerance on left-right crab phase and
-      on crab-main-rf phase differences

-  <0.012o (t<0.08 ps)
- at c=1 mrad & 400 MHz
-  <0.04o (t<0.28 ps)
- at c=0.3 mrad & 400 MHz

## IP offset of 0.2 x*

|  | KEKB | Super-KEKB | ILC | Super-LHC |
| --- | --- | --- | --- | --- |
| x* | 100 m | 70 m | 0.24 m | 11 m |
| c | +/- 11 mrad | +/-15 mrad | +/-5 mrad | +/- 0.5 mrad |
| t | 6 ps | 3 ps  | 0.03 ps | 0.08 ps |

**IP offset of 0.2 ********x*******

- IP offset of
- 0.001 ******x*******

- Comparison of timing tolerance with other applications

*→ **not more difficult than ILC crab cavity*

- Zimmerman

## SC Crab/Deflecting Cavities Efforts Worldwide

- CERN/Karlsruhe SC deflecting cavity for separating the kaon beam 1970’s, 2.86 GHz
- Cornell 1500 MHz crab cavity 1/3 scale models 1991...for very high average current 1 A
- KEK 500 MHz crab cavity with extreme polarization  1993 - Present, 1-2 A beam current,  - Hosoyama WG3
- KEK 2003 new crab cavity design for Super-KEKB,
- 	10 A beam current, 3 mm bunch length,
- 	more heavily damped (coaxial & waveguide)
- Fermilab CKM deflecting cavity - 2000 - present
- Daresbury is studying crab cavities for ILC, 2005
- Crab cavities for ultra-fast light sources (LBNL, Argonne)
- Crab cavities or super-LHC (upgrades)

## Around the World

- LHC
- 1/2 Crossing angle max: 0.15 mrad - 4 mrad,
- frequency: 400 to 1200 MHz,
- E.g. Vc (400 MHz) = 28 MV/mrad => 112 MV max
- CESR-B studies,
- 12 mrad crossing angle,
-  	kick = 2 MV,
- one 500 MHz cavity, Es < 25 MV/m
- KEK-B crab crossing
- Most advanced development
- 500 MHz, Vc = 1.44 MV
- ILC crab crossing
- Crossing angle = 20 mrad, w/o crab lum loss is 80%
-  	kick voltage = 6 MV at 3.9 GHz or 18 MV at 1.3 GHz
- 6.5 MV/m
- Most likely candidate Fermilab 3.9 GHz structure

## Cornell SC Crab Cavity

- Developed in 1991 for B-factories
- Single Cell Model Tested in 1992
- Advances at KEK based on Cornell  concept/development

## Issues Addressed at Cornell

- Kick voltage, peak surface fields
- Input power coupling requirements
- Polarization of crab mode
- Removal of Lower Order Modes
- TM010, TE110 (2 pol), and TM110 wrong pol.
- Use coaxial coupler with choke filter to reject desired crab mode
- Removal of HOM
- Large beam pipe/ferrite dampers
- 1/3 scale Nb cavity made and tested
- Kick field and peak fields achieved

## TM110 Mode Properties

## 500 MHz TM110 Polarized Cavity Mode separation14 MHz

## Nb Cavities Made - First

- After overcoming one low field multipacting barrier we were able to reach Epk = 30 MV/m @ Q = 1.2x109 in one polarization, and Epk = 27.5 MV/m @ Q = 2.3x109 in the other polarization of the TM110 mode. Both maximum fields were limited by field emission.  Needed 25 MV/m

## Second Nb Cavity 

- Reached 22 MV/m
- Q > 109

- QHOM <100
- QLOM < 100

## Input Coupler 

- One requirement...there may be others
- For a 1 mm displacement and a transverse voltage maximum of 2 MV, the effective accelerating voltage is 2 x104 Volts.
- A 1 amp beam current extracts or gives back about 20 (x) kWatts for x mm displacement.
- The required coupling of Qext = 6.4x106/x (mm) can be accomplished by a coaxial coupler similar to input couplers presently used

## KEK Squashed Cavity

## Slide 24

## Slide 25

## Slide 26

## Slide 27

## Slide 28

## Fermilab Deflecting Cavity, 3.9 GHz, Candidate for ILC Crab

## Slide 31

## Slide 32

## Slide 33

## Crab Cavity for Ultrafast Light Source

- The resulting extended x-ray pulse with correlated distribution may be imaged by asymmetrically-cut crystals or aberration-corrected x-ray optics, to result in an x-ray pulse of approximately picosecond duration.
- After the bunch radiates, the kick introduced is cancelled by a downstream deflecting cavity.
- High repetition rates, in principle as high as 500 MHz (ALS bunch rate), may be achievable using this technique, providing an ultrafast x-ray source with characteristics suitable for a variety of time-resolved photoemission and magnetism experiments.

## LHC Crab System Issues

- What are the crossing angle choices: early stage: 0.3 mrad, final stage: 8 mrad? What are the required kick voltages? (next slide)
- What are the crab development/implementation time scales?
- Choice of rf frequency, 400 MHz, 800 MHz or 1200 MHz
- How much free space is there near the interaction region for installing crab cavities?
- Estimate total length of cavity system, assuming filling factor 0.5?
- Single cells or multi-cells (number of cells?)
- How much polarization split needed for the TM110 deflecting mode?
- Factors influencing choice of aperture (reduced by presence of coupler for TM010 mode)
- Optimum geometry for crab cells, choice of safe max surface fields
- Input power coupling requirements, Qext, power handling.
- To avoid deflecting the bunch, so as to keep beams in collision, the phase of the cavities must be highly stable; how much is tolerable?
- How much phase jitter is tolerable to avoid beam-beam performance degradation?

## Super-LHC

| crossing angle | 0.3 mrad | 1 mrad | 8 mrad |
| --- | --- | --- | --- |
| 800 MHz | 2.1 MV | 7.0 MV | 56 MV |
| 400 MHz | 4.2 MV | 13.9 MV | 111 MV |
| 200 MHz | 8.4 MV | 27.9 MV | 223 MV |

- Crab cavity voltage for different c’s & rf frequencies

## Higher Frequency Choice

- Less Total kick voltage required   1/f
- But Wavelength must be >> than bunch length (7.5 cm)
- Need fewer cavities, less free space
- Transverse dimensions smaller
- Aperture smaller, HOMs excitation stronger
- Can use IOT at 1200 MHz, better phase stability
- Higher frequency more tolerant of phase jitter

## LHC Crab System Issues/2

- How to control phase jitter in crab cavity?
- What is the tolerance on crab kick amplitude?
- How much is the phase jitter due to the accelerating cavity, how much is tolerable?
- What bunch length, bunch charge, average current, bunch spacing to use for design of HOM damping? (Given several different luminosity upgrade scenarios for LHC)
- Estimate damping needed to avoid multi-bunch instability, given bunch spacing (e.g. 25 ns?)
- Estimate growth rate of coupled bunch instabilities caused by HOMs
- Compare to acceptable instability growth rate from accelerating LHC cavities.
- Tuning range for tuner
- Microphonics tolerance due to LOM coupler