Possible Methods for Reducing the Trapped Mode Effect in the SLAC Rotatable Collimator for the LHC P...
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Possible Methods for Reducing the Trapped Mode Effect in the SLAC Rotatable Collimator for the LHC Phase II Upgrade. Liling Xiao. 1. Current Model. Courtesy Steven Lundgren. R_beampipe=30.5mm. Jaw’s opening=2mm / 60mm. 2. Two Jaw Gaps – Longitudinal Modes. Current Design.

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Liling xiao

Possible Methods for Reducing the Trapped Mode Effect in the SLAC Rotatable Collimator for the LHC Phase II Upgrade

Liling Xiao

1


Liling xiao

Current Model

Courtesy Steven Lundgren

R_beampipe=30.5mm

Jaw’s opening=2mm / 60mm

L. Xiao

2


Liling xiao

Two Jaw Gaps – Longitudinal Modes

Current Design

The longitudinal modes have similar R/Q for the fully inserted jaws and the fully retracted jaws in the current design.

L. Xiao

3


Liling xiao

Lowest Longitudinal Modes

E-field

Gap=2mm: F=87.4MHz, R/Q=2.10e-1ohm/collimator

Gap=60mm: F=73.6MHz, R/Q=3.65e-1ohm/collimator

In the current design, the narrow EM foils can only perturb the Ez fields along the beam path without changing it significantly when the jaw gap varies.

L. Xiao

4


Liling xiao

Two Jaw Gaps – Transverse Modes

Current Design

The transverse modes have strong R/Q_T for fully inserted jaws than for fully retracted jaws. The transverse modes are gap modes between the two jaws .

L. Xiao

5


Liling xiao

Lowest Transverse Modes

E-field

Gap=2mm, F=62.9MHz, R/Q_T=5.5e6ohm/collimator

Gap=60mm, F=75.4MHz, R/Q_T=3.3e2ohm/collimator

150

6

yoffset@ 0.5mm

yoffset@ 0.5mm

Gap=2mm

Gap=60mm

There are strong Ey fields between the two jaws for fully inserted jaws.

L. Xiao

6


Liling xiao

Current Design

  • The longitudinal modes have similar R/Qs for the fully inserted jaws and for the fully retracted jaws, and the longitudinal modes R/Q depend on the geometry in transition region. Max.R/Q < 3.65e-1 ohm/structure @ 74MHz

  • (CERN Design: Max. R =2380 ohm @ 1.25GHz)

  • The transverse modes have higher R/Qs for the fully inserted jaws than for the fully retracted jaws. The most dangerous transverse modes are gap modes that exist in the straight section. Max. R/Q_T ~ 5.5e+6 ohm/collimator @ 63MHz

  • (CERN Design: Max. R/Q ~ 10e5 ohm/collimator) Problem!!!

L. Xiao

7


Liling xiao

Modified Transition Part VS. Longitudinal Modes

fully inserted jaws

fully retracted jaws

Modified EM Foils

Increasing the height of the EM foils can reduce the lower longitudinal modes R/Q effectively, thus reduce the beam heating.

L. Xiao

8


Liling xiao

Transverse Modes in CERN Collimator

CERN Collimator I

A=140mm

Beam in rectangular waveguide with Fc=c/(2*a)=1.07GHz

Gap Modes: F>Fc

A. Grudiev: AB-Note-2005-042 RF

L. Xiao


Liling xiao

Transverse Modes in SLAC Collimator

SLAC Collimator

F=63MHz

F=190MHz

Beam in parallel plats waveguide with Fc=0

Gap Modes: F=f(Length of jaw)

F=346MHz

L. Xiao


Liling xiao

Modified Straight Part VS. Transverse Modes

Modified SLAC Collimator

fully inserted jaws

fully retracted jaws

h

Adding two plates on the top and bottom to form a closed waveguide for the fully inserted jaws, thus suppressing the dangerous gap modes.

Beam in double-ridge waveguide with Fc~c/(2*h)=2.5GHz for fully inserted jaws

No gap Modes < 2GHz

Beam in two pairs of the parallel plates with larger gaps. Strong fields locate between the jaw surface and the base plates.

L. Xiao


Liling xiao

Lowest Transverse Modes

Without Plates

E-field

Gap=2mm, F=62.9MHz, R/Q_T=5.5e6ohm/collimator

Gap=60mm, F=75.4MHz, R/Q_T=3.3e2ohm/collimator

With Plates

Gap=2mm, F=332MHz

Gap=60mm, F=160MHz,

The strong gap modes are eliminated by adding the two plates both for fully inserted jaws and fully retracted jaws.

L. Xiao

12


Liling xiao

Summary

  • Modifying the transition part can reduce the longitudinal modes R/Q_z.

  • Closed the volume around the two cylinder jaws can eliminate the gap modes below 2 GHz for the fully inserted jaws. The gap modes for fully retracted jaws will be reevaluated .

L. Xiao, March 15, 2010

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