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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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

L. Xiao

2


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


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


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


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


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


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


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


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


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


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


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

13


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