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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 SLAC Rotatable Collimator for the LHC Phase II Upgrade Model

Courtesy Steven Lundgren

R_beampipe=30.5mm

Jaw’s opening=2mm / 60mm

L. Xiao

2


Two Jaw Gaps – Longitudinal Modes SLAC Rotatable Collimator for the LHC Phase II Upgrade

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 SLAC Rotatable Collimator for the LHC Phase II Upgrade

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 SLAC Rotatable Collimator for the LHC Phase II Upgrade

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 SLAC Rotatable Collimator for the LHC Phase II Upgrade

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 SLAC Rotatable Collimator for the LHC Phase II Upgrade

  • 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 SLAC Rotatable Collimator for the LHC Phase II Upgrade 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 SLAC Rotatable Collimator for the LHC Phase II Upgrade

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 Rotatable Collimator for the LHC Phase II Upgrade

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 SLAC Rotatable Collimator for the LHC Phase II Upgrade . 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 SLAC Rotatable Collimator for the LHC Phase II Upgrade

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 SLAC Rotatable Collimator for the LHC Phase II Upgrade

  • 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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