Progress of SuperKEKB Collimator
The 18th KEKB Accelerator Review Committee
March 4th, 2013
Takuya Ishibashi (on behalf of KEKB Vacuum Group)General InfoWe plan to install 10 horizontal and 3 vertical
SuperKEKB type collimators in LER.
At the beginning of the commissioning, 2 horizontal
collimators are going to be installed in LER.
In HER, KEKB type collimators are going to bereused at the same locations as KEKB at thebeginning.
We plan to renew all of the KEKB type collimators
in HER in the future.

IR D02
D03

by Y. Ohnishie+D06Collimator’s History
Ver.4: KEKB type

Ver.5: Fit antechambers. The loss factor issmaller by a factor of 2 compared with Ver.4.
Total length ~1500 mm.

Ver.6: The latest version. Part of the movable heads
are placed in the antechambers. The total length isshorten with the same loss factor as Ver.5.
Total length ~ 1000 mm.

Design Concepts of Ver.6
Part of the movable heads is hidden inside theantechambers to reduce the impedance andincrease maintainability by downsizing thecollimators.
Need to get space for installations of HOM
absorbers and bellows up/downstream of thecollimators by the downsizing.
Have to avoid the trapped-modes.

SR Power Estimation
In the collimator location, the power density ata vertical wall is about 19.3 kW/m, and this isapproximately-same as that at a wiggler regionof LER.
When the ramp’s angle of the wall becomes 30
degrees, the power density is about 9.7 kW/m.
This would be no problem for Copper wall withcooling water.

Material for the Tip
If all bunches in the ring hit the movable head, all materials that weinvestigated would melt.
Materials which have narrow gap between the melting point and themaximum temperature: Be, C, Cr, Mo, Ti, W
We’ve adopted tungsten because of the availability, the junction
property with copper, the electric and thermal conductivity.

→ enable easy replacement of the movable head

Temperature [℃]100000#10000#

Melting PointMaximum Temp

1000#

100#
Al# Be# C# Co# Cr# Cu# Hf# Ir# Mo# Nb# Pd# Pt# Re# Rh# Ru# Ta# Ti# W# Zr#

Structure
The chamber and the heads are made from copper.
The strokes of the movable heads are d=5~25 mm in horizontal and 2~12 mm
in vertical.
(“d” refers to the distance between the central beam axis and the tip of the head.)

The ramp have to be less than 30 degrees because of the heat load by SR.
Tungsten is jointed at the tip of the head with Hot Isostatic Press (HIP, alreadysucceed in the test).
RF contacts are attached between the heads and chambers.
HIPed tungsten on tip

ramp ~23°

cooling channel
tapered beam pipe

bellows

Vertical Collimator
We’re also designing the vertical collimator.

The inside of the antechambers wouldbe tapered to avoid the trapped mode.

Loss & Kick Factor for Different “d”
Impedances are estimated with GdfidL (σz=6 mm).
The radiation length of tungsten is about 3.5 mm, so the longitudinal lengthof the tip has been fixed to 10 mm (> 2RL).
The loss factor is less than 0.1 V/pC per collimator.
The loss factor between d=2 in vertical and d=5 in horizontal is about same.

Loss & Kick Factor for Different Bunch Length

Bunch Length: 6 mm(LER), 5 mm(HER)
The factors are largely dependent on thebunch length.

Impedance (Horizontal)

Impedance (Vertical)

Transverse Mode Coupling
We estimated the threshold of the TMC using actual β value ateach collimator (σz = 6 mm).
When the collimator’s apertures are all closed to the minimum(H: 5 mm, V: 2 mm), the bunch current would exceed thethreshold.
However, there would be no problem when we operate themwithin the designed aperture values.

C1 fs E/e
Ithresh = P
i i ?i ( z )LER

TMC Threshold (mA/bunch)
All Closed

Bunch Current

Realistic Apertures

Horizontal

1.41

13.15

Vertical0.965.10(mA/bunch)
1.44

Bunch Lengthening
thanks to D. Zhou
The longitudinal wake-fields due to collimators are mostly inductive.

We roughly estimated the inductance and the bunch lengthening
using Zotter’s equation.

(zz0)

3zz0

↵Ib Im(Zk /n)ef f R 3p()=02
2⇡(E/e)⌫s0z0

The total inductance of the collimators in LER is about 16.3 nH.
10.9 [nH] in horizontal + 5.4 [nH] in vertical

σz0 = 6 mm, σz = 6.96 mm; σz0 = 5 mm, σz = 6.23 mm
We may need to consider the mitigation method for suppressing theimpedances.
Demin Zhou has just started estimating the lengthening using his
beam tracking code to look into the detail.

Future Plans
A first prototype of the SuperKEKB type horizontal
collimator is in the process of production, and it’sscheduled for completion in March.
We’re going to test the motion of the movable heads,
the mechanical property of the RF contacts, the control
system and so on for the prototype.
At the beginning of the commissioning, we have noplan to install HOM absorbers because the loss factorof the collimator is small.
However, we continue to design the HOM absorbers.
We’re simulating and designing the vertical collimator.