Interaction Region of PEP-II

1ILC MDI workshopJanuary 6-8, 2004

PEP-II IRM. Sullivan

Interaction Region of PEP-II

M. Sullivan

for the

ILC MDI workshopJanuary 6-8, 2005



Outline

•Initial IR design parameters

•Initial beam parameters

•Detector constraints

•IR design

•Present performance and issues

•New beam parameters

•Luminosity background

•Summary



SLAC beam lines



Layout of the PEP-II Ring



Initial IR Design Parameters

• Energy asymmetry of 9 on 3.1 GeV• Head-on collision• Bunch separation of 1.26 m• 25 mm separation between BSC envelopes at

2.8 m (room for a septum magnet)• SR masking

– No direct hits on the detector beam pipe– No surfaces that can one bounce to the detector

beam pipe– This leaves mask tip scattering as the dominate SR

source



Initial Beam Parameters

• Collision Frequency 238 MHz• Number of bunches 1650• Bunch spacing 1.26 m• Charge/bunch (L/H) 5.9x1010 2.1x1010 • IP Beta X,Y 0.50, 0.015 m• Emittance X, Y 48, 1.5 nm-rad• Bunch size (x, y, z) 155, 4.7, 10000 µm• Currents (L/H) 2.1 A 0.75 A• Luminosity 3x1033 cm-2 sec-1



Detector Constraints

• Detector acceptance minimum angle of 300 mrad• Detector magnetic field of 1.5T• Minimum thickness beam pipe

– 4 µm Au– 800 µm Be– 1 mm water– 400 µm Be

• Detector center shifted in Z +0.37 m





Present status• We have achieved y

*s of 10 mm • With x

*s of 30 cm• Bunch lengths are estimated to be 11-13 mm• Total beam currents of 2.450 A on 1.550 A

(1.590 on 2.540 on last shift)• No sign of SR backgrounds• Fairly large luminosity background• Total current in the support tube up to 4.1 A!

• Heating in the IR support tube (from Q2 to Q2) OK so far

• NEG pump heating by HOM power



Past and Present HOM heating

• IR Be bellows• NEG pumps in the LER• Q1/Q2 bellows

• Bellows in region 4• Bellows in region 10



IR Vacuum Chambers



Bellows Detail

Transverse H11 mode can couple through the RF shield

RF shieldBe is coated with about 4 m of Au

Al heat sink

Be to SS braze

SS sleeves

Cu pipe



Forward VTX BLWS Cooling



VTX Bellows Cooling Installation



Lab test data of NEG outgassing as a function of temperature

LER NEG Pump Temperatures

Upstream LER side

NEG Pressure vs NEG Temperature

0

1

10

100

1000

10000

0 100 200 300 400 500 600 700

NEG temperature deg F

NEG

pre

ssur

e (n

Torr

)

350 °F



LEB Fans



HEB Fans



New Beam Parameters

• Collision Frequency 238 MHz• Number of bunches 1710• Bunch spacing 1.26 m• Charge/bunch (L/H) 12.6x1010 3.5x1010 • IP Beta X,Y 0.30, 0.008 m• Emittance X, Y 60, 1.0 nm-rad• Spot size (x, y, z) 134, 2.8, 9000 µm• Currents (L/H) 4.5 A 2.2 A• Luminosity 2x1034 cm-2 sec-1



Issues for the near future

• Run 5 goals that affect the IR• LER current of 3.3 A• HER current of 1.8 A• Higher bunch currents more HOM power• Shorter LER bunch length more HOM power• Lower y

*s to 9 mm• Lower LER x

* to 30 cm • Increase LER emittance to 50 nmrad

Total of 5.1 A !

BSCs



Issues for the farther future

• Ultimate goals that affect the IR• LER current of 4.5 A• HER current of 2.2 A• Still higher bunch currents more HOM power• Still shorter bunch lengths more HOM power• Lower y

*s to 8 mm• Lower LER x

* to 30 cm • Increase LER emittance to 60 nmrad

Total of 6.7 A !!

BSCs



Present machine parameters

LER x*= 30

cm and LER x = 22 nmrad

LER x* may

be a little low, but it is the value for run 5

BSC just clears the present Q2 chamber on the forward side

Plenty of room on the backward side

BSCs



BSCsExpected LER beam size for run 5

LER parameters: x

* = 30 cm x = 50 nmrad

BSC is defined as 15 + 2mm (uncoupled)

BSC violated at the septum

Just enough room for the beam

New chamber being built and RFI by April or May



LEB+HEB HOM Power ~ ib2 z

-2



Be Vertex Pipe and Bellows


PEP-II IRM. SullivanLuminosity versus Crossing Angle

• Normalized Lsp vs. e- angle

0.75

0.80

0.85

0.90

0.95

1.00

1.05

-1000 -500 0 500 1000

XP[e-] (murad)

Lsp

/ Lsp

(XP=

0)

by-4, msrd

by-4, fit

sp. by-2, msrd

sp. by-2, fit

Without parasitic crossing

Withparasitic crossing


PEP-II IRM. SullivanComparisons between stronger B1s,

crossing angles and energy changes• The parasitic crossing separations are:

Present design 3.22 mm30% Stronger B1 (Nd) 3.6-3.8 mm (12-17%)Stronger B1 (volume) 3.5-3.9 mm (9-21%)With +/- 0.5 mrad x angle 3.9-4.0 mm (21-24%)

• The energy differences for the crossing angle option are:

HER 8.9732 to 8.7450 (-2.5%)LER 3.1186 to 3.2000 (+2.6%)

• Increase the energy asymmetry AND remove last B1 sliceVery preliminary look with head-on collisions:HER energy 8.9732 9.427 GeV +5.1% and LER energy 3.1186 2.969 GeV - 4.8% PC separation of 3.55 mm (10%)



Synchrotron radiation masking has been checked OK with a 7mm beta y*

Local beam-gas and coulomb should be essentially unchanged since the geometry is almost the same and the masking is the same. These backgrounds should slowly improve as the total number of A-hrs increases

Radiative Bhabhas as a background that increases as the luminosity increases

Detector Backgrounds in the Future



87.57

6.56

5.5

4

3.532.5

21.51

0.5

4.55

HER Radiative Bhabhas

-7.5 -5 -2.5 0 2.5 5 7.5

0

10

20

30

-10

-20

-30

m

cm

M. SullivanFeb. 8, 2004API88k3_R5_RADBHA_TOT_7_5M

3.1 G

eV

3.1 G

eV

9 GeV

9 GeV



3 2.5

2 1.51

0.5

LER Radiative Bhabhas

-7.5 -5 -2.5 0 2.5 5 7.5

0

10

20

30

-10

-20

-30

m

cm

M. SullivanFeb. 8, 2004API88k3_R5_RADBHA_TOT_7_5M

3.1 G

eV

3.1 G

eV

9 GeV

9 GeV



The PEP-II interaction region has performed very well

The PEP-II accelerator is starting to move into a new area of performance where HOM power will play a much larger role. The higher beam currents and higher bunch currents contribute to the higher HOM power as well as to higher SR power.

So far we have addressed two major issues in the IR concerning HOM heating – Be bellows heating and NEG pump heating. Other regions will bear watching as well as other vacuum components.

The IR upgrade is looking at modifying the B1 bending magnets to improve the beam separation at the 1st parasitic crossing while maintaining head-on collisions. Stronger magnetic material is needed to do this. However, we have not found a material that is comfortably rad-hard.

The option of adding a small crossing angle (+/- 0.5 mrad) using the present hardware can be done by changing the beam energies by about +/- 2.5%.

Detector backgrounds that are a function of the luminosity will become more important as the luminosity increases

Summary

Documents

Interaction Region of PEP-II