First application of a tungsten single-crystal
positron source at the KEKB injector linac

T.Suwada (tsuyoshi.suwada@kek.jp)
Accelerator Laboratory, KEKAbstract
 A new positron-production target has been successfully
employed for generation of the intense positron beam at theKEKB injector linac in September 2006. The target composedof a tungsten single-crystal (10.5 mm thick) is bombarded with
4-GeV incident electrons. The positrons are collected andaccelerated up to 3.5 GeV in the succeeding sections. A
conventional tungsten plate (14 mm thick) has been usedpreviously, and the positron-production efficiency (PPE), theratio of the number of positrons to the number of the incident
electrons, was 0.20, and the crystal target increased to 0.25. The
increase of the PPE has boosted the positron intensity to its
maximum since the beginning of KEKB operation. Now thisnew positron source is stably operating and is contributing toincreasing the integrated luminosity of the KEKB B-factory.
KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Introduction
 In order to achieve high luminosities, positron sources must beboosted towards next generations of B-factories and e+e- linearcolliders.
 Conventional heavy-metal targets limit to increase the beam
power of primary electron beams due to the allowable heatload on the target.
 Crystal-assisted positron source is one of the bright schemes forhigh-intensity e+ production.
 A new crystal-assisted positron source was first proposed by
Chehab (IPNS), et al. in 1989. (R. Chehab, et al., PAC’89, Chicago, IL,USA, Mar. 1989, p.283)

 Yoshida, et al., demonstrated a clear enhancement of the e+yield in a tungsten crystal target using a 1.2-GeV electronbeam. (K. Yoshida, et al., Phys. Rev. Lett. 80, 1437, 1998)

KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Previous Experimental Results:
Rocking Curves (tungsten crystal targets, crystal axis <111>) at
Ee-= 4 GeV and Pe+= 20MeV/c
1400
9-mm-thick tungsten crystal,

Relative Positron Yield[arbitrary unit]

1350-+

Ee =4 GeV, Pe =20 MeV/c

1300
1250
12001150
1100
1050
1000-100-50

050100150

Rotational Angle [mrad]· (Left) Variations of the positron yield as a function of the rotational angle(rocking curve) of the crystal target. The broad peak width is mainly due tomultiple scattering in tungsten crystal.
· (Right) Variations of the peak width (FWHM) as a function of the target
thickness. The peak width is ~40 mrad (FWHM) for the 9-mm-thick tungsten
crystal at 4 GeV (see T.Suwada, et al., presented at LINAC’06, Knoxville, TN, US, Aug.
21-25, 2006).
KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Previous Experimental Results:
Positron production enhancement and target-thickness
optimization at Ee-= 4 GeV and Pe+= 20MeV/c
Positron Production Efficiency

0.04
Tungsten Crystal
Standard Tungsten Plate

0.035
0.03
0.025
0.02
0.015
0.01
0.005
005101520Target Thickness [mm]· (Left) Variations of the positron-yield enhancement as a function of the target thickness. Theenhancements decrease with the increase of the target thickness, and at the crystal thicknessof 14 mm, the positron yield almost agrees with that from a conventional 4X0 thick tungsten
plate. (Right) Variations of the positron-production efficiency as a function of the target
thickness. At the 9mm thick, the positron yield is ~26% larger than that for a conventional
tungsten. The optimum thickness is reduced to be about 10 mm at 4 GeV due to the crystal
effect while the optimum thickness of a conventional tungsten target is 14 mm.
KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Layout of the KEKB positron source

Primary electron beam
· The positrons are generated by a 4-GeV primary electron beam(~10 nC/bunch) impinging on a conventional tungsten target.
· The average beam power is 2 kW at a maximum repetition rate of 50 Hz.
· The typical transverse normalized emittances are 660 mm·mrad(horizontal) and360 mm·mrad(vertical) on average.
· The typical beam size is 0.7 mm (rms) in radius.
· The horizontal (vertical) angular spread at the target is estimated to be 0.2(0.1) mrad, where theseangular spreads need to be controlled within the critical angle for axial channeling (0.61mrad at4GeV) in tungsten crystal.

Positron capture section (Quarter-wave transformer)· It comprises a 45-mm-long pulse solenoid (2 T), an 8-m-long DC solenoid (0.4 T), and two 1-mlong and two 2-m-long acceleration structures.
· The electrons along with the positrons generated from the target are stopped by apositron/electron separator (chicane) comprised four rectangular magnets and a beam stopper at
the center of the chicane (see A.Enomoto, et al., EPAC’92, vol.1,1992, p.524).
KEK Linac/Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Tungsten Crystal Target: Crystal target fabrication after postmachining(Left) Mechanical drawing of the target assembly. The tungsten crystal is fixed to the cylindricalcopper body with a diameter of 50 mm for water cooling. The heat deposited on the crystal target
is conducted through a cooling water channel (water flow1.5l/min) composed of a copper pipe
(4mm in diameter). Electrons (blue arrow) impinge the tungsten crystal target and they areconverted to the electrons (blue arrow) and positrons (red arrow).
(Right) Target assembly after the post-machining process installed in a vacuum chamber seenfrom downstream. Two thermocouples are mounted 7.5 mm away from the center of the tungstencrystal, and a small hole with a 3 mm diameter is penetrated through the copper body for thetransport of the electron beam.
KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Crystal target fabrication: Crystal axis measurement by an Xray Bragg reflection
The target assembly itself was precisely machined in order to keep the precise alignment between
the crystal axis and the central axis of the target assembly based on the crystal-axis measurement.

N

Crystal assemblyW

Detector

EX-raysS

N Crystal axis

Turn tabledirection

4.9mradW

Laue picture oftungsten crystal

E
7.7mradS

Inclination directionof the crystal axis tothe central axis of
the copper body
KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Crystal target fabrication: Fabrication process of the crystal
target assembly machined by a lathe· (Left) Target assembly fixed at the center of a positioning jig seen from upstream. In
this machining procedure, the relative inclination angles between the crystal axis and thecentral axis of the copper body are corrected based on the Brag reflection measurement
results. First, the stainless-steel body was precisely machined by a lathe and finally thecopper body was similarly machined.
(Right) Target assembly seen from sideways.
KEK Linac/Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Alignment of the target assembly: Alignment and reproducibility tests at
the installation of the target assembly into a vacuum chamber

Error estimation for each test
items

Reproducib
ility error

Linear actuator in & out

0.41 mrad

Target chamber mounting on &off

0.41 mrad

Transportation test

0.41 mrad

Remounting of the mirror

1.81 mrad

Other systematic errors

2 mradX-ray axis-measurement±1 mradIn total±3 mrad

KEK Linac/
Tsuyoshi Suwada· (Left) Alignment procedure of the target
assembly with a precise alignment telescopewith an angular resolution of 0.5 mrad.
· After this fundamental alignment
procedure, further alignment tests weremade in order to check the reproducibility ofthe target alignment (a) with a linear
actuator in and out (40 times), (b) with thetarget chamber mounting on and off (3times), (c) the transportation test of thetarget chamber, and (d) the remounting testof an alignment mirror.

KEKB-MAC, Mar.19-20, 2007

Performance of the crystal target: Positron-production
efficiency measurements

Number of Beam Pulses

1200
1000

Tungsten Crystal Target
Previously-used Tungsten Target

for 1st e+ bunch

800
600
400
200
00.1

0.12 0.14 0.16 0.18

0.20.22 0.24 0.26

Positron-Production Efficiency

Positron-Production Efficiency
Results of Tungsten Crystal Target
PPE(1st)=0.25±0.01,
PPE(2nd)=0.26±0.01
Results of Previous Tungsten Target
PPE(1st)=0.2±0.01,
PPE(2nd)=0.2±0.01
KEK Linac/Tsuyoshi Suwada· (Left) Positron-production efficiencies (PPE)of the 1st bunch measured for each beam pulse.
The data were obtained after adjusting theincident angles of the primary electron beamwith upstream steering magnets. For the sake ofcomparison, the data of the previous tungsten
plate (June 2006) were plotted. The solid lines
are gaussian-function fits of the data. The PPE
is defined by Ne+ / Ne-, where Ne+ is thenumber of positrons captured in the positron
capture section and Ne- is the number of theprimary electrons.
· The results show that the increase of thepositrons is 25 ±2% (28 ±2%) for the 1st (2nd)bunch on average. The results are consistent
with those obtained in the previous
experiments.

KEKB-MAC, Mar.19-20, 2007

Performance of the crystal target: Operational performance
0.4· (Left) Time traces of the positron-production
0.3
efficiencies of the two bunches averaged every
0.25
five days after the start-up of KEKB operation
0.2
in summer 2006.
0.15· The emittance-measurement results of the0.1
0.4
2nd bunch
0.35
positron beams are consistent with those0.3
obtained for the previously-used tungsten
0.25
plate. This means that the transverse
0.2
emittances are mainly determined by the0.15
acceptance of the positron capture section.
0.1
10/Sep.
24/Sep.
8/Oct.
5/Nov.
22/Oct.
/2006 · The integrated electron flux hitting theDate
crystal target has amounted to about 5.5×107
Normalized emittances of positrons
nC/mm2 (average one-bunch charges of 7.7
Horizontal
nC/bunch at 4 GeV) for the KEKB two-monthγεx= 1440 ±40mm·mrad (1st),
operation. No damage on the crystal
γεx= 1500 ±80mm·mrad (2nd),
structure was observed after the irradiation.
VerticalClear answers will be obtained in futureγεy= 1640 ±110mm·mrad (1st),
operational experience of the KEKB.
γεy= 1740 ±60mm·mrad (2nd),
1st bunch

Positron-Production Efficiency
2nd bunch
1st bunch

0.35

KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Performance of the crystal target: Linearity of the positron
intensity to the primary-electron intensity
Positron Beam Intensity [nC/bunch]

2· (Left) Positron beam intensity plottedversus the primary electron beam intensity
in a bunch. The solid line through the dataindicates a linear-function fit of the data.
· The present results show that the positronyields increase linearly with the increase of
the primary electron intensity without any
abnormal behavior within the experimental
errors.

Tungsten Crystal Target

one bunch operation
1.5

10.5

2

3

45

6

78

Primary-Electron Beam Intensity[nC/bunch]

The average instantaneous charges
is 7.7 nC/bunch at 4 GeV.

KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Performance of the crystal target: Temperature-risemeasurements
2.5

Temperature Rise [degree/nC]· The temperature rise was measured with twothermocouples mounted inside the copper body.
2· The average temperature rise is ΔT~13.2oC
1.5at a beam repetition rate of 50 Hz under onebunch operation condition with an average bunch
1charge of 7.8 nC/bunch.
· The results show that the temperature rise of the0.5
crystal target changes linearly as a function of thebeam repetition rate, and that the heat load
00102030
40
5060
normalized by the primary electron charges isBeam Repetition Rate [Hz]clearly reduced by ~20% in comparison with the· (Top) Variations of the temperature rise previously-used tungsten target.
of the tungsten crystal target as a function· The measured heat loads are consistent withof the beam repetition rate under onesimulation results performed by Artru, et al. (X.
bunch operation. In this figure theTungsten Crystal Target
Previously-used Tungsten Plate

temperature rise is normalized by theaverage beam charges of the primaryelectron beam. For the sake ofcomparison, the data of the previouslyused tungsten target are plotted.
KEK Linac/Tsuyoshi Suwada

Artru, et al., Phys. Rev. ST Accel. Beams 6, 091003 (2003)).

KEKB-MAC, Mar.19-20, 2007

Performance of the crystal target: Operational experiences The new positron source has been stably operating without any significant
reduction of the positron-production efficiency during two months.
 For long-term KEKB operation, it would be useful to apply a feedback control tostabilize the incident angles of the primary electron beam in the direction of thecrystal target axis.
 The integrated electron flux hitting the crystal target has amounted to ~5.5×107
nC/mm2.
 As for the radiation damage effect, Artru et al. tested on a 0.3-mm-thick tungsten
crystal placed in front of the SLC positron source up to the integrated electron fluxof 3.2×108 nC/mm2. No damage to the crystal structure was found after thisirradiation.
 We expect that the present tungsten crystal target can be used for one year at least
without any serious radiation damage according to their results.

KEK Linac/
Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007

Conclusions
 We have successfully applied a new tungsten single-crystal target for generationof the intense positron beam at the KEKB positron source.
 The positron intensity increased by ~25% in comparison with that obtained fromthe previously-used conventional tungsten target.
 On the contrary, the heat load on the crystal target reduced by ~20%.
 The tungsten crystal target has boosted the positron intensity to its maximum
since the beginning of the KEKB operation in 1999.
 This is a first application of the crystal target to high-energy electron/positronlinacs.
 Useful information on radiation damage and stability of the crystal target will beobtained through KEKB operation.
 The present result encourages us to consider the application of crystal targets inthe next generation of the B-factories and e+e- linear colliders.

KEK Linac/Tsuyoshi Suwada

KEKB-MAC, Mar.19-20, 2007