T.Suwada Accelerator Laboratory, KEK

T.Suwada (tsuyoshi.suwada@kek.jp) Accelerator Laboratory, KEK
Operational Performace on Positron Production from Tungsten Monocrystalline Target at the KEKB Injector Linac このプレゼンテーションでは、出席者間で討論をし、アクションアイテムを作成する場合があります。PowerPoint を使って、プレゼンテーションの実行中にアクションアイテムを作成するには ... スライドショーの実行中に control + クリック [会議メモ] をクリック [アクションアイテム] タブをクリックアクションアイテムを入力 [OK] をクリックこのようにすると、入力したアクションアイテムを集めたスライドが、プレゼンテーションの最後に自動的に作成されます。 T.Suwada Accelerator Laboratory, KEK

the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008
Introduction Summary of the previous systematic studies on the positron-production efficiencies (PPEs) with tungsten crystals (Wc) We carried out the systematic studies on the PPEs with tungsten crystals having various thickness using 4- and 8-GeV electron beams at the KEKB injector during Installation and performance of the 1st Wc target We optimized the thickness of the tungsten crystal at 4-GeV. After developing acrystal-axis alignment technique, we installed a Wc target at the positron source of the KEKB injector linac in September 2006. Operational performance on the e+ production during a year The first operation of the e+ production with a Wc target has been performed from Sep to June 2007 (~10 months). The 2nd operation of the e+ production with another new Wc target has been again started in Oct KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Experimental Results at Linac Test Beam Line: Rocking Curves (tungsten crystal targets, crystal axis <111>) at Ee-= 4 GeV and Pe+= 20MeV/c 2.2mm-thick Wc 5.3mm-thick Wc 9mm-thick Wc · Variations of the positron yield as a function of the rotational angle (rocking curve) of the crystal target. We can see clear enhancements in the positron production depending on various crystal thicknesses. The broad peak widths may mainly come from multiple scattering in the crystal target. The solid lines indicate the simulation results based on coherent bremsstrahlung process (by Tomsk group). KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Experimental Results at Linac Test Beam Line: Peak widths (tungsten crystal targets, crystal axis <111>) at Ee-= 4 GeV and Pe+= 20MeV/c · 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 , 2006). Target thickness 10mm for the KEKB e+ source pw~40 mrad>> cb, cr Critical angle for channeling cr~0.61 mrad (Wc, E=4GeV) Critical angle for coherent brems. cb~13 mrad (Wc, E=4GeV) Critical angle cb Critical angle cr The obtained peak widths are much larger than the critical angle for axial channeling due to multiple scattering!! KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Experimental Results at Linac Test Beam Line: Positron production enhancement at Ee-= 4 GeV Enhancements depending on the positron momentum Pe+ [MeV/c] 2.2-mm Wc 5.3-mm Wc 9-mm Wc 5 3.3±0.1 2.2±0.1 1.5±0.2 10 3.6±0.3 2.3±0.1 15 3.5±0.1 1.7±0.3 20 3.7±0.1 1.5±0.1 En=Ye+on-axis/Ye+off-axis Expected for KEKB Wc Expected for Wa Normalized by the positron yield from amorphous tungsten · (Left) Variations of the positron-yield enhancement as a function of the target thickness at Pe+=20MeV/c and Ee-=4GeV. The enhancements decrease with the increase of the target thickness due to multiple scattering, and at the crystal thickness of 14 mm, the positron yield almost agrees with that from a conventional 14-mm-thick (4X0) tungsten plate. · (Right) Variations of the positron-yield enhancement as a function of the positron momentum. The results are very consistent with those of simulations. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Experimental Results at Linac Test Beam Line: Energy dependence of PPE & Optimization of the target thickness at Ee-= 4 GeV Enhancements depending on the electron energy (2.2-mm-thick Wc) Positron production efficiencies depending on the target thickness En=5.4 Optimized Wc t=10mm Yield diff.~26% En=3.6 Expected for LC R=5.4/3.6 ~1.5 Optimized Wa t=14mm (KEKB old) KEKB · Variations of the positron-yield enhancement as a function of the electron energy at Pe+=20MeV/c for the 2.2-mm-thick tungsten crystal target. The enhancements increase monotonically with the increase of the electron energy. The results are very consistent with those of simulations (sim1 (Novosibirsk), sim2 (Tomsk)). ·Variations of the positron-production efficiency as a function of the target thickness at Pe+=20MeV/c. The results shows that the optimum thickness of ~10mm gives the maximum e+ yield at KEKB. You can also see the effective radiation length for the crystal is reduced in comparison with that of the amorphous due to crystal effects. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Positron Source of the KEKB Injector Linac
Pulse Coil W Target e+ W Target & Pulse Coil DC solenoids & Accelerating sections e+ beam e+ beam Primary e- beam Primary e- beam e- beam Q~10nC/bunch, E=4GeV Chicane Positron capture section based on QWT Primary e-beam e-beam e-beam BPM1 BPM2 BPM3 Wc target ST1 ST2 KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Tungsten Crystal Target: Crystal preparation
Tungsten crystal target assembly after HIP technique · Tungsten crystals with several thicknesses were fabricated at collaborating Tomsk Polytechnic University. · They were developed based on the solid-state re-crystallisation process of large-size crystal ingots. · Also, a technique for sample cutting along crystallographic planes without any crystal quality damage was developed. · The surface mosaic spreads of the crystal structure on both the beam-entrance and -exit sides were measured by an X-ray scattering with 0.5mrad. Sample picture Fabricated tungsten crystals with dimensions 5mm5mm12mm and 5mm5mm14mm KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Tungsten Crystal Target: Joining of tungsten crystal to a copper body by a Hot Isostatic Pressing (HIP) technique Tungsten crystal target assembly after HIP technique · HIP technique is a mechanical joining technique, which mechanically joins different kinds of metal together in a few atomic layers between their metal surfaces under high pressure and high temperature conditions. · HIP conditions : high temperature (1050 oC) and high pressure (150 Mpa) for 3 hours. The dimensions of the tungsten crystal target : 5mm5mm10.5t mm for KEKB KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Crystal target fabrication: Crystal axis measurement by an X-ray Bragg reflection N Crystal assembly Detector W E X-rays S Turn table N Crystal axis direction 4.9mrad Laue picture of tungsten crystal W E 7.7mrad S Inclination direction of the crystal axis to the central axis of the copper body KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Crystal target fabrication: Fabrication of the crystal target axis-maching 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 the central 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 the copper body was similarly machined. (Right) Target assembly seen from sideways. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Tungsten Crystal Target: Crystal target fabrication after post-machining Final target assembly 50 mm (Left) Mechanical drawing of the target assembly. The tungsten crystal is fixed to the cylindrical copper 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 are converted to the electrons (blue arrow) and positrons (red arrow). (Right) Target assembly after the post-machining process installed in a vacuum chamber seen from downstream. Two thermocouples are mounted 7.5 mm away from the center of the tungsten crystal, and a small hole with a 3 mm diameter is penetrated through the copper body for the transport of the electron beam. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Error estimation for each test items
Alignment of the target assembly: Alignment and its reproducibility tests of the crystal target installed on a girder · (Left) Alignment procedure of the target assembly with a precise alignment telescope with an angular resolution of 0.5 mrad. · After this fundamental alignment procedure, further alignment tests were made in order to check the reproducibility of the target alignment (a) with a linear actuator in and out (40 times), (b) with the target chamber mounting on and off (3 times), (c) the transportation test of the target chamber, and (d) the remounting test of an alignment mirror. Error estimation for each test items Reproducibility error Linear actuator in & out 0.41 mrad Target chamber mounting on & off Transportation test Remounting of the mirror 1.81 mrad Other systematic errors 2 mrad X-ray axis-measurement 1 mrad In total 3 mrad The axis misalignment in total causes only a small amount of yield loss (~2%) in the e+ production. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Performance of the crystal target: Positron-production efficiency measurements · (Left) Positron-production efficiencies (PPE) of the 1st bunch measured for each beam pulse. The data were obtained after adjusting the incident angles of the primary electron beam with upstream steering magnets. For the sake of comparison, 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 the number of positrons captured in the positron capture section and Ne- is the number of the primary electrons. · The results show that the increase of the positrons is 25 ±2% (28 ±2%) for the 1st (2nd) bunch on average. The results are consistent with those obtained in the previous experiments. for 1st e+ bunch 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 the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Performance of the crystal target: Linear relation checks in e+ production yield & temperature rise 1 bunch operation 1 bunch operation · (Right) The temperature rise was measured with two thermocouples mounted inside the copper body. · The average temperature rise is T~13.2oC at a beam repetition rate of 50 Hz (average bunch charge ~7.8 nC). · The results show that the temperature rise of the crystal target changes linearly as a function of the beam repetition rate, and that the steady-state heat load normalized by the primary electron charges is clearly reduced by ~20% in comparison with the previously-used tungsten target. · (Left) Positron beam intensity plotted versus the primary electron beam intensity in a bunch. The solid line through the data indicates a linear-function fit of the data. · The present results show that the positron yields increase linearly with the increase of the primary electron intensity without any abnormal behaviors within the experimental errors. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Short summary in installation & performance of the crystal target
The crystal target was successfully installed in the KEKB positron source with the alignment precisions of ±3mrad in total. The obtained positron-production efficiency for the crystal target is 25 ±2% (28 ±2%) for the 1st (2nd) bunch on average. The results are consistent with those obtained in the previous experiments. On the other hand, the PPE for the amorphous target is 20% on average. The increase of the PPE is ~25%. The steady-state heat load normalized by the primary electron charges is clearly reduced by ~20% in comparison with the previously-used tungsten target. The positron yields increase linearly as a function of the primary electron intensity up to ~8nC/bunch without any abnormal behaviors. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Long-term time traces of Positron-Production Efficiency (PPE) in Tungsten Crystal (Sep Jul. 2007) Sep. 2006 (for 1st bunch) Max.PPE~0.26 Min.PPE~0.14 Long-term drift of PPE in the range of of 0.14~0.26 (60%). Jul. 2007 KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Long-term time traces of Positron-Production Efficiency (PPE) in Amorphous Tungsten (Oct Jul. 2006) Oct. 2005 We can see a long-term drift of Wa PPE is the range of of 0.16~0.23 (35%) (not very large!!). Jul. 2006 KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Long-term time traces of Primary e- Charges in Tungsten Crystal (Sep Jul. 2007) Sep. 2006 beam rescan We can see a long-term drift of the primary e- charges in the range of of 7.3~9 nC (21%). Max.Qe-~9nC Min.Qe-~7.3nC Jul. 2007 KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Long-term time traces of e+ Charges in Tungsten Crystal (Sep Jul. 2007) Sep. 2006 beam rescan Max.Qe+~2nC Min.Qe+~1.2nC We can see a long-term drift of the primary e+ charges in the range of of 1.2~2 nC (53%). Jul. 2007 KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Long-term time traces of e- Injection Angle in Tungsten Crystal (Sep Jul. 2007) beam rescan Sep. 2006 Inj. Angle ±0.5mrad We can see relatively stable injection angles (<±0.4mrad) during this long period. Jul. 2007 KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Short summary in performance of the crystal target
PPEs, the e+ and e- intensities (the injection angles have been relatively stable) were largely varied during one-year long-term operation. PPEs: 14~26%, Qe+: 53%(1.2~2nC), Qe-: 21%(7.3~9nC), Injection angles (both x and y): < ±0.4 mrad The integrated electron flux hitting the crystal target has been amounted to ~5109 nC/mm2 for this period. No significant damages to the crystal structure were found after this irradiation. Although I checked the correlations between the PPEs and some beam parameters (beam charges, hitting positions, injection angles of the primary e-…) at the target, no clear correlations were found. Other unknown parameters (the beam sizes, energies, energy spreads, bunch length,…, etc. at the target ) may cause these variations while we can not directly measure them. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Aperture Analysis with 3 BPMs
Here, we give some results in terms of the PPE reduction based on a simple aperture analysis. Goodness of orbit linearity (that is, root-mean-squares orbit positions) Gl=∑√(xi- x0)2/3, x0= (∑ xi)/3 BPM1(SP174) BPM2(SP17C4) BPM3(SP21K5) e+ target capture section Good orbit, Gl=0 Aperture no beam loss at BPM3 e- beam e+ beam x1 x2 x3 Orbit kink & beam loss at BPM3 Bad orbit, Gl>0 e- beam e+ beam x1 x2 x3 KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Results of Aperture Analysis with 3 BPMs
Based on the measured distributions, a certain limited aperture may reduce the PPE. It indicates that the allowable aperture is in the region of Goodness of less than 0.5mm, and over the limit, the PPE is strongly reduced. The PPE reduction behavior is similar for both the Wc and Wa target, while the PPE level of the Wc target within the aperture is clearly higher than that of the Wa target. The results show that the radiation damage of the crystal target may not be very hard. The plot points compose the sampled in each region and also include two-bunch data for both Wa and Wc target. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Conclusions We have successfully applied a new tungsten single-crystal target for generation of the intense positron beam at the KEKB positron source. The positron intensity increased by ~25% in comparison with that obtained from the 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/positron linacs. We found no significant damages to the crystal structure after one-year KEKB operation, and however, we found a long-term fluctuation on the positron intensities due to the long-term drifts (or variations) of the primary electrons. The present results encourage us to consider the application of crystal targets in the next generation of the B-factories and e+e- linear colliders. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Experimental Studies at KEK
e+ targets tested Tungsten crystal (Wc), Diamond, Si, etc. 4, 8 GeV e- Target configuration for Wc e+ momentum acceptance P/P~2.4% (FWHM) Geometrical acceptance ~1msr at Pe+=20MeV/c. 8 GeV e- Target configuration for Diamond, Si Beam charge ~0.1/bunch KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Radiation processes in single-crystal
Simulations by Lyon group, Photon Energy Spectrum 2 GeV e- Crystal W Amorphous W 20 GeV e- Crystal W Amorphous W KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

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) and 360 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 these angular spreads need to be controlled within the critical angle for axial channeling (0.61mrad at 4GeV) in tungsten crystal. Positron production target · The previously-used 14-mm-thick (4X0) conventional tungsten plate was replaced by a 10.5-mm-thick tungsten crystal in Sep without any significant modification of the accelerator layout. · The new tungsten-crystal target was successfully installed without any rotational devices after developing the crystal-target fabrication technique and the crystal-axis alignment technique. 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-m-long and two 2-m-long acceleration structures. · The electrons along with the positrons generated from the target are stopped by a positron/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 the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

Performance of the crystal target: Operational performance
· (Left) Time traces of the positron-production efficiencies of the two bunches averaged every five days after the start-up of KEKB operation in summer 2006. · The emittance-measurement results of the positron beams are consistent with those obtained for the previously-used tungsten plate. · The integrated electron flux hitting the crystal target has amounted to about 5.5107 nC/mm2 (average one-bunch charges of 7.7 nC/bunch at 4 GeV) for the KEKB two-month operation. No damage on the crystal structure was observed after the irradiation. KEK Linac/ Tsuyoshi Suwada the 3rd ILC Positron Source Meeting, LAL-Orsay, France, Dec.1-2, 2008

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