positron sources using channeling for ilc & clic

R.Chehab/Posipol2008/Hiroshima, june 2008

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POSITRON SOURCES USING CHANNELING FOR ILC & CLIC

R.Chehab, X.Artru, M.Chevallier, IPNL/IN2P3/CNRS, Universite Lyon 1

A.Variola, A.Vivoli, LAL/IN2P3/CNRS, Universite Paris-Sud

V.M.Strakhovenko, BINP-NovosibirskL.Rinolfi, CERN


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INTRODUCTION * Necessity to consider conventional-like sources * Good results and perspectives for crystal sources using

electron channeling in axially oriented crystals {experiments at CERN and KEK }

* Studies on energy deposition in crystal and amorphous targets confirm the interest of crystals for the total energy deposited [X.Artru et al PRST-AB 6 (2003)091003] which is less than in an amorphous targets giving the same yield.

* Simulations and experiments showed that an hybrid scheme (crystal radiator + amorphous converter) is advantageous. Such a solution allows a limited energy deposition in the crystal, preserving its qualities (as the available crystal potential decreases with temperature rise).


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THE BASIC SCHEME FOR ILC & CLIC

e-

Crystal Amorphous

e+, e-, e-

e+

“x” meters

Only the photons are impinging on the converter: that limits the energy deposition in the amorphous target. The yield is less than if the particles coming from the crystal were also impinging on the amorphous target

Radiator Converter


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* THE CRYSTAL : Photons generated by electrons in channeling conditions and above barrier.

* The crystal, axially oriented, may be W, Si, Ge, C(d)… In our case, we choose W in <111> orientation * THE AMORPHOUS CONVERTER: it is made of W * THE DISTANCE RADIATOR-CONVERTER: it is of some

meters; here it is 2 meters. It allows the use of sweeping magnet in between. Another possibility is to select also charged particles coming from the radiator (e+, e-) with energy larger than Etreshold to increase the yield e+/e-

* IMPINGING ENERGIES: E- = 3, 4, 5 and 10 GeV


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crystal

e-

ERL Option

e-,e+

Target

Capture Section (AMD, Accelerator, Solenoid)+ Bunch Compressor

Up to 270 MeV

Up to 2.4 / 5 GeVsuperconducting linac

with quadrupole focusing

e+

3 to 10 GeV superconducting linac

e-

GENERAL SCHEME: For CLIC, the preaccelerator may stops at 200 Mev; the DR is at 2.4 GeV


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SIMULATIONS Incident beam: electron beam energy between 3 and 10 GeV and

transverse rms radius of 1 and 2.5 mm have been considered Targets: (a) Crystals of 1, 2 and 4 mm thick: for E-=10GeV Amorphous target of 8 mm (b) Crystals of 1.4, 2.4 et 4.4 mm thick for E- =3, 4 and 5 GeV Amorphous target of 10 mm thick Capture system: an Adiabatic Matching Device with a magnetic

field decreasing from 6 Teslas to 0.5 Teslas on 50 cms. Iris aperture is ~20mm radius. Accelerating field is 18 MeV/m peak value [SW]

Outputs: Simulations have been carried out corresponding to the general scheme. The yield e+/e-, the transverse emittance as the longitudinal emittance have been determined at the end of the solenoid (270 MeV). The capture efficiency has also been determined.


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RESULTS Selected results are concerning: 1) An incident energy E- of 10 GeV with 1 and 2.5 mm

rms beam radius and a 1 mm thick W crystal on <111> axis. The amorphous target is 8 mm thick [ILC source].

2) Incident electron energies of 3, 4 and 5 GeV with 1 and 2.5 mm rms radius and 1.4 mm thick W crystal. The amorphous target is 10 mm thick [CLIC source].

The results of the simulations are summarized in the table below


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TABLE OF SIMULATION RESULTS FOR DIFFERENT ELECTRON ENERGIES


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POSITRON YIELD FOR DIFFERENT INCIDENT ENERGIES

Two rms radius values for the incident e- beam: 1 and 2.5 mm

The yield values are associated to hybrid sources defined above:10 GeV/1mm crystal & 8mm am.3, 4, 5 GeV/ 1.4 mm crystal & 10mm am.

Accepted yield

Incident energy


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THE PROBLEM OF PEDD

The local and almost instantaneous energy deposition in a target (for instance during a pulse duration) may be very critical for the target survival. Indeed, due to inhomogeneous energy deposition in the target, thermal gradients causing mechanical stresses lead to target destruction as by shock waves. After the SLC target destruction, analyses showed that a maximum value of 35 J/g (in tungsten) must not be exceeded. So, an accurate simulation of the energy deposited in the target has to be worked out dividing the target in elementary domains (typically, annular disks with radius increments of tenths of mm and thickness of tenths of Xo). The energy deposited in each domain is calculated and comparisons made with the maximum allowed value. The PEDD is strongly depending on the incident beam intensity and energy and on its transverse dimensions; it depends also on the thickness of the target.


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SOME PEDD VALUES We give here some values of the PEDD corresponding to one

electron in GeV/cm3 and will precise later the actual values for precise applications (ILC and CLIC).

From X.Artru et al: Polarized and unpolarized positron sources… (NIMB,2008)

10 GeV

5 GeV

10 GeV

5 GeV


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SOME PRELIMINARY CONCLUSIONS * Concerning the positron yield: the highest the

incident energy, the highest the yield. The accepted yield is higher with smaller electron beam radii (geometrical acceptance)

* Concerning the PEDD: the larger the incident beam size, the smaller the PEDD

=> a compromise is to be worked out to have enough yield with limited PEDD. The choices will be made depending on the particularities of the colliders.


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POSITRON SOURCES USING CHANNELING FOR ILC & CLIC1-CLIC

Hybrid scheme


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1-CLIC: ACCEPTED POSITRON YIELD * For an incident e- beam with = 1 mm => 1 e+/e- * For an incident e- beam with = 2.5 mm => = 0.9 e+/e- PEDD Assuming an incident e- pulse of 2.34 1012 e-, we have : CRYSTAL AMORPHOUS PEDD/e- PEDD/total PEDD/e- PEDD/total (GeV/cm3/e-) J/g (GeV/cm3/e-) J/g

mm 2 38 2.5 48.5 =2.5mm 0.35 6.8 0.8 15.5 An entirely amorphous target, 9 mm thick, with the same incident e-

beam would have provided the same accepted yield and a PEDD of 150 J/g (=1mm) or 40 J/g (=2.5 mm). This shows the advantages of an hybrid scheme leading to a unique target with a PEDD < 35 J/g using an e- beam with = 2.5 mm.


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SIMULATION RESULTS AT END OF CLIC PREACCELERATOR

TRANSVERSE EMITTANCES AT 270 MeV (mm); for -=2.5mm, the rms emittance areasare only slightly different, the shapes being the same (see figures below).

Blue: 80%

Red: rms

x=10mmmrad

y=11mmmrad


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TRANSVERSE EMITTANCES FOR CLIC (2.5mm)

Blue: 80%Red: rms

x=11mmmrady=12mmmrad


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LONGITUDINAL EMITTANCE AT END OF CLIC PREACCELERATOR [1mm]

Blue: 80%Red: rmsz =9.5cmMeV

For 2.5 mm, the emittance shape is very similar and the rms area almost the same


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PROPOSED POSITRON TARGET FOR CLIC

e-

Crystal Amorphous

e+, e-, e-

e+

2 m

(5 GeV)

W: 1.4 mm thick W: 10 mm thick

With an incident beam of 2.34 1012 e-/pulse, we expect 2.1 1012 e+/pulse at 270 MeV (pulse of 156 ns) Or 6.7 109 e+/bunch


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2-A CRYSTAL SOURCE FOR ILC At energies of some GeV the electron motion in the axial fields of an

aligned crystal may be compared to the electron motion in an helical undulator. However:

* in the crystal the transverse motion is not regular: instead of a circle we have the so-called rosette motion * The period of motion in an undulator is that of the undulator ~ cm. The emitted photon wavelength is ~The period of motion in a crystal is ’~m;the photon wavelength is ’~’/2

There is a factor 10-4 on the wavelengths (’/’) and so, a factor 10-2 on

the needed energies. So, instead of an energy of 200 GeV as for the

undulator, we need only some GeV. This system can be tested before the

LC is completed. * In order to have a high yield, we choose 10 GeV as incident electron energy.


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ILC: what is changed from undulator to crystale+ target The magnetic undulator is replaced

by an atomic undulator /crystal

incident electron energy is 20Times less=> 10 GeV

This is not a proposed scheme, but only a presentation of the modifications


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INCIDENT BEAM: an incident electron beam of 10 GeV TARGETS: CRYSTAL: a 1 mm thick W crystal <111> orientation AMORPHOUS: a 8 mm thick amorphous target CAPTURE SYSTEM: AMD with decreasing field from 6 to 0.5 Tesla on

50 cms Accelerating field is 18 MeV/m, peak [SW] RESULTS: accepted yield: 1.8 e+/e- (mm) 1.5 e+/e- (2.5 mm) PEDD: assuming an incident e- bunch of 2. 1010 e- crystal amorphous PEDD/e- PEDD/bunch PEDD/e-

PEDD/bunch mm 2 GeV/cm3 0.33 J/g/bunch 7.5 GeV/cm3 1.25 J/g/bunch 2.5mm 0.35 GeV/cm3 0.058 J/g/bunch 2 GeV/cm3 0.33 J/g/bunch

It is quite clear that the hybrid target cannot sustain the 2820 bunches and that distributed targets system must be considered.


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EMITTANCES AT END OF ILC PREACCELERATOR

Transverse emittances at 270 MeV (mm)

Blue: 80%

Red: rms

x,y = 11 mmmrad rms


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TRANSVERSE EMITTANCES AT ILC PREACCELERATOR

(2.5 mm)

Blue: 80%Red: rms

x=11mmmrad

y= 12 mmmrad


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EMITTANCES AT THE END OF ILC PREACCELERATOR LONGITUDINAL EMITTANCE [1mm]

Blue: 80%

Red: rms

z=9.4cmMeV (rms)


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PROPOSED SCHEME FOR ILC: ONLY IF MULTITARGET

e-

Crystalmultitarget

Amorphousmultitarget

e+, e-, e-

e+

2 meters

With incident beam of 2.1010 e-/bunch the expected accepted e+ yield is of 3.7 1010 e+/bunch at 270 MeV1mm). It is of 3.1010 e+/bunch for 2.5mm


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SUMMARY AND DISCUSSION * Hybrid schemes using a crystal as a radiator and an amorphous target

where only coming from the crystal are sent, have been studied for CLIC and ILC as “conventional” solutions. The simulation results showed accepted yields in accordance with the requirements. The emittances at the end of the preaccelerator (270 MeV) can be improved through optimization of the capture section and bunch compression in the longitudinal phase space; energy compression can also been considered in view of efficient stacking in the damping ring.

* One of the main concerns for positron targets is the energy deposited. As demonstrated in recent papers, the total energy deposited is less for a purely cristalline target than for the equivalent (giving the same yield) amorphous target. The Peak Energy Deposition Density (PEDD) is comparable. But using hybrid targets with a separation between the crystal and amorphous parts giving the possibillity of sweeping off part or all the charged particles, allows lowering of the PEDD and henceforth, avoiding multi-target use. That solution can be worked out with CLIC. Concerning ILC, the very intense pulse (> 1013e-) precludes use of a unique hybrid target: both crystal and amorphous systems are multitargets; that makes the crystal solution hard to work out.


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SUMMARY AND DISCUSSION (continued) * Optimization ? The optimization of the hybrid scheme concerns the - the choice of a threshold in energy allowing the charged particles coming out from the crystal to impinge on the amorphous target, - the optimization of the distance between the crystal and

the amorphous target Both optimizations have consequences not only on the yield

but also on the PEDD. Concerning the incident beam, its energy, intensity as well as

its transverse size may be optimized with respect to the yield and PEDD.

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