1. P.Sievers/POSIPOL2011/Beijing 1 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE FOR ILC P.Sievers (CERN), X.Artru, R.Chehab, M.Chevallier (IPNL), O.Dadoun, A.Variola (LAL), C.Xu (IHEP and LAL), V.M.Strakhovenko (BINP) Thanks to L.Rinolfi, A.Vivoli (CERN), T.Kamitani, T.Omori, T.Suwada, J.Urakawa (KEK),T.Takahashi (Hiroshima-U)

2. P.Sievers/POSIPOL2011/Beijing 2 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE PLAN * 1- Introduction * 2- The hybrid positron source: a recall * 3- The granular amorphous converter * 4- The simulation results * 5- Technical Solutions * 6- Summary and discussion

3. P.Sievers/POSIPOL2011/Beijing 3 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE 1-INTRODUCTION The hybrid positron source uses an oriented crystal where channeling radiation and also coherent bremsstrahlung provide a large amount of soft photons and an amorphous target, where these photons materialize into e+e- pairs [ theoretical works from BKS* and others and Experiments at CERN and KEK]. The hybrid source allows important decrease of the PEDD (Peak Energy Deposition Density) provided the charged particles emerging from the crystal are swept off. However, the average heating of the target has still to be lowered. For this, rotating target wheels have been considered. High rotation speeds lead to technical difficulties (Eddy currents brought by pulsed magnetic lenses, rotating vacuum seals,..). One is therefore led to limit the wheel speed, hence the tolerable amount of thermal power deposited in each target of the wheel: that implies also an appropriate time structure of the beam. An interesting solution using granular targets, developed for Neutrino factories projects and in Spallation sources [See P.Sievers’s references] can be also interesting for positron sources. We present here an hybrid positron source using an amorphous granular substance as converter. This principal will be applied to the ILC project. *BKS: Baier-Katkov-Strakhovenko

4. P.Sievers/POSIPOL2011/Beijing 4 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE 2-THE HYBRID POSITRON SOURCE: A RECALL A High energy electron beam (5 to 10 GeV) is directed along the crystal axis. Photons, electrons and positrons are generated. A sweeping magnet takes off the charged particles and only the photons impinge on the amorphous converter, placed about 2 m further downstream. For the ILC we consider an e-beam at 10 GeV with a transverse rms-width of σ=2.5 mm. R.Chehab, V.M.Strakhovenko, A.Variola

5. P.Sievers/POSIPOL2011/Beijing 5 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE 3-THE GRANULAR AMORPHOUS CONVERTER As already pointed out (see P.Pugnat, P.Sievers) [J.Phys.G.Nucl.Part.Phys.29 (2003)1797-1800] a granular converter made of small spheres of ~ mm radius offers the advantages of presenting a relatively high [surface/volume] ratio which is interesting for the power dissipation. Staggered rows of spheres have been considered, which leads to an effective density of about 72 % of pure W. The densest possible packing would be 85 % with special arrangement. A comparison has been carried out between the granular and the compact targets concerning the yield and deposited energy.

6. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE 4-THE SIMULATION RESULTS Simulations have been made with two dedicated programs: - the simulation program of V.M.Strakhovenko (VMS), taking into account the crystal effects and embedded into GEANT4. - the program G4FOT using the simulation program FOT of X.Artru, and embedded into GEANT4. Both dedicated programs are in good agreement. We shall present the results without specifying the origin of the program. Results are presented for the total and the accepted yield, the positron energy, the transverse size distribution and emittance, the time duration of the bunch,… A table giving the compared features of granular and compact targets is given below. P.Sievers/POSIPOL2011/Beijing 6

7. 7 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE COMPARISON: GRANULAR & COMPACT thicknessyieldPEDD(ΔE)depN-layers spheres number Effective density unitymm e+/e- GeV/cm 3 / e- MeV/e-g.cm -3 compact 813.32.1852319.3 Granular r=1mm 10.16 12.181.88446386413.9 Granular r= 0.5mm 11.6013.452.336137806413.9 The number of sphere layers is counted along the longitudinal axis

8. P.Sievers/POSIPOL2011/Beijing 8 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE Comments on the table: * The granular target is thicker than the compact for the same yield, as the effective density is lower * The energy deposited increases with the thickness * The PEDD values are depending on the elementary volume; for volumes from 1 to 4 mm 3 the differences are within 20 %. * The effective density of the two granular targets (r=1 mm and r= 0.5 mm) is the same as every granular target is divided in a number of mini-cubes, containing, each, 8 spheres. * An additional table gives information on the positron characteristics ( mean energy, transverse dimension and momentum, time duration,..) * The choice of the granular targets have been made to approach the values of the positron yield of the compact, 8 mm target.

9. P.Sievers/POSIPOL2011/Beijing 9 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE COMPARISON BETWEEN GRANULAR AND COMPACT TARGETS We can notice that the PEDD values are different for the 3 cases. A probable explanation is due to the difference of elementary volumes where it is determined: Granular (r=0.5 mm): volume=0.5 mm 3 and PEDD= 2.33 Compact volume= 1 mm 3 and PEDD= 2.18 Granular (r=1mm): volume =4 mm 3 and PEDD = 1.88 VMS-ILC Granular r=1 mm l = 10.165 mm Layers = 3 Granular r=0.5 mm L =11.6 mm Layers=7 Compact l = 8 mm position Target endCapture endTarget endCapture endTarget end Captur e end rmsx/mm3,416768,57813,418458,579333,142298,545 Px/MeV6,443693,076276,362282,831376,467213,125 Energy/MeV24,240619,008123,941218,632524,069618,77 Time/ps11,314640,501411,245342,555910,783443,33 meanEnergy/MeV28,14319,478127,370318,745727,959318,74 yielde+/e-12,18844,297213,45444,643213,264,53 PEDD pedd/(GeV/cm^3/ e-)1,878582,338012,18018

10. P.Sievers/POSIPOL2011/Beijing 10 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE COMPARISON GRANULAR/COMPACT TARGET The yield and total deposited energy/incident electron have been simulated for a compact and a granular target (w.r.t. the number of layers ). We can see, here, that a granular target with 3 layers has slightly lower yield than the compact one (8 mm thick) and the energy deposited is also lower. The 3-layer granularity seems convenient..

11. A GRANULAR CONVERSION TARGET… SIMULATION RESULTS FOR THE PHOTON BEAM ON THE GRANULAR TARGET The transverse (x-y) dimensions and the marginal photon distribution in x-plane are represented below: the γ have been created by an electron beam of 10 GeV (having an rms radius of 2.5 mm ) impinging on a 1mm thick tungsten crystal oriented on its axis. P.Sievers/POSIPOL2011/Beijing 11 X-Y distribution of γ beam on granular targetMarginal photon distribution on X-plane

12. P.Sievers/POSIPOL2011/Beijing 12 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE SIMULATION RESULTS FOR THE POSITRON BEAM: the ILC case The positrons created in the granular target (3 layers) are captured by an AMD. The AMD has the following characteristics:6T-0.5Tand L=50cm. Energy spectrum, transverse dimensions and momentum are given. At the target exit After capture and 1 meter acceleration

13. A GRANULAR CONVERSION TARGET… POSITRON EMITTANCE WITH THE GRANULAR TARGET The target is made of 3 layers of spheres (Φ= 2 mm); the case is for ILC P.Sievers/POSIPOL2011/Beijing 13

14. P.Sievers/POSIPOL2011/Beijing 14 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE SIMULATION RESULTS: ENERGY DEPOSITED AND PEDD (ILC) The precise configuration is described below for the case of ILC. In order to lower the amount of energy deposited per pulse and per second - we divide the original pulse into many minipulses - we choose a multi-target system on a rotating wheel The pulse configuration is that of Omori/KEK known as the 300 Hz solution. The new ILC data is taken with ~ 1300 bunches shared into 13 Mini trains with a separation of 3.3 ms. Each mini train contains 100 bunches. The duration of the macro pulse is 40 ms, leaving 160 ms for damping in DR.

15. P.Sievers/POSIPOL2011/Beijing 15 A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE THE SIMULATION RESULTS: ENERGY DEPOSITED AND PEDD (ILC) For the chosen system (300 Hz) We have determined the energy deposition density in the (x,z) plane; where z is the propagation axis. The spheres have 1 mm radius and correspond to a volume of 4mm 3. The largest value (PEDD) is 1.8 GeV/cm 3 /e-; it is slightly lower than that of a compact target. The total energy deposited in the target is ~ 446 MeV/e- for the 3 layers target.

16. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE The case of small spheres (r=0.5 mm) We report the energy deposition density The maximum PEDD is About 2.33 GeV/cm3/e- The PEDD is higher than in the case r=1 mm (1.88). P.Sievers/POSIPOL2011/Beijing 16

17. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE 5- The heating and cooling of a granular target (for ILC) In granular targets, consisting of densely packed spheres with sizes smaller than the rms- transverse beam profile of 3.5 mm, only small and essentially linear temperature gradients will be created across each individual sphere. Since this will be able to expand relatively freely it will be submitted to negligible thermal stresses. Thermally induced shocks can be neglected when: R/2<<to.c; R: radius of the sphere of 1 mm, to: Pulse duration of a micro pulse of 0.6 micro seconds; c: velocity of sound in Tungsten of c=4.10 3 m/s. For ILC this is satisfied in good approximation. Another important feature of the granular target is that the deposited heat can be evacuated rapidly and at the location of heat deposition by the cooling fluid passing between the spheres. P.Sievers/POSIPOL2011/Beijing 17

18. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE A GRANULAR TARGET DISTRIBUTED ON A ROTATING WHEEL A stationary target would receive a too large amount of deposited energy, leading to serious heating problems. -To reduce the adiabatic temperature rises induced by one macro pulse, the beam energy can be diluted by sweeping, rotating the target. Since the deposited beam energy is concentrated within a diameter of about 1 cm ( see previous plot), with a linear velocity of the rim of the rotating wheel of about 3 m/s, the rim is displaced by 1 cm over 3.3 ms, so that the energy of each micro pulse is separated from the adjacent, following micro pulse. Only very little pile up occurs. The calculated maximum temperature rise in the target rim, resulting from the PEDD per micro pulse, is not exceeding 222 K. Details on the wheel and on the target container (Be and Ti) are presented below. P.Sievers/POSIPOL2011/Beijing 18

19. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE THE GRANULAR WHEEL TARGET P.Sievers/POSIPOL2011/Beijing 19

20. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE The Granular Target Container The W spheres are filling a ring container made of a light element as Titanium, Beryllium in order to lower the energy deposition and the multiple scattering. The entrance and exit windows (1 mm thick in beam direction) are Be whereas the upper and lower container structures are made of Ti. The data obtained for the hybrid source in ILC conditions [E- =10 GeV, L(crystal) = 1 mm, Granular target with 3 layers] is: Energy deposited PEDD MeV/e- GeV/cm 3 /e- Be/up 0.12 0.0016 Be/down 10.6 0.17 Ti/up 0.14 0.0021 Ti/down 0.14 0.0021 The beam parameters are for ILC: E-=10 GeV; L(crystal)=1mm; Distance Rad-Conv =2 m The PEDD in the downstream window has been calculated as 30.6 J/g per micro pulse which leads to an adiabatic temperature rise of 16.3 K/micro pulse. This low temperature is clearly due to the low ionization rate dE/dx in Be. Also the thermally induced stress per micro pulse are about 30 MPa, 13% of its yield strength. Material fatigue with about 3 Mio thermal cycles per 100 days of continuous operation and radiation damage may however limit their lifetime. Similarly, fatigue of the W-spheres, although considered as stress free, may occur due to the temperature pulses of 222 K/micro pulses, again at a rate of 3 Mio/100 days. P.Sievers/POSIPOL2011/Beijing 20

21. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE COOLING OF THE ROTATING WHEEL -The minimum periphery of the rotating wheel would be 13 cm, i.e. a diameter of 4 cm, to distribute uniformly a macro pulse over its rim over 40 ms. This results in a rotation frequency of 23 Hz or 1400 rpm, which may be difficult to achieve with rotating seals. Therefore a larger wheel with a diameter of 58 cm is considered. With a velocity of the rim of 3.25 m/s, allowing for comfortable separation between micro pulses, this leads to a rotation at 1.786 Hz or 107 r.p.m. which will be much easier to engineer. With this configuration and taking into account the repetition frequency of the macro pulses of 5 Hz, it results that, in the average, each sphere will be hit every 5 turns, i.e. every 2.8 s, leading to 3 Mio. hits per 100 days of operation. The total, average power, to be removed from a wheel (and from a stationary target) is about 10 kW. With Helium, pressurized to 1 MPa and with an entrance velocity of 10 m/s, a He mass flow of 30 gr/s is required which would result in an average temperature rise of 84 °C at the exit of the He- flow. Temporary and locally, values of 250 °C in the He may, however, occur at the hottest sphere just after a micro pulse. Clearly, the wheel must be made vacuum tight and resist to the internal He-pressure of 1 MPa. Moreover, with the above specified He-cooling, the time constant of the exponential decrease in temperature of an adiabatically heated sphere is about 100 ms. Thus, a sphere will be cooled to practically zero before being hit again after 2.8 s. P.Sievers/POSIPOL2011/Beijing 21

22. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE PROBLEM OF EDDY CURRENTS WITH ROTATING WHEEL The interference of the magnetic fringe field of the flux concentrator with the adjacent rotating, metallic structure of the wheel have been studied for very fast rotating wheels ( I.Bailey et al, Durham ILC e+ Meeting, Oct. 2009). Thus at the above assumed velocity of several m/s these problems should be much reduced. Moreover, the high electrical resistivity of the pack of W-spheres and that of the Ti-alloy should help. Further studies will be required, in particular of eddy currents, possibly induced in the Be-disk and among the W-spheres. P.Sievers/POSIPOL2011/Beijing 22

23. A GRANULAR CONVERSION TARGET FOR THE HYBRID POSITRON SOURCE AN ALTERNATIVE SOLUTION TO THE WHEEL: THE PENDULUM - To avoid rotating seals, wobbling or trolling targets have been devised (Durham ILC e+ Meeting, Oct. 2009) where the displacement of the target structure from the outside into the vacuum is made via flexible, vacuum tight bellows. The injection of the cooling fluid can thus be ensured through a rigid, non rotating structure. In the following we consider a “Pendulum Target”, where the required displacement and velocity is provided by the sinusoidal oscillation of the target. The width of the target is 13 cm, providing space for 13 micro pulses with a diameter of 1 cm each and being displaced, when the beam is hitting the target, at a velocity of about 3 m/s over +/- 7.5 degrees. The total swing is +/- 23 degrees, allowing for comfortable inversion of the direction of the movement during the “off beam” time of 160 ms. The same range in angle must be sustained by the bellows oscillating at 2.5 Hz. Since in this configuration the average time between hits of the same target spot is about 0.2 s as compared to the rotating wheel with 2.8 s, average temperatures higher by about 100 K will result. This might, if necessary, be compensated by an improved He cooling. Clearly, prototyping will be required, also to assess the life time of the spheres, the windows and the bellows, submitted to 40 Mio. cycles over 100 days of continuous operation. P.Sievers/POSIPOL2011/Beijing 23

24. A GRANULAR CONVERSION TARGET… THE PENDULUM GRANULAR TARGET P.Sievers/POSIPOL2011/Beijing 24

25. A GRANULAR CONVERSION TARGET… CONCLUSIONS * The hybrid target with a granular amorphous converter exhibits: - a good quality positron beam (yield, emittance) generated by photons from channeling radiation - a lower PEDD resulting from the hybrid configuration - an efficient thermal dissipation process due to the granular character. In particular, the enlargement of the macro pulse to 40 ms duration in the above considered « 300 Hz-KEK-System » and leaving a gap of « no-beam » of 160 ms, sufficient for efficient damping, opens the possibility to reduce the thermal load from the macro pulse on the target by displacing it laterally through rotation or trooling it. Lateral velocities of about 3 m/s for this displacement are sufficient to reduce the local energy deposition density by a factor of 13. Using granular targets, consisting of an ensemble of Tungsten spheres of 2 mm diameter or below and cooled by a Helium gas stream passing between the spheres, tolerable temperatures for Tungsten, Helium and the target container are achieved. In addition to a rotating wheel, a pendulum target is considered, by which the use of rotating vacuum and He-seals is avoided. Both target types seem to represent viable solutions for the positron production in the considered ILC scheme. P.Sievers/POSIPOL2011/Beijing 25