1. Status: Cooling of the ILC e+ target by thermal radiation POSIPOL 2015, Cockroft Institute, Daresbury, UK 2 nd September 2015 Felix Dietrich (DESY), Sabine Riemann (DESY), Peter Sievers (CERN), Gudrid Moortgat-Pick, Andriy Ushakov (Hamburg U)

2. Radiative thermal cooling of e+ target Status –Target temperature –Design considerations, mechanical issues, eddy currents –Our next steps Summary Short remark on helicity reversal Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 2

3. Ti6Al4V Radiator (Cu) Cooler (Cu) photons 3 Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling Radiative cooling – considered so far Rotating target wheel consists of Ti rim (e+ target) and Cu (radiator) Heat path: –thermal conduction Ti  Cu wheel –Thermal radiation of Cu to stationary water cooled coolers Target, radiator and cooler are in vacuum Cooling area can be easily increased by additional fins Felix Dietrich

4. e+ target for E cm = 250 … 500GeV E cm and luminosity determine deposited energy (P e+ ≤30%) Target wheel design appropriate for all E cm ? –radiative surface A sufficient to remove deposited energies W for all E cm ? –Thermal contact Ti-Cu radiator is important (size and quality) Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 4 E beam [GeV] E dep [kW]  T max /pulse [K] E dep [kW]  T max /pulse [K] Nominal luminosityHigh luminosity 1205.066-- 175 (ILC EDMS) 3.9125-- 250 (ILC EDMS) 2.01304.1195 2502.3854.6165 A. Ushakov, Update 2015 A. Ushakov, 2015

5.  = Stefan-Boltzmann constant = 5.67 × 10 -8 W/(m 2 K 4 )  = emissivity = 0.7 G = geometric form factor = 1 Area A of thermal radiation Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 5 T[°C] A[m 2 ] Studies so far(ANSYS) : Felix Dietrich: –A ≈ 2.8 m 2 –‘outer’ radiator with fins + cooler – P = 2kW, 4kW Andriy Ushakov: –A = 1.76 m 2 1.51 m 2 (Cu) + 0.25 m 2 (Ti) –Full copper disc, no fins; radiation ambient –P = 2.3kW To be checked: Pol upgrade

6. Heat transfer target - radiator Temperature distribution in Ti target depends on –deposited energy –height of target rim –heat transfer from Ti to Cu radiator, i.e. contact area target-radiator Details see Felix Dietrich’s talk. heat transfer determines also time until equilibrium temperature is reached: –Photon beam hits after ~7seconds the same position at the target –Thermal diffusion vs time: x Ti = 0.68 cm for 7s; 2.2 cm for 70s x Cu = 2.8 cm for 7s; 9.0 cm for 70s –7s are not sufficient to remove the heat from target –temperature accumulates over many bunch trains up to equilibrium  Temperature gradient from beam path area to contact surface to radiator Heat flux density through Ti – Ci contact –Max power per bunch train: 7kJ/5 = 1.4kJ  average energy deposition in 7s at same target area: 200 W  heat flux density Ti  Cu is 12.5W/cm 2 for 16 cm 2 Ti-Cu contact surface Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 6 Ti Cu

7. Equilibrium temperature distribution in target Temperature evolution over time, ANSYS (see also Felix’ talk): E cm = 500GeV; E dep = 2.3 kW Radiator area ~2.8 m 2 After ~3 hours equilibrium temperature distribution reached Target height  max temperature T max ≈ 680°C (a = 5cm) T max ≈ 570°C (a = 4cm)  Distance beam path to Ti-Cu contact is important Temperature in Cu (near Ti) ≈280°C (a = 5cm), ≈270°C (a = 4cm) Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 7 2cm a a = 5cm a = 4cm Ti Cu Felix Dietrich Max average temperature in Ti target (~ area along beam path)

8. So far, Felix considered a cooling area of 2.8 m 2  Max average temperature in target goes down if almost full disc is included in radiative cooling (+ 0.7m 2 without fins) Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 8

9. Heat flux through Ti-Cu contact Andriy Ushakov (2014) P = 5.17 kW,  Cu = 0.7,  Ti = 0.25  max heat flux thru Ti-Cu contact ≈ 10 W/cm 2 Felix Dietrich : P = 2.3 kW,  Cu =  Ti = 0.7 Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 9 Ti Cu 2cm 4cm Ti Cu 2cm 5cm Ti Cu Ti Cu

10. Stress at Ti-Cu contact Thermal expansion and resulting distortion/stress at the Ti-Cu contact depends on temperature profile in radiator and target  Ti = 8.6 × 10 -6 /K  Cu = 16.5 × 10 -6 /K –Max temperature in Ti wheel ~500°C   h ~ 0.43% –Max temperature in Cu radiator (near Ti-Cu contact) is ~200°C   r ~ 0.33% Additionally, stress due to centrifugal force –Not yet taken into account Hea t transfer at contact depends on –Surface roughness –Contact pressure –Are there substances, vacuum-capable, to minimize thermal resistance? –Bolted Ti-Cu contact seems to be better than brazed contact contact must stand thermal fluctuations without degradation Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 10 Simulation (prel.) by A. Ushakov brazed Ti-Cu contact assumed P= 5170W,  Cu = 0.7,  Ti = 0.25  v.Mises stress <200MPa

11. Stress at Ti-Cu contact Simulation (prel.) by A. Ushakov brazed Ti-Cu contact assumed Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 11 v.Mises tress at Ti –Cu contact <200MPa Max values (ANSYS) appear at outermost region of contact. Ti target Cu radiator

12. Stress at Ti-Cu contact Thermal expansion and resulting distortion/stress at the Ti-Cu contact depends on temperature profile in radiator and target  Ti = 8.6 × 10 -6 /K  Cu = 16.5 × 10 -6 /K –Max temperature in Ti wheel ~500°C   h ~ 0.43% –Max temperature in Cu radiator (near Ti-Cu contact) is ~200°C   r ~ 0.33% Hea t transfer at contact depends on –Surface roughness –Contact pressure –Are there substances, vacuum-capable, to minimize thermal resistance? Contact must stand thermal fluctuations without degradation  Bolted Ti-Cu contact is ok, brazed contact is not –bolting at 10 MPa could allow frictionless, lateral thermal expansion maintaining the at the thermal contact (P. Sievers) To be done: implement ‘real’ contact in ANSYS simulations. experimental tests Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 12

13. Mechanical aspects Mass of the wheel (Cu radiator) ~100 kg Energy E stored in the wheel ≈ 0.5MJ  safety rules ! I = moment of inertia ~ 20kgm 2  = angular frequency Stress (average) in radiator due to centrifugal force ~112 MPa (1/0.4 = formfactor for this shape of flywheels) Stress at the radiator fins due to centrifugal force – mass of one fin: ~4kg –Centrifugal force: F = m  2 r ~ 80 kN –Stress at each fin = F/A ~ 5 MPa Next steps: Alternative (lighter) radiator material? Design optimization with smaller (no?) fins Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 13

14. In principle, Ti keeps its stability in the temperature range 200 - 635°C (http://form-technik.biz/titan-material-eigenschaften/)http://form-technik.biz/titan-material-eigenschaften/ Stability of Cu decreases with temperature, but this shouldn’t be a problem for T ~ 200 and choice of special alloy –Permanent stress due to tangential force  creeping effects ?  to be checked Thermal expansion  slightly changed momentum of inertia  wheel slows down slightly if no correction Non uniform average temperatures around the wheel  non symmetric thermal deformations  unbalances –Temperature raise up to equilibrium (~3h) could cause imbalances –unbalances have to be monitored and corrected; has to be discussed with engineers Long term material degradation due to irradiation must be tested experimentally Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 14

15. Eddy currents B field at the target  additional heat load Eddy current tests at UK Zang, http://research- archive.liv.ac.uk/1445/2/ZangLei_Sept2010_1445.pdf,.http://research- archive.liv.ac.uk/1445/2/ZangLei_Sept2010_1445.pdf Bailey et al., Proceedings of IPAC2010, Kyoto, Japan  expect ~8 kW energy deposition in the Ti rim rotating with 2000rpm through a field of B = 0.5T ILC: duty cycle of 5×1/1000 (5×1ms), –heat load is only 8W per bunch train distributed to a length of ~10cm of the spinning rim Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 15

16. Eddy current estimations (P. Sievers) Input: Target velocity v = 100 m/s magnetic pulse: flat top, duration of 1 ms; rise and fall times of about <0.2 ms. B = 0.5T at target, considered as constant over radius of 10 mm repetition rate 5 Hz. target cross section: height h = 2 cm, thickness d = 1.4 cm Electrical conductivity (Ti-alloy): σ= 5.3×10 5  m) Results: –Torque  = F×R = 37 Nm –Average power: W = F v = 7.4 kW; duty cycle 5∙10 -3  W = 37 W –High momentum of inertia of the wheel  slow mechanical response Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 16

17. Eddy currents … In addition: intermittent force (~100N in 1ms) on the target shaft and bearings resulting from the pulsed braking power  tight control of the wheel velocity, the motor torque and damping if possible  vibrations could be important and have to be studied Conclusion No problem with eddy currents at undulator source. In case of longer duty cycle values have to be updated. Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 17

18. Magnetic Bearings No news yet Experience exists over 30 years for the use of magnetic bearings (see P. Sievers’ talk @POSIPOL’14) Industrial suppliers are SKF/Gemany/Calgary/Canada and KFZ/Juelich/Germany Loads above 100 kg with more than 7000 rpm are possible Temperatures of up to 300°C can be accepted by the bearings Active vibration control of the axis at the magnetic bearings is available Thermal barriers should be arranged to prevent heat flowing into the rotation axis Very precise velocity control is standard Prototype with magnetic bearings for cooling with sliding pads is also benchmark for radiative cooling  We should agree the parameters to be tested Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 18

19. Our next steps Temperature evolution in the whole system stress at the wheel (including radiator) optimize system (target + radiator) to be flexible for all energies and luminosities –Thermal contact target-radiator –Aspects important for magnetic bearings (forces, imbalances,…) –Figure out safety factors Target material tests –We get support from German Ministry of Science –Period: July 2015 – 2019 (manpower) –DESY Zeuthen: 1 postdoc 2016/17 (2 years) Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 19

20. Material tests at Mainz First step: Tests at MAMI –3.5kW beam power, E=3.5MeV –No problems with radiation protection –Study material response to cyclic load Later step Tests at MESA (>2017), E≈15MeV Simulations to prepare tests will be done by DESY/ Uni HH; preparation and set up of experiments will be done in Mainz Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 20 K. Aulenbacher

21. Summary Radiative cooling should work, no showstopper identified Scheme is under study & optimization –Prototyping : Desired: design an experimental mock up in real size DESY/Uni HH: no hardware resources Collaboration with ANL/… ?! –Material tests at Mainz (e- beam) Also important: polarization issues –Upgrade to higher polarization –Spin flipper Our Resources –Manpower: Felix Dietrich (master student, until autumn 2016), Andriy Ushakov (contract until 2017(+)), SR. + postdoc @ Zeuthen from 2016 Peter Sievers 21 Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling

22. Last but not least: helicity reversal for e+ ILC project meeting at DESY, 28 Sept 2015: Nick Walker presented news from ILC AD&I status … spin flipper… OTDR:ur statement concerning spin flipper: Helicity reversal is essential to control systematic errors  second rotator line is required Design exists – see L.I. Malysheva et al., “The Design of spin-rotator with a possibility of helicity switching for polarized positron at the ILC,” Proc. IPAC’12, (TUPPR003) Depolarization in damping ring is very difficult –See talks of L. Malysheva and Valentyn Kovalenko at previous e+ source meetings Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 22

23. Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 23 N. Walker, B. List; DESY ILC project meeting 28 Sep 2015

24. Spin flipper L.I. Malysheva et al., http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/TUPPR003.PDF http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/TUPPR003.PDF Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 24 –Studies done so far for sc solenoid (8.32m) –Have to be updated for nc 40m solenoid

25. Last but not least: helicity reversal for e+ (cont’d) ILC project meeting at DESY, 28 Sept 2015: Nick Walker presented news from ILC AD&I status … spin flipper… Our statement concerning spin flipper: Helicity reversal is essential to enhance luminosity and to control systematic errors  second rotator line is required Design exists – see L.I. Malysheva et al., “The Design of spin-rotator with a possibility of helicity switching for polarized positron at the ILC,” Proc. IPAC’12, (TUPPR003) http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/TUPPR003.PDF http://accelconf.web.cern.ch/AccelConf/IPAC2012/papers/TUPPR003.PDF Depolarization in damping ring is very difficult –See talks of L. Malysheva and Valentyn Kovalenko at previous e+ source meetings Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 25

26. Thank you! Riemann, Dietrich, Sievers, Ushakov POSIPOL 2015: Status radiative thermal e+ target cooling 26