1. Alessandro Variola LAL Orsay Journées cavités passives Outlook Introduction: Compton effect Polarization and Polarimetry Polarized positron sources & FP cavities Nuclear isotopes detection Conclusions

2. Alessandro Variola LAL Orsay Journées cavités passives Introduction Thomson diffusion and Compton effect Kinematical collision between an electron and a photon. Neglecting the recoil therefore taking into account m e >>   : THOMSON diffusion

3. Alessandro Variola LAL Orsay Journées cavités passives Compton Cross section If the recoil is not negligible the diffused photon undergoes a frequency shift and the differential cross section is [Klein Nishina] (in the case in which the polarization is not taken into account) : And the frequency shift in the center of mass frame IMPORTANT 1 frequency 1 angle

4. Alessandro Variola LAL Orsay Journées cavités passives Frequency shift: In the lab frame: two boosts, relativistic and Doppler effect. COMPTON BACKSCATTERING

5. Alessandro Variola LAL Orsay Journées cavités passives Very interesting : The scattered photon “acquires” a part of the electron energy=> frequency boost. The maximum is for head-on collisions where the backscattered photon (  ) => 4  2, from Lorentz and Doppler. This is called CUT-OFF. THIS IS THE REAL INTEREST FOR HIGH ENERGY PHYSICS APPLICATIONS : With relative low energy electrons it is possible to produce high energy gammas Very interesting : The emission cone is relativistic shrinked =>  Very interesting : Taking into account a single particle collision there is a univocal relationship between the energy and the angle of the scattered photon => energy selection. With polarized laser energy=>polarization Energy Spectrum

6. Alessandro Variola LAL Orsay Journées cavités passives

7. Alessandro Variola LAL Orsay Journées cavités passives In real world : electron bunches impinging on laser pulses Luminosity geometrical factor

8. Alessandro Variola LAL Orsay Journées cavités passives Small laser spot size &2 mirrors cavity  unstable resonator (concentric resonator) BUT  astigmatic & linearly polarised eigen-modes Stable solution: 4 mirror cavity as in Femto lasers Non-planar 4 mirrors cavity  Astigmatism reduced & ~circularly polarised eigenmodes Toward small laser spot size e - beam Laser input

9. Alessandro Variola LAL Orsay Journées cavités passives In HEP application the flux is supposed to be too much for the coatings. A crossing angle is foreseen

10. Alessandro Variola LAL Orsay Journées cavités passives Polarization dependence

11. Alessandro Variola LAL Orsay Journées cavités passives 1 st trivial Application This is good for Polarimetry Mesuring the cross section asymmetry In this example only Pz….

12. Alessandro Variola LAL Orsay Journées cavités passives Optical cavity Photon detector Electron detector Dipoles APPLICATIONS: 1-Compton Polarimeter. Example Jlab (D.Gasket) Compton polarimeter uses high gain Fabry-Perot cavity to create ~ 1 kW of laser power in IR (1064 nm) Detects both scattered electron and backscattered g  2 independent measurements, coincidences used to calibrate  detector Systematic errors quoted at 1% level Upgrade in progress to achieve same (or better?) precision at ~ 1GeV –IR  Green laser –Increase segmentation of electron detector

13. Alessandro Variola LAL Orsay Journées cavités passives Diaphragm effect & monochromatization: polarization dependence Example: Very convergent beam

14. Alessandro Variola LAL Orsay Journées cavités passives Diaphragm => If laser is polarized Energy and polarization selection

15. Alessandro Variola LAL Orsay Journées cavités passives APPLICATION: 2-Generation of Polarized Positrons How to make polarised positrons: 1) Compton effect. If the laser is polarized the polarization is conserved in the backscattered photon 2)Polarised gammas impinge on a target => pairs are created in the nuclear field of the material (and polarization of the gamma is conserved…) 3)Pairs are separated, positron are captured and re-accelerated to the damping rings 4)In future lepton colliders the required amount of positrons per bunch is large….Stacking is necessary 5)Need to play on the Repetition frequency and on the accumulation in the same bunch Why Polarized positrons. 1 st : In some physics channel polarization act like a filter so it affect the rate (not luminosity!!!) 2 nd : lot of different Physics cases have been worked out for polarized positron at the new lepton colliders

16. Alessandro Variola LAL Orsay Journées cavités passives In the target and after: Pairs are created They lose energy and are multiple scattered At the exit : huge energy spread and exponential decay of the spectrum population, cut off energy close to the max energy of the gammas, huge angular divergence (~ to p transv ) In a positron capture system only a certain fraction of the spectrum can be accepted with a constant energy acceptance ( ~ 30-40 MeV)…higher the energy- higher the polarization-lower the population (yield) These are the reasons for which : 1) Very low energy gammas (~ few MeV > than 1) NOT OK (losses in the target and final divergence…) 2) Very high energy gammas NOT OK (very low Yield) 3) Compromise 10-40 MeV => Electron energies from few 100 MeV to 10 GeV (depending on the lasers)

17. Alessandro Variola LAL Orsay Journées cavités passives High energy photon Pair creation Polarized e+ and e- Capture System With fixed energy window acceptance Multiple scattering and energy losses spectrum Cut off = impinging gamma energy Low E High E More visual

18. Alessandro Variola LAL Orsay Journées cavités passives Positron sources needs Example ILC : 2 10 10 positron / bunch ~ 3000 bunch in a 1.2 msec train 5 Hz And what is the efficiency: 1)Compton production (depends on laser power, bunch current, spot sizes at the IP) (~10% very good – 100% risk to go in non linear Compton) 2)Pair creation + positron capture (few percent) 3)Transport ~ 50 % Going back => per bunch I need ~ 10 12 gammas per collision!!!!!! And we need at least 15000 of such a collision in 1 second …(or much collision with less gammas…we will see how to do…)

19. Alessandro Variola LAL Orsay Journées cavités passives IN THIS CONTEXT: What is the problem of a Compton source? For  m, Photon/collision =  n e n g f overlap where  = =6.65 10 -29 m So let’s have an estimate : In an electron bunch 1 nC (6.25 10exp9), laser of 1 J @ 1 eV ~5 10 exp18 So multiplying and taking into account a section of 1 mm 2 we have 2 Mega photons per collision in the whole spectrum!!!!!!! (100  m - 10exp8, 10  m - 10exp10) If laser 1 W (tech constraints)=> 1Hz => 1nA current = >  is not low for a QED process but it is for high energy applications Like the polarised positron sources. On the other side it is ok for polarimetry SO BASIC IDEA: COUPLING BETWEEN HIGH CHARGE ELECTRON BUNCHES WITH LASER PULSED AMPLIFIED IN FABRY PEROT CAVITIES (if not we would need lasers of ~ MW average power…)

20. Alessandro Variola LAL Orsay Journées cavités passives 2 BASIC IDEAS For COMPTON Polarised Positron Sources 1 st = accumulation ring, high frep, high current. Complex…..

21. Alessandro Variola LAL Orsay Journées cavités passives What laser and cavity? 1) Bunches in ring must be reused => Compton recoil minimized for the energy spread : High energy beams and high wavelength cavities 2) Bunches in ring are long but can have high charge (up to 10 nC) : effect the crossing angle. Laser pulses can be few ps. Beam wait can be few tenths of microns 3) Dream : FP cavity for  >> 1  m with “reasonable power” depending on the main parameter : the collision repetition frequency……because in electron rings the beam cools with a characteristic cooling time. The cavity is stable (accelerator environment) and the waist is few tenths of microns (not less…convolutions) It would be wonderful (real Dream) to decide HOW to distribute the average power (continuous pulses or trains). For example 1 MW can be 2 10 6 pulses of 0.5 J distributed with 1000 trains (1 kHz) of 2000 pulses..etc ec

22. Alessandro Variola LAL Orsay Journées cavités passives Polarised positron source – Compton cavities + ERL. Positron damping ring Linac 1.5 GeVLinac 4.75 GeV Target Capture Post Acceleration 250 MeV Compton cavities + bunch compressor Electron re-circulation 2 nd ERL

23. Alessandro Variola LAL Orsay Journées cavités passives What laser and cavity? 1) Bunches in ERL are not reused =>Maximize the flux and the enrgy in dependence of the accelerator energy (not recoil problems) 2) Bunches in ERL can be very short (~ 100 fs) but lower charge: Interest to have also FP cavity pulses short to compensate 3) Dream : FP cavity.  adapted to the constraints. Power/pulse maximized and if possible working in “burst mode”. Stable and waist few tenths of microns (scales with energy for emittance and for photons divergence)

24. Alessandro Variola LAL Orsay Journées cavités passives 3 rd application

25. Alessandro Variola LAL Orsay Journées cavités passives R.Hajima

26. Alessandro Variola LAL Orsay Journées cavités passives R.Hajima

27. Alessandro Variola LAL Orsay Journées cavités passives TEST : MightyLaser Collaboration : LAL, CELIA, LMA, KEK An high finesse 4 mirrors cavity is installed in ATF (accelerator test facility). Japanese machine for the production, transport And focalization for nanometric beams This will allow: 1)Lock an high average power fiber laser With an high finesse cavity 2) Synchronize with a low emittance beam 3) Gamma production and detection, calorimetry 4) This will be the first gamma factory

28. Alessandro Variola LAL Orsay Journées cavités passives Conclusions 1) Compton effect has important applications in HEP…example polarimetry Polarized positron = frontier of the new generation of high energy accelerator physics 2) To do it is DIFFICULT…but COMPTON EFFECT can be a solution 3) 2 schemes : Ring and ERL => different requirements in pulse length and 4) in principle we need ~ 1 MW at disposition in the cavity 5) We can do it with lasers ? Not at my knowledge… 6) FP cavities are the key element together with the high charge accelerator (with gain ~ 10000 we can get back to few hundreds watt lasers…) The DREAM CAVITY allows to play with the most important parameter, the collision repetition frequency, as a free parameter. This allows a complete matching with the electron machine requirements. Moreover it allows to store a huge power and to focalize it in a small waist (~10-20  m) remaining stable. The mirrors has to withstand the power and the radiation environment….. Another important field of application is the detection of radioactive isotopes A first step will be the experiment in KEK THANK YOU FOR YOUR ATTENTION

29. Alessandro Variola LAL Orsay Journées cavités passives

30. Alessandro Variola LAL Orsay Journées cavités passives Gamma’s intensity vs. time. Laser flash energy Wlas = 15 mJ, collision angle  col = 6 , laser beam waist  las = 40  (rms), repetition rate frep = 100 Hz. Example

31. Alessandro Variola LAL Orsay Journées cavités passives Compton scheme: We can subdivide the scheme into different phases: a) Production (rep frequency, FP cavity) b) Capture (AMD magnetic field, target) + polarisation selection c) Stacking in the damping ring (3D emittance, rep frequency for cooling) Point a) requires high cross section (charge per bunch, light pulse. Limit = Non linear regime) and low rep freq (pump laser of the cavity) Point b) requires low frep (or train of pulses) for pulsed magnet, short bunch length, forward production for the acceptance. Point c) requires very good 3D emittance and low frep So talking about Compton collision, we need (at the same current ) an ERL machine that increase the charge per bunch (as much as we can) and decreases the frep (from 10 to 75 MHz).

32. Alessandro Variola LAL Orsay Journées cavités passives JLabAES JLAB CornellDares. ERLP JAERI Th.Ionic BINP Th.Ionic BoeingLANL AES LUXAES BNL 4GLS DC DcNCRF SRF 1.50.751.3 0.50.180.4330.71.30.71.3RF (GHz) 0.0750.751.30.080.01 (0.083) 0.011 (0.09) 0.0270.033 (0.35) 1.30.351.3frep 0.133 0.0770.080.51.74.753.01.01.40.08Q (nC) 10100 6.55 (40)20 (150) 32100 (1050 1300500100I (mA) <71.2<11.53032~762.10.5  (  m) 3.26.3245015ERL bl (ps) 44 3020531610Laser bl (ps) 527 Laser wl (nm) Looking at this table…ERL is much more than a concrete solution !

33. Alessandro Variola LAL Orsay Journées cavités passives e-e- Vacuum vessel for KEK Injection laser 100 W @ 100 MHz = 1  Joule If the cavity gain is 10000 in the cavity 10 mJ/pulse circulating

34. Alessandro Variola LAL Orsay Journées cavités passives Technical general considerations 1) In a Compton machine all the parameters are linked. The “glue” is the repetition frequency. For both system (electrons & photons) the systems are completely different following this parameters. This is particularly true if we divide the two domains ~10 MHz< frep< ~10 MHz 2) The energy spectrum is continuous up to the cut frequency. The reduction of accepted flux vs the accepted energy spread is almost linear. (DIAPHRAGM) 3) In linear regime Compton can be seen as purely kinematic => The beam energy spread acquired by the beam is equivalent to the Compton spectrum. Reutilisation of the beam for a multi-turn machine must carefully take into account this effect. And this is strictly linked to the light power performances. Higher the power” => more difficult to re-collide (Bunch lengthening) 4)This fix the machine philosophy. 1 st question: do we want to re-use the beam (at least more than 1000 collisions) or not -> ring, LINAC or ERL? This is a machine that definitively works in a low ratio (gamma scattered/ electron in the bunch) with a consequent flux. 5) This is a difficult machine and set up. We have to start from the SIMPLEST possible scheme and improve it when necessary. EVERY weird idea MUST be supported by a careful evaluation of the impact. For example : multi injections – (How to do it), FP cavity with lot of circulating pulses ( the phases between different pulses in the PDH signal is taken into account ?), long living beams (IBS, Toushcek), high charge beam in the ring (space charge tune)..etc etc

35. Alessandro Variola LAL Orsay Journées cavités passives LINAC+ LASERRING+ FPCW LINAC + FPCW SC LINAC+FP Charge~ 3nC1 nC0.1 nC FrepPulsed ILC= 15000 train /sec More than 10exp810exp7-10exp8 Emittance1-2 p mm mrad? TBA~ 7-8 p mm mrad7-8 p mm mrad En spread~0.1%? TBA0.1% Bunch length1-2 psFew ps. Higher the reusing=longer the bunch 3-4 ps Beam dumpNot a problemDepend from Injection- extraction frep.In principle no problem. Problem (500 kW)Same problem but not for ERL LaserLinked to the frep and pulsed or not… See VUV FEL Fiber - YAG Number of possible Compton collision 10exp4> 10 exp810 exp8 < CurrentMicro amps? TBA (0.x Amperes)~ 10 mA Power needed ($) And cost Low Average Medium High Medium Average High (cryoplant) Needed R&DNo but low fluxYes for all the components Yes for the cavities and the FP cavities+laser FP cavities+laser GUNRF DC+RF e - machine DIFFICULTIES Reach very low emittance and nC RING design + injection. Compton dynamics CW cavities + GUN TECHNOLOGY GUN TECHNOLOGY A feeling about the parameters and the difficulties

36. Alessandro Variola LAL Orsay Journées cavités passives LINAC + Laser Advantages : Based on existing technologies (also if challenging if we push the limits), no high RF Power required, easy design of the interaction region (head on), Charge per bunch and energy per laser pulse. Good emittances and en spread so high focalisation in the interaction region. Dimensions. Disadvantages: frep (LOW FLUX factor at least 10exp2-10 exp3) RING + FP Advantages : Very high frep. CW mode. Possible head on or angle. Charge per bunch (if possible). Pulsed injector. Dimensions. Disadvantages: Very difficult design. All parameters are linked. TBE: IBS, Space charge tune, lifetime, injection, focalisation in the IR (Chromatic effect). Complexity of the q-poles system. CW WARM LINAC Advantages : High frep. No SC technology required. Demonstrator possible at 10 MeV. Connection with AMD e+ source so cavity design. Disadvantages : HIGH power required. Gun technology (JLAB), Beam dump. Dimensions CW SC LINAC (ERL and push PULL) Advantages : High frep (high flux), two photon line possible (so all the FP cycles used and two patients treated)), beam dump, low RF power Disadvantages : Gun technology (JLAB), SC technology (CEA, IPNO). Dimensions, Cost General overview.

37. Alessandro Variola LAL Orsay Journées cavités passives RING At present the ring is the preferred solution and it is under study (C.Bruni and A.Lolergue) IBS scales like gamma EXP3. Taking into account the energy of 50 MeV and having simple scaling the lifetime is less than 1 sec. I think that we can see the ring as a “multiple” re- circulator where “multiple” is a lot…. In this case the emittance is determined by the source (injector). This start to be challenging. Without cooling also electrons have memory… Space charge tune has to be considered Impedances Fast injection (and extraction?). How to do it? CSR ? I would exclude the exercise of ramping in the ring. Injection at 50 MeV (or the decided energy). For regimes of more than 1000 collisions/bunch minimum total additive en spread = 0.1 %. For 20 msec ~few % Very low average beta not good for IBS, space charge. Compton additive energy spread can be huge => bunch length and D=0 collision point. Bunch length is correlated to luminosity by the crossing angle 4 or 8 dipoles has to be evaluated. Preferred 4 but 8 will make the FP cavity easier It seems that the ring can be a low cost and easy technology solution BUT difficult as far as beam dynamics is concerned

38. Alessandro Variola LAL Orsay Journées cavités passives Factor ~ 30 IMPORTANT. To be coupled with the divergence Anyway we have to think that this scales with the SQRT Of the beta function so the effect are less drastic for little beam sizes. Cain Simulations

39. Alessandro Variola LAL Orsay Journées cavités passives Comments on diaphragm and Energy spectrum Diaphragm is useful to select energies and angles. This is very important for the polarisation selection. For the energy selection this is not true. A careful evaluation about the total effect of filtering and use of monochromators has to be carried out! Easy analytical form if Selection as to be done Only with diaphragm : In our case beta> than few cm In theory we are safe…..