Outlook Introduction Compton effect Polarization and Polarimetry Polarized

• • • Outlook Introduction: Compton effect Polarization and Polarimetry Polarized positron sources & FP cavities Nuclear isotopes detection Conclusions 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 me >> wg: THOMSON diffusion 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 <i, f> is not taken into account) : And the frequency shift in the center of mass frame Alessandro Variola LAL Orsay Journées cavités passives IMPORTANT 1 frequency 1 angle

Frequency shift: In the lab frame: two boosts, relativistic and Doppler effect. COMPTON BACKSCATTERING 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 (q=p) => 4 g 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 => q ~1/g 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 Alessandro Variola LAL Orsay Journées cavités passives

Alessandro Variola LAL Orsay Journées cavités passives

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

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

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

Polarization dependence 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…. Alessandro Variola LAL Orsay Journées cavités passives

APPLICATIONS: 1 -Compton Polarimeter. Example Jlab • • (D. Gasket) Compton polarimeter uses high gain Fabry-Perot cavity to create ~ 1 k. W of laser power in IR (1064 nm) Detects both scattered electron and backscattered g 2 independent measurements, coincidences used to calibrate g detector Systematic errors quoted at 1% level Upgrade in progress to achieve same (or better? ) precision at ~ 1 Ge. V – IR Green laser – Increase segmentation of electron detector r etector on d Electr n de Photo al Optic cavity es Dipol Alessandro Variola LAL Orsay Journées cavités passives

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

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

APPLICATION: 2 -Generation of Polarized Positrons 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 • • • 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 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 ptransv) In a positron capture system only a certain fraction of the spectrum can be accepted with a constant energy acceptance ( ~ 30 -40 Me. V)…higher the energyhigher the polarization-lower the population (yield) These are the reasons for which : 1) Very low energy gammas (~ few Me. V > 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 Me. V => Electron energies from few 100 Me. V to 10 Ge. V (depending on the lasers) Alessandro Variola LAL Orsay Journées cavités passives

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

• Positron sources needs Example ILC : 2 1010 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 ~ 1012 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…) Alessandro Variola LAL Orsay Journées cavités passives

• IN THIS CONTEXT: What is the problem of a Compton source? For gw<<m, Photon/collision = s ne ng foverlap where s = 10 -29 m =6. 65 So let’s have an estimate : In an electron bunch 1 n. C (6. 25 10 exp 9), laser of 1 J @ 1 e. V ~5 10 exp 18 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 10 exp 8, 10 m - 10 exp 10) If laser 1 W (tech constraints)=> 1 Hz => 1 n. A 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…) Alessandro Variola LAL Orsay Journées cavités passives

• 2 BASIC IDEAS For COMPTON Polarised Positron Sources • 1 st = accumulation ring, high frep, high current. • Complex…. . 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 n. C) : effect the crossing angle. Laser pulses can be few ps. Beam wait can be few tenths of microns • 3) Dream : FP cavity for l >> 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 2 106 pulses of 0. 5 J distributed with 1000 trains (1 k. Hz) of 2000 pulses. . etc ec Alessandro Variola LAL Orsay Journées cavités passives

2 nd ERL Polarised positron source – Compton cavities + ERL. Positron damping ring Electron re-circulation Linac 1. 5 Ge. V Compton cavities + bunch compressor Target Linac 4. 75 Ge. V Post Acceleration 250 Me. V Capture 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. l 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) Alessandro Variola LAL Orsay Journées cavités passives

3 rd application Alessandro Variola LAL Orsay Journées cavités passives

Alessandro Variola LAL Orsay Journées cavités passives R. Hajima

Alessandro Variola LAL Orsay Journées cavités passives R. Hajima

TEST : Mighty. Laser 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 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 l 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 • • • Alessandro Variola LAL Orsay Journées cavités passives

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Example Gamma’s intensity vs. time. Laser flash energy Wlas = 15 m. J, collision angle col = 6 , laser beam waist las = 40 (rms), repetition rate frep = 100 Hz. 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 (3 D 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 3 D 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). Alessandro Variola LAL Orsay Journées cavités passives

Looking at this table…ERL is much more than a concrete solution ! JLab AES JLAB Cornell Dares. ERLP JAERI BINP Boeing LANL AES LUX AES BNL 4 GLS Th. Ionic DC DC DC Dc NCRF SRF 1. 5 0. 75 1. 3 0. 5 0. 18 0. 433 0. 7 1. 3 RF (GHz) 0. 075 0. 75 1. 3 0. 08 0. 01 (0. 083) 0. 011 (0. 09) 0. 027 0. 033 (0. 35) 1. 3 0. 35 1. 3 frep 0. 133 0. 077 0. 08 0. 5 1. 7 4. 75 3. 0 1. 4 0. 08 Q (n. C) 10 100 6. 5 5 (40) 20 (150) 32 100 (1050 1300 500 100 I (m. A) <7 1. 2 <1 1. 5 30 32 ~7 6 2. 1 0. 5 e ( m) 3. 2 6. 3 2 4 44 44 30 20 53 16 527 527 527 50 15 Alessandro Variola LAL Orsay Journées cavités passives 527 ERL bl (ps) 10 Laser bl (ps) 527 Laser wl (nm)

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

A feeling about the parameters and the difficulties LINAC+ LASER RING+ FP CW LINAC + FP CW SC LINAC+FP Charge ~ 3 n. C 1 n. C 0. 1 n. C Frep Pulsed ILC= 15000 train /sec More than 10 exp 8 10 exp 7 -10 exp 8 Emittance 1 -2 p mm mrad ? TBA ~ 7 -8 p mm mrad En spread ~0. 1% ? TBA 0. 1% Bunch length 1 -2 ps Few ps. Higher the reusing=longer the bunch 3 -4 ps 3 -4 ps Beam dump Not a problem Depend from Injectionextraction frep. In principle no problem. Problem (500 k. W) Same problem but not for ERL Laser Linked to the frep and pulsed or not… See VUV FEL Fiber - YAG Number of possible Compton collision 10 exp 4 > 10 exp 8 < Current Micro amps ? TBA (0. x Amperes) ~ 10 m. A Power needed ($) And cost Low Average Medium High Medium Average High (cryoplant) Needed R&D No but low flux Yes for all the components Yes for the cavities and the FP cavities+laser GUN RF RF DC+RF e- machine DIFFICULTIES Reach very low emittance and n. C Alessandro RING design Variola + CW cavities + GUN LAL Orsay injection. Compton TECHNOLOGY Journées cavités passives dynamics GUN TECHNOLOGY

General overview. • • • 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 10 exp 2 -10 exp 3) 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 Me. V. 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). Alessandro Variola Dimensions, Cost 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 EXP 3. Taking into account the energy of 50 Me. V and having simple scaling the lifetime is less than 1 sec. I think that we can see the ring as a “multiple” recirculator 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 Me. V (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 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. Alessandro Variola LAL Orsay Journées cavités passives Cain Simulations

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! Alessandro Variola LAL Orsay Journées cavités passives Easy analytical form if Selection as to be done Only with diaphragm : In our case beta> than few cm In theory we are safe…. .