Alessandro Variola LAL Orsay Journées cavités passives Outlook Introduction: Compton effect Polarization and Polarimetry Polarized positron sources & FP

Alessandro VariolaLAL Orsay

Journées cavités passives

• Outlook• Introduction: Compton effect• Polarization and Polarimetry• Polarized positron sources & FP

cavities• Nuclear isotopes detection• Conclusions



Introduction Thomson diffusion and Compton effect

• Kinematical collision between an electron and a photon.

Neglecting the recoil therefore taking into account me >> :

THOMSON diffusion

][m 10 0.665 3

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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) :

2

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Sinr

d

d

i

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And the frequency shift in the center of mass frameIMPORTANT1 frequency 1 angle



Frequency shift:In the lab frame: two boosts, relativistic and

Doppler effect.

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COMPTON BACKSCATTERING



• 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





In real world : electron bunches impinging on laser pulses

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SinCos

CosfggNNRate

zzexxeyye

rep

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Luminositygeometrical factor



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 cavityAstigmatism reduced & ~circularly polarised eigenmodes

Toward small laser spot size

e- beam

Laser input



Rate vs Angle & Bunch Length (no crab)

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0,4

0,6

0,8

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Crossing Angle [deg]

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3mm

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In HEP application the flux is supposed to betoo much for the coatings. A crossing angle is foreseen



Polarization dependence

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1st trivialApplicationThis is good for PolarimetryMesuring the cross section asymmetry

In this example only Pz….



Optical cavity

Photon detectorElectron 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



Diaphragm effect & monochromatization: polarization dependence

Example:Very convergent beam



Diaphragm => If laser is polarizedEnergy and polarizationselection



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.1st : In some physics channel polarization act like a filter so it affect the rate (not luminosity!!!)2nd : lot of different Physics cases have been worked out for polarized positron at the new lepton colliders



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 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)



High energy photon

Pair creation

Polarized e+ and e-

Capture SystemWith fixed energy windowacceptance

Multiple scatteringand energy losses

spectrumCut off = impinging gamma energy

Low E

Low E

High E

More visual



• Positron sources needs

Example ILC : 2 1010 positron / bunch~ 3000 bunch in a 1.2 msec train5 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…)



• IN THIS CONTEXT: What is the problem of a Compton source?

For m, Photon/collision = ne ng foverlap where = =6.65 10-29m

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 mm2 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

applicationsLike 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…)

3

8 20r



•2 BASIC IDEAS For COMPTON Polarised Positron Sources

•1st = accumulation ring, high frep, high current. •Complex…..



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 theaverage power (continuous pulses or trains).

For example 1 MW can be 2 106 pulses of 0.5 J distributed with 1000 trains (1 kHz) of 2000 pulses..etc ec



Polarised positron source – Compton cavities + ERL.

Positron damping ring

Linac 1.5 GeV Linac 4.75 GeV

Target

Capture

Post Acceleration 250 MeV

Compton cavities+ bunch compressor

Electron re-circulation

2 nd ERL



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)



3rd application



R.Hajima


Journées cavités passivesR.Hajima



TEST : MightyLaser

Collaboration : LAL, CELIA, LMA, KEK An high finesse 4 mirrors cavity is installed inATF (accelerator test facility).Japanese machine for the production, transportAnd focalization for nanometric beams

This will allow:

1) Lock an high average power fiber laserWith an high finesse cavity2) Synchronize with a low emittance beam3) Gamma production and detection, calorimetry4) This will be the first gamma factory



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





0.000 0.002 0.004 0.006 0.008 0.0104.00E+012

5.00E+012

6.00E+012

7.00E+012

8.00E+012

9.00E+012

1.00E+013

1.10E+013

1.20E+013 dn / dt

t (ms)

0.000 0.002 0.004 0.006 0.008 0.0100.00E+000

5.00E-008

1.00E-007

1.50E-007

2.00E-007

2.50E-007

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x

t (ms)0.000 0.002 0.004 0.006 0.008 0.010

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6.0x10-6

7.0x10-6

8.0x10-6

9.0x10-6

1.0x10-5

1.1x10-5

s

t (ms)

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



Compton scheme:

• We can subdivide the scheme into different phases:a) Production (rep frequency, FP cavity)b) Capture (AMD magnetic field, target) + polarisation selectionc) 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).



JLab AESJLAB

Cornell

Dares.ERLP

JAERITh.Ionic

BINPTh.Ionic

Boeing

LANLAES

LUX AESBNL

4GLS

DC DC DC DC DC Dc NCRF NCRF NCRF SRF SRF

1.5 0.75 1.3 1.3 0.5 0.18 0.433 0.7 1.3 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 0.133 0.077 0.08 0.5 1.7 4.75 3.0 1.0 1.4 0.08 Q (nC)

10 100 100 6.5 5 (40) 20 (150)

32 100 (1050

1300 500 100 I (mA)

<7 1.2 <1 1.5 30 32 ~7 6 2.1 0.5 (m)

3.2 6.3 2 4 50 15 ERL bl(ps)

44 44 30 20 53 16 10 Laser bl (ps)

527 527 527 527 527 527 527 527 Laser wl (nm)

Looking at this table…ERL is much more than a concrete solution !



e-

Vacuum vessel for KEK

Injection laser100 W @ 100 MHz= 1 JouleIf the cavity gain is 10000in the cavity 10 mJ/pulse circulating



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. 1st 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



LINAC+ LASER RING+ FP CW LINAC + FP CW SC LINAC+FP

Charge ~ 3nC 1 nC 0.1 nC 0.1 nC

Frep Pulsed

ILC= 15000 train /sec

More than 10exp8 10exp7-10exp8 10exp7-10exp8

Emittance 1-2 p mm mrad ? TBA ~ 7-8 p mm mrad 7-8 p mm mrad

En spread ~0.1% ? TBA 0.1% 0.1%

Bunch length 1-2 ps Few ps. Higher the reusing=longer the bunch

3-4 ps 3-4ps

Beam dump Not a problem Depend from Injection-extraction frep.In principle no problem.

Problem (500 kW) Same problem but not for ERL

Laser Linked to the frep and pulsed or not…

See VUV FEL

Fiber - YAG Fiber - YAG Fiber - YAG

Number of possible Compton collision

10exp4 > 10 exp8 10 exp8 < 10 exp8 <

Current Micro amps ? TBA (0.x Amperes) ~ 10 mA ~ 10 mA

Power needed ($)

And cost

Low

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

FP cavities+laser

GUN RF RF DC+RF 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



• 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.



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



flux vs beam size (0degrees 5 ps) emit=5 10exp-8

020406080

100120140160

50 100 150 200 250

sigma beam (microns)

flu

x (

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ex

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)

Série1

Factor ~ 30 IMPORTANT. To be coupled with the divergenceAnyway we have to think that this scales with the SQRTOf the beta function so the effect are less drastic for little beam sizes.

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!

])2(

8[ S

22

2

Laser

cutCos

Easy analytical form ifSelection as to be doneOnly with diaphragm :

4

1

In our case beta> than few cmIn theory we are safe…..

Documents

Alessandro Variola LAL Orsay Journées cavités passives Outlook Introduction: Compton effect Polarization and Polarimetry Polarized positron sources & FP