1. Compton effect and ThomX What possible future?
Alessandro Variola (LAL Orsay)
Compton effect and ThomX What possible future?
IN2P3
Les deux infinis

2. ThomX source: CBS Source demonstrator
Cycle = 20 msec
RF Pulse = 3 ms
Energy = MeV
Laser
and FP cavity

3. 1) Compton effect Compton backscattering
The Project
Compton backscattering
- CBS is by far the most efficient photon energy booster (bumper cars effect !!!)
wdiff=4g2 wlaser, ThomX => g~100 => we can obtain high energy photons by means of a relatively low energy electron accelerator => compact and cheap
- Thomson cross section is ‘weak’: cm2
Tunability : laser / linear - beam energy / quadratic
Small accelerators: from X to g rays. Large application possibilities
The whole spectrum is emitted
Temporal characteristics given by the ‘space-time’ geometry of the collision point

4. 2) Compton Effect
- Directivity = > f= 1/g around the electron propagation
Energy vs Angles => diaphragm = monochromaticity (%).
ATTENTION: monochromaticity/brillance is a convolution of different effects: aperture angle, beams energy spread, emittances and b*, Rayleigh length, laser intensity
- Polarization if needed
- Backscattered photon (cut off) => collision angle dependence (factor 2)

5. Light source 1) Flux !! Average and instantaneous 2) Bandwidth
3) Coherence
4) Divergence (small..but also bigger)
Short pulses
Temporal resolved pulse (multibunch)
Directivity (painting… see 4)
Tunability

6. CBS source parameter : Frep
Collision Repetition Frequency
High average flux
Configuration
MHz, 100 kHz
Electrons :
Laser :
Ring
High average power laser
SC Linac
FP cavity
ERL
The collision frep has a strong impact on other hidden parameters too. For example, the FP cavity mirrors
diameters…(the cavity has l~frep, and l is proportional to the waist and therefore to the beam spot on the mirrors…).

7. ERL: As linac but 2 IP possible (push-pull), no DUMP problems
Ring / Re-use the beam:
‘Classical technology’, easy to integrate, multi Ips, very high frep, cost
Compton Dynamics [En spread %], IP access, Operations, Radioprotection, Bandwidth f(ring) [%]
SC Linac:
High frep, short pulses, low emittance (low charge), IP access, Bandwidth
Technology integration, Source, DUMP (Radioprotection), single X line, expert operation
ERL: As linac but 2 IP possible (push-pull), no DUMP problems
Cost and expert operation
C.Bruni

8. CBS source parameters, Frep
Collision Repetition Frequency
Low average flux
High Instantaneous flux
Configuration
100 Hz, 10 kHz
Laser:
Electrons:
High peak power laser
Warm Linac
Regenerative cavity (~ 2J integrated in 100 pass) or re-circulator (0.n J 20 pass)

9. 1 bunch per train: High power laser (J/Hz)
Linac :
2 operation modes
1 bunch per train: High power laser (J/Hz)
Up to 10 nC. En spread and emittance proportional to the charge/bunch. Therefore, when the charge increases (instantaneous flux), the bandwidth is affected (reduction of the brilliance). Time resolved experiments. Head on collisions possible !!!!!
Multibunch per train: High power laser + cavity (100mJ/n 10 pass/ n 10 Hz)
Band dependent (L,S,C,X). Bunches: from 20 to 100 per train. Charge n 100 pC (nC S/L). High brilliance
High charge, short pulses, very low emittance and energy spread, IP access, Bandwidth
Technology integration, Source, DUMP (for train operation) (Radioprotection), single X line, expert operation
C.Bruni

10. Sources characteristics, Summary
LINAC+ LASER
RING+ FP
CW SC LINAC + FP
CW ERL LINAC+FP
Charge
Up to 10 nC
1 nC
N 100 pC
Frep
Pulsed 100Hz
N 100 puse/train
More than 10exp8
10exp7-10exp8
Emittance
1-2 p mm mrad
Eq emittance. Proportional to
f laser and b*
~ 7-8 p mm mrad
7-8 p mm mrad
En spread
~0.1%
Eq energy spread. Proportional to SQRT f laser and g
Less than %
Bunch length
1-2 ps
Equilibrium …Few ps. The more you reuse it, the longer the bunch
3-4 ps
3-4ps
Beam dump
Not a problem
Depends on injection-extraction frep. In principle no problem.
Problem (n 100 kW)
Same problems, but not for ERL
Cavity
Regenerative / multipass recirculator
Fabry Perot
Laser
Linked to the frep and pulsed or not…
See VUV FEL
Fiber - YAG
Number of possible Compton collisions
10exp4
> 10 exp8
10 exp8 <
Current
Micro amps
> mAmps
~ 10 mA
Power needed ($)
and cost
Low
Average
Medium
High (cryoplant)
Needed R&D
Multipass cavities
Yes for all components
Yes for the cavities and the FP cavities+laser
FP cavities+laser
GUN RF
DC+RF

11. X ray characteristics LINAC+ LASER RING+ FP CW SC LINAC + FP
CW ERL LINAC+FP
Average Flux
* / ** (multibunch)
****
***
Peak flux **
Bandwidth
Short pulses
Temporal resolved pulse (multibunch)
Directivity (painting) *
Tunability
Cost
Operations and integration

12. ThomX. What possible extensions for the future?
The Thomx choice (storage ring) is the result of a compromise among
flux, cost and integration constraints.
But in ThomX we have:
A second IP
A Linac up to 70 MeV (50 baseline)
Extraction line up to 70 MeV (50 baseline). Energy spread and emittance
deteriorated
So what other solution or option can be implemented to provide other
X rays beams characteristics?

13. The multi-line ThomX 2 1 3 1) Second IP
- It can be just a second IP. Slight modifications on the injection line to extract the X.
It is inside dipoles. CSR mm waves.
Integration of a p/2 collision geometry.
The spectrum cut off is ½ but we have very short pulses (n 10 fs)
2) Linac, changing the gun laser.
Single pulse (3 nC) on a high power laser (~ J) at low Frep (n Hz).
Peak brilliance and time resolved experiments
Trains with multibunch. Good bandwidth with diaphragms.
Good average flux (no diaphragm)
Short pulse (less than ps) in head on collision (full energy cut-off)
Extraction line 2 1 3

14. Conclusions THOMX is an ‘open future’ project !!!!!!!!!
- Compton effect can be used in different configurations
- Target different users
- From this point of view ThomX has some flexibility as far as the warm
RF technology is concerned
- Three new users line are possible:
New IP
End of Linac
Extraction
These lines can provide photons for a different ‘kind’ of experimental
techniques not aiming at a high average flux :
High peak brilliance
Short pulses
mm waves
Time resolved experiments
THOMX is an ‘open future’ project !!!!!!!!!