1. ERL accelerator review. Parameters for a Compton source
N. A. Vinokurov
Budker INP, Novosibirsk, Russia

2. There are several applications of electron beams, when the used beam parameters are still “good enough”:
Colliding beams and internal targets
Electron cooling
FEL and microwave tubes
X-ray and gamma sources

3. In such cases it is desirable to decelerate the used beam (or to use it many times, as in storage rings).
It (i) increases the beam average current and (ii) reduce radiation hazard.
Beam power: P = I· T/e ,
I – beam current, T – particle kinetic energy, e – electron charge.

4. Pcoll = I·(|Ucath| - |Ucoll|)
Electrostatic energy recovery
|Ucath| > |Ucoll|
Pcoll = I·(|Ucath| - |Ucoll|)

5. The simplest radiofrequency (RF) energy recovery
Problems - high power passing through the RF coupling elements and RF losses in both accelerator and “decelerator”.

6. Energy recovery linacs (ERLs) with the same cavity energy recovery
Problems: a – colliding beams, b – focusing of two beams with different energies in the RF accelerator.

7. 3 ERLs are in operation now. All they works for FEL.
Jefferson Lab. (USA) and JAERI (Japan) ERLs use superconducting RF.
Novosibirsk ERL uses normal-conducting RF.

8. Threshold currents of some instabilities Transverse beam breakup
Longitudinal instability
This slide shows the threshold currents of two most dangerous instabilities: regenerative beam breakup and longitudinal instability. Not going deep into details, I’d like to draw you attention to cavity quality factor in denominators, which leads to substantially lower threshold currents for superconducting systems. Typically these currents are of the order of milliampere for superconducting accelerators and instabilities must be suppressed for high current applications, whereas normal conducting systems can operate with currents of up to 100 milliamperes taking no notice [‘noutis] of it.
[1] E. Pozdeev et al., Multipass beam breakup in energy recovery linacs, NIM A 557, (2006), p
[2] N. A. Vinokurov et al., Proc. of SPIE Vol. 2988, p. 221 (1997).

9. Energy to 160 MeV
Ave. Current mA
Charge pC
Transverse Emittance <8 mm-mrad
Energy Spread %
Bunch length ps FWHM
Longitudinal Emittance <80 kV-ps

10. First stage: submillimeter (THz) FEL

11. Multiturn ERL may decrease cost of RF system
1 - injector, 2 - accelerating RF structure, degree bends, 4 – undulator, 5 – beam dump, 6 – mirrors of optical resonator

12. Full scale Novosibirsk FEL (bottom view)
Four tracks in horizontal plane with two IR FELs (under construction)
Lasing (2)
Common for all FELs
accelerating system
(exists)
Lasing (4)
One track in vertical plane
with terahertz FEL (exists)
Lasing (1) 12

13. Some remarks on ERL for Compton gamma source

14. 1. If the probability to radiate photon for electron is about 1, the probability of radiation of 2 and 3 photons is also significant. → Several percent energy spread of used beam.
It rejects multiturn scheme and requires the decrease of energy spread during deceleration.

15. The beam with 90 MeV energy spread after deceleration from 1.5 GeV.
Blue - R56 =4.5, T566=0.
Red R56=5, T566 = 32.
To transport the beam with high energy spread to linac the Compton insertion have to be installed at the end of 360-degree loop of ERL.

16. 2. Due to focusing problem the minimum injection energy for “standard” ERL is about ten times less, than maximum particle energy. For 1.5 GeV it corresponds to 150 MeV. Therefore either cascade scheme, or ERL with colliding beams are necessary.

17. MARS (Multiturn Accelerator-Recuperator Source)
8 GeV
2.6 GeV
E = 1.8 GeV
~1Å
~12Å
G. N. Kulipanov, A. N. Skrinsky and N. A. Vinokurov, 1997
Injector
Beam
dump

18. First linac has 5-10 MeV energy and does not use energy recovery.
4. MARS:
Cascade scheme of injection
First linac has 5-10 MeV energy and does not use energy recovery.
To booster linacs (65 MeV and 730 MeV energy gain) energy recovery used.
E = 1.8 GeV
E = 730 MeV
E = 65 MeV
5 MeV
1 – 3 mA
1 – 15 kW
Injector
Beam dump 18

19. 3. The non-zero crossing angle of light and electrons reduce probability of photon scattering per one electron by the factor
Therefore it would be better not to use it.
Instead, it is possible to deflect electron beam.

20. Different lasers illuminate electron beam at different straight sections separated with small bending magnets. Positrons are picked from many targets and merged to one beam after that (for example, using RF kickers).

21. Different lasers illuminate rotating electron beam at different angle positions. Then the repetition rate of laser pulses is less than the repetition rate of electron bunches. Positrons are picked from many targets and merged to one beam after that (using RF kickers).

22. 4. The maximum ERL current is now 30 mA. But, 1A current seems feasible.