Synchrotron radiation at eRHIC

Yichao Jing, Oleg Chubar, Vladimir N. Litvinenko

Outline• Introduction of eRHIC and its vacuum chamber.

• Synchrotron radiation (SR) in ARCs (power and flux).

• Neutron generated by SR penetration through chamber.

• Preliminary synchrotron radiation (SR) in IRs.

• Summary.

EIC RD workshop 2012

eRHIC : an upgrade of RHIC

EIC RD workshop 2012

Add e- accelerator to the existing $2B RHIC

eRHIC layout and e- beam parameter

EIC RD workshop 2012

eRHIC is a multi-pass energy recovery linac. In routine operation, the electron beam will be accelerated to its top energy at 30 GeV then collide with ion beams.

Energy, GeV 30

Bunch charge (nC) 0.5

Beam current (mA) 12.6

Polarization 80%

Peak luminosity (cm-2s-1) 1 X 1035

Machine parameters for top energy:

eRHIC ARC and chamber design

Courtesy of M.Mapes

EIC RD workshop 2012 yjing@bnl.gov

Energy, GeV 10 20 30Dipole

strengths, T 0.143 0.285 0.427Beam current,

mA 50 50 12.6

Different passes of beams (diff energies) are separated vertically. Asymmetric chamber design prevents back scattered secondary particles from influencing e- beam. Cooling channels are placed where synchrotron radiation power is highest.

Outside of ERL

SR calculation setup

EIC RD workshop 2012 yjing@bnl.gov

e- beam offset (cm)

6.5

Distance from radiation (m)

6.05

Incident angle(mrad)

19.9

To the first order, e- beam’s trajectory is determined by dipole magnets thus should be paralleled to vacuum chamber. The synchrotron radiation is emitted in the tangential direction at the point where electrons are bent in dipoles. The distance of SR travels before it reaches vacuum chamber and the electron beam properties (energy & current) determine its power density and spectral flux on the chamber.

Distance >> source size (e- beam size) Far field!!

Heat load and spectrum for 1st stage eRHIC (5 – 10 GeV)

Heat load on vacuum chamber:

For E = 10 GeV, I = 50 mA, dP/ds = 0.098 W/mm.

For E = 5 GeV, I = 50 mA, dP/ds = 0.0058 W/mm.

EIC RD workshop 2012 yjing@bnl.gov

Photons with relatively low energy (< MeV’s) are generated. Can be stopped by chamber.

Heat load on vacuum chamber:

For E = 30 GeV, I = 12.6 mA, dP/ds = 2.02 W/mm.

For E = 20 GeV, I = 50 mA, dP/ds = 1.58 W/mm

EIC RD workshop 2012 yjing@bnl.gov

Heat load and spectrum for full stage eRHIC (20 – 30 GeV)

Photons with large energy (> MeV’s) are generated. Can penetrate chamber and induce severe background if not shielded.

EIC RD workshop 2012 yjing@bnl.gov

How photons behave in chamber? (power attenuation)

Power decays exponentially (attenuation length) into chamber. Higher energy photons have longer attenuation length thus can penetrate longer distance before losing power.

Overall power can be absorbed with a 1 or 2 cm thick Al chamber.

Some high energy photons can still penetrate!!

EIC RD workshop 2012 yjing@bnl.gov

How photons behave in chamber? (spectral attenuation)

Low energy photon flux drops dramatically – can easily be shielded. The remnant of high energy photon (a few times of critical photon energy) could be problematic.

For 10 GeV, the total number of esaping photons in entire tunnel is 5.7e-5 ph/sec.For 30 GeV, it is 23882.5 ph/sec!!

Need additional shielding for 30 GeV!

Neutron generation in chamber

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Photons travel in Al chamber can induce process:

Cross section of such photonuclear reaction is:

Courtesy of A. Tonchev

Integrated with the photon flux:

Total number of neutron rate is 1.1e-6 n/sec.

Situation at interaction region (IR)

Courtesy of D. Trbojevic

Quadrupole triplet:

1st: B1=45.65 (T/m);2nd: B1=-42.8 (T/m);3rd: B1=36.67 (T/m).

EIC RD workshop 2012 yjing@bnl.gov

Synchrotron radiation from strong quadrupoles depends on particle position within the beam. Radiations from different positions overlap and add up incoherently to the final spectrum. We have to track every macro particle’s trajectory and integrate the wave front among all particles.

Intense computation!!

Particle trajectory matching (SRW)

Beam envelope is matched with quadrupole kicks to Interaction point in SRW.

100000 macro particles with initial random gaussian distribution were tracked along beam line in paralleled python code based on SRWLIB.

EIC RD workshop 2012 yjing@bnl.gov

Photon flux @ IP

Higher energy photons have smaller radiation cone – more concentrated.

Flux decays exponentially with amplitude. However, it’s not negligible at large amplitude (>4 mm). Collimators and possible secondary emission

need to be calculated further using GEANT4 or FLUKA.

Line power density is estimated to be ~ 50 mW/mm, much less than in the ARC’s. Power dissipation is not a big problem.

EIC RD workshop 2012 yjing@bnl.gov

Photon flux @ IP (cont’d)

When photon energy is low (wavelength is long compare to magnets’ lengths), single electron emission features can be observed (with peaks at 1/γ). When photon energy is high, the cone of radiation has a very small opening angle that all photons follow the beam trajectory (a very small spot at IP).

SR patterns come from incoherently overlapping and averaging over all electrons.

Deeper interpretation is needed about the background.

EIC RD workshop 2012 yjing@bnl.gov

Same as previous plot but iteration stops at early stage

(fewer macro particles). Particle trajectory can be

clearly viewed from synchrotron radiation.

At low energy, cone is big so radiation overlaps each

other and smears out.

At high energy, cone is so small that radiation

follows beam trajectory and focus to a small spot.

Most overlapping happens at the center which leaves a

bright spot.

10

keV

100

keV

500

keV

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Photon flux @ s = 10 m

EIC RD workshop 2012 yjing@bnl.gov

Summary• We calculated the power and spectral distribution of synchrotron

radiation at eRHIC for both ARCs and IRs in synchrotron radiation workshop (SRW).

• For first stage eRHIC (5 – 10 GeV), Al vacuum chamber is capable of preventing photons from escaping. For top energy operation (30 GeV), additional shielding is needed to stop high energy photons (~ MeV).

• Neutron generation through photonuclear reaction is negligible.

• Complete study of synchrotron radiation in IRs is a work in progress. Future studies of secondary particle emission and background requires additional simulation tools like GEANT4 and/or FLUKA.

EIC RD workshop 2012 yjing@bnl.gov

EIC RD workshop 2012 yjing@bnl.gov

EIC RD workshop 2012 yjing@bnl.gov