ALPHA Storage Ring

Indiana UniversityXiaoying Pang

Summary

• Purpose• Wiggler design• Injection and Extraction• RF cavity• Touschek lifetime• Vacuum• Photon source

Our Purpose• Provide radiation effect experiments for NASA Debunch the rf linac beam bunches

• Compact X-ray photon source based on Inverse Compton scattering (ICSX)

Advantage: low cost, easy operation. Difficulties: long damping time,

negative horizontal damping partition, space charge effect, beam lifetime issue.

Design Typical operational energy : 20MeV ~100MeV, maximum : 600MeV

Dipole (existing) 1) effective length 2m 2) bending radius = 1.273m 3) edge angle 12⁰ vertical focusing

Wiggler (three dipoles) 1) modify damping partition number horizontal betatron

motion stable (without wigglers Jx = -0.3 ) 2) tune momentum compaction factor

C = 20 mT = 66.6 ns

3m

Wiggler Design

• Three gradient rectangular dipoles with B1 /B0=1.9 m−1 , where B1=(dBz /dx)x=0

• The minimum vertical gap is 40mm.• Maximum field strength is 1.67 kG at 25MeV, 6.7kG at

100MeV.

0.1m 0.1m

0.2m

zero momentum compaction factor

positive horizontaldamping partition number

Wiggler Design

A linac beam can be debunched ofits rf structure in

one turn if |c|≥0.5

When the wiggler is turned off, w=∞

Qausi-isochronous condition

Wiggler Design25MeV

At 50 MeV , the horizontal damping time can be 10s.

Injection• Two bumpers and a Lamberston septum are

used • Use electrostatic kickers with kicker rise time

10ns• Beams are injected from a linac with 25MeV

(up to 60 MeV)• Phase space painting

kicker 1 kicker2

iii

ico ss

sx

)(cossin2

)()(

2

1

0sinsin

0coscos

:orbit closed localizedfor Condition

212211

212211

and 2

11221

dipole

21

septumseptum

At the kickers’ location:

At the septum’s location:

s = 0.818, mrad Xco(septum) 32 mm

beam pipebeam pipe

25mm 2-5mm

1mm 5mm or 2mm

2mm

X=0

BSeptum

Amax = 25mm

75.1x

x

mm1.0Injection Scheme (Accumulation)

Injection Efficiency vs Fractional Tune

= 0.75

= 0.73

= 0.77

When tune is off 0.75, the ideal 4-injection-turn per closed orbit location is not guaranteed

= 0.667

= 0.6 or 0.8

Number of Injection Turns per Closed Orbit Location

Number of Injection Turns per Closed Orbit Location

x

x ’

Septum5 injections per ellipse

x

x ’

Septum4 injections per ellipse

More Ellipses!

Number of Injection Turns per Closed Orbit Location

Total number of Injection Turns

co, max

A = 21 mm

Beam Size = 42 mm

x = 25 mm

co, max

A = 25 mm

Beam Size = 50 mm

x = 32 mm

Electrostatic kicker will be used:

Kicker Strength

field electric gapE

light of speed c

kicker theoflength L

MeV 60at ][2.0

where,

TmB

Bc

LEk

For one turn injection and extraction, the integrated field strength is 0.60 MV at 25 MeV electron beam energy. Choosing a length of L=0.5 m, the applied voltage on two plate is 60 kV.

Kicker Strength

32

50

5

142

2.84 /

co

gap

x mm

L cm

g cm

V kV

E MV m

Sample Injectionsconstraint aperture without )injection turn 10( 75.1xv

Watch the beam at septum

Beam profile evolution around the ring

Considering the aperture: Let’s take into the consideration of the apertures at the bending dipoles and electrostatic kickers. Set the aperture radius at dipoles to be 100mm = 0.1m, at kickers to be 25mm. The total injection numbers will decrease.

With about 10 turns of injection, 50ns bunch length and 0.5A linac current, we can achieve:

JMVnCQEEnergy

nCnsAQ

6.1560250

25010505.0

Extraction by Lambertson extraction magnet

100MeV.at 1.02T and MeV, 25at 0.25T Bstrength field The

dipole.adjacent theavoid toseptum by the deflected 35' need we

'

.2/sept.)(kick

wherespacedrift theof middle in the septum put the We

x

B

lBx z

3535

The Lambertson septum is used to extract beam.

InjectionSeptum

Extraction

1.4m 1.4m0.2m

35

RF cavity

• Revolution frequency 15MHz• In the operational mode of debunching no RF

cavity is needed.• For beam physics study with quasi-

isochronous condition, we can modify the existing MPI cavity to make it operate at h=1, f=15MHz

MPI Cavity• Was built for proton acceleration with frequency from 2 to 10MHz• A quarter-wave –like cavity, is loaded with 10 ferrite rings with

quadrupole field bias.• Major RF tuning is achieved by parallel external capacitors. With an

external capacitance Cext =290 pF , the cavity was tested up to 11.4MHz, the resulting shunt impedance was about 1k.

Diameter of the cavity ~0.55m; Length ~0.6m

In the future

• For 15MHz operation, we need to reduce the external capacitance to about 120pF or the number of ferrite rings in the cavity.

• Reconfigure the ferrite rings to maximize the shunt impedance for a possible 3kV voltage.

• We will built a 90 MHz rf cavity for harmonic h=6 (or 494 MHz, h=33)in order to achieve a bunch length of the order of 10ps for short-pulse X-rays

Touschek lifetime Toucheck lifetime is sensitiveto the parameter:

is the rf bucket height, is the horizontal momen-tum spread can range from 0.001 to 1.

we will need a lifetime of 1h or more.

It can also be varied by changing the momentum compaction factor

where,

Vacuum

energy. offunction

a as emittance mequilibriu the

calculatecan we,2.9

25MeV,for 9.48 Choosing

)()/(

)/(

:emittance mequilibriu The

CO 40% H 60%

:ncompositio Vacuum

n compositio

vacuumon the depends :g

pressure. vacuum:P

emission.photon to

due excitation quantum :G

time.damping :

where,2

12

:equationevolution emittance The

dilution. emittance of source

another is scattering gas Beam

1

0

50

10

20

2

mg

nTorrPg

gPGdt

d

g

Emittances are dominated by pressure in low energy,become natural emittances at high energy.

Compact Photon Source

• The X-ray is generated by laser- electron scattering at the chicane magnet.

X-ray• The energy of the scattering photon is:

where, EL is the laser energy, c is the electron speed, is angle of

the scattered X-ray photon, * is the crossing angle of the laser and the electron beam, for head on collision, *=.

is a small correction term.

• The scattered X-ray photons are confined to a cone of 1/ with respect to the electron beam direction

• The bending angle of the chicane magnet can vary from zero to 110mrad.

• The scattered X-ray can easily be separated from the circulating

electron beam at a distance 25cm from the collision point.

Photon Brilliance

• The brilliance of the back scattering X-ray photon is:

• The X-ray flux is given by: where L is the luminosity. For head-on

collision, the luminosity is:• Brilliance 1/x

2z2

Quadrupole Triplet

Fitting results

Best results Parameters

• Currently the ALPHA ring is under construction.

• We will start the experiments on RF cavity soon.