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.