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High Intensity Polarized Electron
Gun Studies at MIT-Bates
10/01/2008 PESP2008
Evgeni Tsentalovich
MIT
1
OUTLINE
•
•
•
•
Introduction, motivation
Major challenges
Developments at MIT-Bates
Conclusion
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2
eRHIC (Linac-ring version)
Requires a polarized electron source with an extremely
high current
Currently
achieved
Luminosity ~ 2.6  1033 cm 2s 1
 I(average) ~ 250 mA
I(peak) ~ 100 A
200 µA
10 A
High polarization → strained GaAs → QE ~ 0.5%
I(mA )  (nm )  Plaser (W)  QE(%) / 124
Average laser power ~ 80 W (fresh crystal)
1W
Hundreds Watts might be needed as crystal loses QE
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3
Main challenges
High average current – cathode
damage by ion bombardment
Solution:
High peak current – surface charge
saturation (QE drops at high light
intensity); space charge saturation
Cathode with
very large area
25
0.25
20
0.2
15
0.15
10
0.1
5
0.05
0
0
0
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10
20
30
Laser Power, Watts
40
QE,%
Peak current, mA
High heat load on the cathode – tens of
Watts of laser power
50
4
Ion Damage
Ion damage is inversely proportional to emitting area
anode
residual gas
Ionized residual gas
strikes photocathode
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cathode
Ion damage distributed
over larger area
5
Damage location
Electrons and ions follow different trajectories.
Usually, ions tend to damage central area of
the cathode.
JLAB data
Ring-like cathodes ?
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6
Ion Trapping in CW Beam
Cathode
Anode
Beam line
Ions produced below the anode are trapped in the
electron beam. Half of them will drift toward the gun
and get accelerated in the cathode-anode gap
toward the crystal.
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7
JLAB results for anode biasing
Anode = 0 V
Support = 0 V
Charge = 172 C
Anode = 2 kV
Support = 300 V
Charge 175 C
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DQE
DQE
8
High Intensity Gun Studies at
MIT/Bates
• The project investigates the feasibility of extracting very
high (tens, perhaps hundreds of mA) current from the
gun.
• The project addresses issues of high average current
and high heat load on the cathode.
• Phase I – studies of ion damage, design and
construction of the cathode cooler, gun simulations.
• Phase II – design and construction of the gun and the
beam line, beam tests.
10/01/2008 PESP2008
9
Ion Damage Studies - Apparatus
• Existing gun.
• New diode array laser (~808 nm, P up to 45 W).
• Existing test beam line. This beam line was not designed
for high current and beam losses of 5-10% are typical.
These losses produce out-gassing, and reduce the
lifetime by both poisoning the cathode and ion
bombardment. Relatively low lifetime and significant ion
damage allowed to conduct the measurements fast.
• CW current – one can expect ion trapping.
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Ring-shaped Laser Beam
Fiber
L1
Axicon
L2
Cathode
Axicon (conical lens) in combination with a converging lens (L2)
produces ring-shaped beam in the focal plane of L2. Lens L1
reduces the laser beam divergence (25 from the fiber). Without
axicon, a very small beam spot will be produced. QE could be
mapped by moving the L2
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Axicon-based System Simulations
20
15
10
5
0
-5
-10
-15
-20
L1
-25
-200
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Axicon
200
L2
600
1000
1400
12
Axicon-based System Simulations
5
3
1
-1
-3
-5
-5
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-3
-1
1
3
5
13
Beam Profile (no axicon)
Measured using razor blade
technique and inverse Abel
transformation
40
35
30
35-40
25
30-35
25-30
20-25
20
FWHM<.5 mm
15-20
10-15
15
5-10
0-5
10
5
S  .1 mm
1.3
0.9
0.5
-1.5
1.5
1.3
1.1
-1.1
0.9
0.7
0.3
-0.7
0.1
-0.1
-0.5
-0.3
-0.9
-0.3
0.5
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-0.7
-1.3
0.1
-1.1
-1.5
0
2
14
Axicon Beam Profile
9-10
10
8-9
9
7-8
8
6-7
7
5-6
6
4-5
5
3
4
2.4
3
2
1.8
1
2-3
1-2
0-1
1.2
0
0.6
-0.6
-1.2
-1.8
-3
S  9 mm
2
3
2.4
-2.4
1.8
1.2
0.6
0
-0.6
-1.2
0
-1.8
-2.4
-3
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3-4
15
Axicon Beam Profile
8-10
6-8
4-6
2-4
0-2
3
2.6
2.2
1.8
1.4
1
0.6
0.2
-0.2
-0.6
-1
-1.4
-1.8
-2.2
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-2.6
-3
3
2.8
2.6
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
-1.8
-2
-2.2
-2.4
-2.6
-2.8
-3
16
Axicon Beam Profile
10
8
6
4
2
0
-4
-3
-2
-1
0
1
2
3
4
R,mm
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QE map of the Fresh Crystal
a0
QE, %
1.8
1.6
1.4
1.2
1
0.8
0.6
1.6-1.8
1.4-1.6
1.2-1.4
1-1.2
0.8-1
0.6-0.8
0.4-0.6
0.2-0.4
0.4
0-0.2
0.2
0
5
4
3
2
1
0
-1
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-2
-3
-3
-4
-4
-5 -5
-2
-1
0
1
2
3
4
5
18
QE change (small spot in the center)
a13-a12
1
0.9
0.8
0.7
0.9-1
Run 12.32 C
0.8-0.9
0.7-0.8
0.6
0.6-0.7
0.5-0.6
0.5
0.4
0.3
0.4-0.5
0.3-0.4
0.2-0.3
0.1-0.2
0-0.1
0.2
0.1
0
5
4
3
2
1
0
-1
-2
-2
0
2
4
5
-3
-3
-4
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-1
1
3
-4
-5 -5
19
QE change (run with axicon)
a9-a8
1
Run 17.35 C
0.9
0.8
0.9-1
0.8-0.9
0.7
0.7-0.8
0.6-0.7
0.6
0.5
0.4
0.5-0.6
0.4-0.5
0.3-0.4
0.2-0.3
0.1-0.2
0.3
0-0.1
0.2
0.1
0
5
4
3
2
1
0
0
-1
-2
-2
3
5
-1
-3
-3
-4
-4
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1
2
4
-5
-5
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QE change (axicon, anode biased 1 kV)
a11-a10
1.2
Run 17.62 C
1
0.8
1-1.2
0.8-1
0.6
0.6-0.8
0.4-0.6
0.2-0.4
0-0.2
0.4
0.2
5
4
3
0
2
1
0
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-1
-2
-3
-4
-5 -5
-4
-3
-2
-1
0
1
2
3
4
5
21
QE change (large spot in the center)
a15-a14
Run 17.46 C
0.9
0.8-0.9
0.8
0.7-0.8
0.7
0.6-0.7
0.5-0.6
0.6
0.4-0.5
0.3-0.4
0.5
0.2-0.3
0.4
0.1-0.2
0-0.1
0.3
0.2
0.1
5
0
3
5
4
1
3
2
-1
1
0
-3
-1
-2
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-5
-3
-4
-5
22
QE change (small spot in the corner)
a17-a16
Run 16.84 C
1
0.9
0.8
0.9-1
0.8-0.9
0.7-0.8
0.7
0.6
0.5
0.6-0.7
0.5-0.6
0.4-0.5
0.3-0.4
0.2-0.3
0.4
0.3
0.1-0.2
0-0.1
0.2
5
0.1
3
0
1
-1
-3
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-5
-5
-4
-3
-2
-1
0
1
2
3
4
5
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Radial distribution
axicon, 17.4 C
axicon, 17.4 C, anode 1 kV
small spot, 12.3 C
large spot, 17.5 C
laser(small)
laser (axicon)
1
0.8
0.6
0.4
0.2
0
-6
-4
-2
0
2
4
6
R,mm
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Lifetime
Axicon, anode grounded, t~120h
Large spot X1.4, t~60h
Axicon, anode at 1 kV, t~230h
small spot (center) X17, t~50h
QE, %
1.5
1
0.5
0
0
10
20
30
40
50
Time, hours
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25
High Intensity Run (1 mA)
•
•
•
•
•
Achieved .5 mA with laser power of .25 W (QE=.34 %)
Achieved 1 mA with laser power of 1.16 W (QE=.15%)
Gun vacuum pressure rise (factor of 10)
Current dropped to 132 A in 1 hour
At laser power of 1.16 W, QE degrades even without HV ! –
Overheating.
• Thermal estimate (thermal conductivity through the stalk only
~.01-.025 W/degree
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Conclusion
• Ion damage is concentrated near the center of
the cathode in every configuration.
• Ring-shaped beam allows to improve the lifetime
significantly.
• Biasing the anode improves the lifetime of the
CW beam.
• Active cooling is a “must” for laser powers
exceeding 1 W.
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New optics
1
Old optics:
0.8
Small spot
Axicon
0.6
S  .1 mm 2
S  9 mm 2
New optics S  170 mm 2
0.4
0.2
0
-10
-5
0
5
10
R,mm
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Gun Simulation
• Large emitting area produces large
emittance
• Although emittance is less important for
eRHIC, large beam could result in beam
losses near the gun.
• The main purpose of the simulations is to
minimize the beam losses in the gun and
beam line.
• The second goal – ion distribution
optimization
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Gun Simulation
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Gun Simulations - Ions
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Gun Simulations - Ions
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Gun Simulations - Ions
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Cathode Cooling
• The conceptual design of the test chamber
is completed.
• The test chamber will validate the
adequacy of the cooling power, HV and
high vacuum compatibility and vacuum
cathode handling with manipulators.
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34
Cathode Cooling
HV
Water in
Water out
Manipulator
Crystal
Cathode
10/01/2008 PESP2008
Laser
35
DBR – Equipped Crystal
For instance, talk by L. Gerchikov, St. Petersburg, at PESP 2007
h
“Normal”
cathode
GaAs
Substrate
Cathode with
Distributed
Bragg
Reflector
(DBR)
GaAs
Substrate
e
Buffer
RDBR= 1
SL
BBR
RGaAs= 0.3
h
e
DBR
Buffer
SL
BBR
In “normal” cathode, only 30% of light is reflected. In DBRequipped cathode 99% of light is reflected.
10/01/2008 PESP2008
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