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Study of Secondary Emission Enhanced Photoinjector
Xiangyun Chang1, Ilan Ben-Zvi1,2, Andrew Burrill1, Peter D. Johnson2 Jörg Kewisch1 Triveni S. Rao3 and YongXiang
Zhao2
Collider-Accelerator Department1, Physics Department2, Instrumentation Division3
Brookhaven National LaboratoryUpton NY 11973 USA
Introduction
Schematic diagram of a secondary emission enhanced photoinjector
The advantages of the Secondary Emission Enhanced Photoinjector
• 1. Reduction of the number of primary electrons by the large SEY, i.e. a very low laser power requirement in the photocathode producing the primaries.
• 2. Protection of the cathode from possible contamination from the gun, allowing the use of large quantum efficiency but sensitive cathodes.
• 3. Protection of the gun from possible contamination by the cathode, allowing the use of superconducting gun cavities.
• 4. Production of high average currents, up to ampere class.
• 5. Expected long lifetime
Design considerations • The metal layer
– The choice of material.• One would like to use material which has
high electrical conductivity (σ) to reduce the resistance of the film.
• On the other hand one would like use material which big long Continuous Slowing Down Approximation (CSDA) range parameter RCSDA to reduce the energy loss in the metal film
• The Al is the best choice. 24 /105.310 cmgkeVRAl
mkeVRkeVL AlSTP 3.1/1010
• RF penetration of the Al film for 700MHz MW
It is proven that the RF penetration of the metal film is very small for our application.
So, Al film not only carries out the replenishment current but also the large RF current. This makes the thicker Al film more important.
mAl
Al
1.32
AlAlt
• The secondary electron yield (SEY)
SEY measurement for reflection mode
A. Shih, J. Yater, P. Pehrsson, J. Butler, C. Hor, and R. Abrams
J. Appl. Phys., Vol. 82, No. 4, 15 August 1997It is expected that almost all the secondary electrons produced would come out from the diamond for our transmission mode. We will assume a conservative SEY value of 300 at 4keV for our application.
• The thickness of diamond window– One would like to use thicker diamond to
improve the thermal conduction of the window.– On the other hand, for our RF application the
diamond thickness is limited due to the allowed transmission time for the secondary electrons and the limited electron drifting velocity in diamond.
– The allowed transmission time is 120~200ps for our case.
– The drift velocity is smvD /107.2 5
mvpst DDmd 32120
• Diamond thermal conductivity
Diamond has the highest thermal conductivity. The thermal conductivity is determined by the grain size, boundary, umklapp process and impurity of the film.
. Grain size and surface roughness of our 30μm thick diamond sample
• The impurity problem – Impurities: Boron (p-type), Nitrogen (n-type),
Hydrogen (n-type), Phosphorus (n-type), Lithium (n-type) and Sodium (n-type).
– Heating problem: • Free electrons on diamond conduction band (Nitrogen
doping). Under the strong RF field, it will behave the same way like the secondary electrons. So, not only produces extra heat but also the background current and back-bombardment.
• If the holes on the valence band (Boron doping) is big, it will only produce the extra heat.
– Field shielding problem:• If the concentration of free electrons or holes is too
high, it can be thought as a bad quality conductor. So, the RF field attenuates inside diamond surface and then affects the drift velocity of the secondary electron
• Limits on impurities:– The concentration of the free electrons or holes should be
well below the secondary electron density in diamond. For RHIC cooling, charge/bunch=20nC, the density is about:
– The Hydrogen, Phosphorus and other alkali elements impurities form strong chemical bonds with C with no free electron. So, this impurities are not important .
– Nitrogen:1ppm or more ( ) The activation energy can be 1.7eV or more. The portion exited to conduction band by thermal at T=500K:
– Boron:
• Boron doping concentration can be easily less than
313419
9
/101030106.1
1020cmn
317 /1078.11 cmppm
18107.1
exp
Tk
eV
n
n
BN
e
4109.137.0
exp
Tk
eV
n
n
BB
h
314 /10 cm
Diamond temperature • RHIC e-cooling project:Charge=20nC/bunchRepetition frequency=9.4MHzRadius R>10mmPrimary electron energy EPri=10keVDiamond thickness rDmd=30μm
Al thickness tAl=800nm
Peak RF field on cathode E0=15MV/m
SEY=300Temperature on diamond edge Tedge=80K
Primary electron pulse length PlsPri=10deg
Temperature distribution for Cooling
0
20
40
60
80
100
120
140
160
180
200
0 5 10 15 20
R (mm)
T (
K) R=10mm
R=12.5mm
R=15mm
R 10mm 12.5mm 15mm
Primary power=6.2667(W
)6.2667(W
)6.2667(W
)
Secondary power=7.5536(W
)7.5536(W
)7.5536(W
)
RF power=7.5241(W
)19.9958(
W)48.5851(
W)
Replenishment power=
0.0422(W)
0.0459(W)
0.0538(W)
Total power=21.3866(
W)33.8620(
W)62.4592(
W)
Edge Temperature effectTemperature distribution for Cooling (tAl=800nm)
0
50
100
150
200
250
300
0 5 10 15
R (mm)
T (
K) Tedge=80K
Tedge=140K
Tedge=200K
Tedge 80K 140K 200K
Primary power= 6.2667(W) 6.2667(W)6.2667(W
)
Secondary power= 7.5536(W) 7.5536(W)7.5536(W
)
RF power=19.9958(W
)30.0569(W
)42.0461(
W)
Replenishment power= 0.0459(W) 0.0690(W)
0.0966(W)
Total power=33.8620(W
)43.9461(W
)55.9630(
W)
Al thickness effect
Temperature distribution for Cooling
0
20
40
60
80
100
120
140
160
180
200
0 5 10 15
R (mm)
T (
K) tAl=800nm
tAl=600nm
tAl=400nm
• Energy Recovery Linac (ERL) project in BNLCharge=1.42nC/bunchRepetition frequency=703MHzRadius R~5mmPrimary electron energy EPri=10keVDiamond thickness rDmd=30μmAl thickness tAl=800nmPeak RF field on cathode E0=15MV/mSEY=300Temperature on diamond edge Tedge=80KPrimary electron pulse length PlsPri=10deg
Temperature distribution for ERL
0
50
100
150
200
250
300
0 2 4 6 8
R (mm)
T (
K) R=2.5mm
R=5mm
R=7.5mm
R 2.5mm 5mm 7.5mm
Primary power=33.3333(
W)33.3333(
W)33.3333(
W)
Secondary power=40.1786(
W)40.1786(
W)40.1786(
W)
RF power= 0.0460(W) 0.6691(W) 3.3975(W)
Replenishment power= 0.0250(W) 0.0227(W) 0.0228(W)
Total power=73.5829(
W)74.2037(
W)76.9322(
W)
Secondary electron beam quality
• Bunch broadeningDrift velocity is saturated if E>2MV/m, the broadening is
determined by the straggling distance of primary electron in diamond:
This is equivalent to a 2.6ps long laser pulse
• Limit on charge density
For RHIC cooling example with R=12.5mm, Q<< 65nCFor ERL example with R=5mm, Q<<10nC
mkeVRkeVD CCStrg 41.0/10%2510
24 /1082.210 cmgkeVRC
002/ ERQESPC
• Secondary electron temperature
Under
00
W
LeD
Le
TWTWveE
dt
Wd
dt
Wd
1
2 2538 0.41i r m r mE a E a
mMVE /100
eVTe 4.0
eVmMVmTSurface 1/101.0
Experiments
Conclusion
• The study and calculations show great feasibility of Secondary Emission Enhanced Photoinjector.
• The new approach has many advantages.