Photocathode%201.5%20(1,%203.5)%20cell%20superconducting%20RF%20gun%20with%20electric%20and%20magnetic%20RF%20focusing

1
The calculation results obtained by SuperLANS and
ASTRA codes
B u d k e r I N P-FZR
Photocathode 1.5 (1, 3.5) cell superconducting RF
gun with electric and magnetic RF focusing
Transversal normalized rms emittance (no thermal
emittance) 0.62 p mm mrad Bunch charge


1 nC Laser pulse duration / Laser pulse rise time
20
ps / 1 ps Axis peak induction of TE mode

0.3 Tesla Surface peak induction of TE
and TM modes
0.132 Tesla Acceleration frequency

1300 MHz Axis peak
field of acceleration mode
50 MV/m
Electron bunch energy

4.62 MeV Energy spread (minimum, rms)

0.32
V.N.Volkov_at_inp.nsk.su
2
RF gun geometry. What are the electric and
magnetic RF focusing?
Electric RF focusing region
Magnetic RF focusing region
Cu
T 78K
T78K
Scaled cathode region
1 Heat sink 2 Choke cell 3 Photocathode Cu
stalk 4 Cathode cell 5 Electric TM field
pattern 6 Magnetic TE field pattern 7 Cavity
full cell 8 TE mode coupler (90º routed) 9 TM
mode coupler pipe
3
Other injector parameters
Acceleration field frequency 1300 MHz
Acceleration peak field at the cavity axis 50 MV/m
Launch phase of bunch centre (optimized) 55º
Laser spot radius at the photocathode (optimized) 1.5 mm
Depth of photocathode Cu stem in back cavity wall (optimized to create optimal RF focusing) 2 mm
External quality factor of input coupler (Qext) 3.79105
Input power / Average beam current assuming the Qext 320 kW / 70 mA
Dissipated 1300 MHz power at Cu pipe of input coupler assuming the Qext and TCu78K 9.4 W
Dissipated 1300 MHz power at cavity Nb wall (assuming unloaded quality factor Qo1010, 2K) 12.13 W
Dissipated 1300 MHz power at photocathode Cu stem assuming TCu78K 5 W
Surface peak field at the photocathode 32.8 MV/m
Frequency of magnetic focusing TE mode 3788 MHz
Axis peak induction of TE mode (optimized) 0.3 Tesla
Maximum vector sum of surface induction of 1300 and 3788 MHz (the limit is 0.18 Tesla) 0.132 Tesla
Surface peak induction of TE mode 0.108 Tesla
Ratio of Peak Induction on the surface and on the axis (RPI) 0.358
Unloaded Quality factor of TE mode assuming the Qo for 1300 MHz 0.85108
Dissipated RF power of TE mode at cavity Nb wall 13.43 W
Dissipated RF power of TE mode at Cu pipe of input coupler assuming TCu78K 3.63 W
Transversal normalized emittance of bunch (thermal emittance is not taken into account) 0.62 p mm mrad
Full emittance thermal emittance of Cs2Te photocathode (0.64 mm) is taken into account 0.89 p mm mrad
Axis coordinate of emittance minimum disposition from the cathode 0.85 m
4
RF fields in the cavity /SLANS codThe vectors of
TE and TM fields are ortogonal
F3788 MHz
Peak fields
E 50 MV/m axis
BTE 0.3 T axis
BTM 0.128 T surface
BTE 0.108 T surface
BTMBTE 0.132 T surface
F1300 MHz
5
High order TE modes selection for low Ratio of
Peak Induction (RPI) at the surface and at the
axis
TE021
TE011
F2572.5 MHz
F3787.8 MHz
F, MHz RPI
2572.5 0.539
3787.8 0.358
3899.7 0.819
3947.2 0.863
Pipe cut off TE frequency 5226 MHz
F3899.7 MHz
F3947.2 MHz
6
Emittance dependence from TE field phase
Set examples
en transversal normalized rms emittance eav-
average emittance Ae emittance amplitude fTE
TE mode phase ?o - constant phase BTE TE mode
peak induction at the axis, T R laser spot
radius at the photocathode, mm ?TM launch phase
(here ?TM50º at maximum bunch
energy)
Set examples

eav, mm 0.805 0.712
Ae, mm 0.212 0.08
BTE, T 0.28 0.3
R, mm 1.0 1.5
1
2
7
Parameter scanning for emittance minimization
/ASTRA cod TE induction (BTE), laser spot size
(R), launch phase (?TM)
Average emittance, mm
Emittance amplitude, mm
Optimum
0.32 0.30 0.28 0.26
0.32 0.30 0.28 0.26
fTM46.3º BTE0.29 T R1.5 mm emin0.7mm


0.7

BTE,T
BTE, T
Sensitivity
for Den5 DBTE0.03T DR0.6 mm D?TM10º
1.0 1.25 1.5 1.75
R,mm
1.0 1.25 1.5 1.75
R, mm
Launch phase scanning
Extreme values fTM
Average emittance, mm 0.62 55º
Emittance amplitude, mm 0! 60º
Energy, MeV 4.62 50º
Energy spread, KeV 15 42º
8
Bunch time evolution
Bunch cross section, mm
Bunch rotates by magnetic TE field
60 cm drift
Phase space, KeV/c
Bunch rotation is subtracted here
X, mm
9
Emittance compensation instanceswithout any RF
focusing, with only electric RF focusing, with
only magnetic RF focusing, with sum - electric
and magnetic RF focusing
Optimized settings performances Without any RF focusing Electric RF focusing only Magnetic RF focusing only Electric and magnetic RF focusing
en, p mm mrad 3.66 1.49 1.28 0.62
(en2 eth2)1/2 3.76 1.72 1.44 0.89
R (laser), mm 2 2 1.5 1.5
fTM, deg 49.4º 46.3º 49.4º 55º
Cathode depth, mm 0 2 0 2
BTE (axis, peak), T 0 0 0.3 0.3
B (surf., peak), T 0.128 0.128 0.132 0.132
p mm mrad - Cs2Te photocathode thermal
normalized emittance K.Floettmann studed
10
1 cell superconducting RF gun (DROSSEL) with
electric and magnetic RF focusing
Optimized performances
Bunch transv. norm.emitt., mm 0.510.52
Emitt. minimum disposition, m 0.265
Average Energy, MeV 2.26
Launch phase of 1300 MHz 25.0º
Laser spot radius, mm 1.5
BTE (peak,, axis), T 0.300
BTE (peak, surface), T 0.168
RPI 0.56
BTM (surface), mT 0.123
BTM BTE, mT 0.173
TE021
Emittance TE compensation
11
3.5 cell superconducting RF gun with electric
and magnetic RF focusing
TE021
Bunch transv. norm.emitt., mm 0.780.98
Emitt. minimum disposition, m 4.25
Average Energy, MeV 8.82
Launch phase of TM 1300 MHz 74.6º
Laser spot radius, mm 1.5
BTE (peak, axis), T 0.324
BTE (peak, surface), T 0.136
RPI 0.42
BTM (surface), mT 0.115
BTM BTE, mT 0.144
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Conclusions

• Emittance compensation by the electric and
  magnetic RF focusing as well as a high
  accelerating gradient are the key factors in
  getting a small emittance with a large charge.
• Either electric or magnetic RF focusing diminish
  the emittance more than twice. And together
  about 6 times.
• The peak induction of magnetic field on the axis
  is about 0.3 T. And sum of magnetic fields on
  cavity surface is less than the limit of 0.18 T.
• The induction of peak magnetic field on cavity
  surface proved to be small due to vector
  summation of orthogonal TE and TM fields. Also
  because of an unoverlapping of their peak fields
  on the surface.
• TE021 mode has a smallest ratio of magnetic peak
  induction on the surface to the peak induction on
  the axis.
• The dependence of emittance from TE phase has
  oscillatory view. There are RF gun parameter
  settins at which the oscillatory amplitude
  becomes zero.
• Transversal emittance remains small in wide range
  of RF gun settings.

13
Acknowledgments

• The author would like to thank
• Dietmar Janssen (FZR),
• Klauss Floettmann (DESY),
• Victor Petrov (BINP)
• for helping in the work.