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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)
Bunch charge
Laser pulse duration / Laser pulse rise time
Axis peak induction of TE mode
Surface peak induction of TE and TM modes
Acceleration frequency
0.62 π mm mrad
1 nC
20 ps / 1 ps
0.3 Tesla
0.132 Tesla
1300 MHz
Axis peak field of acceleration mode
50 MV/m
Electron bunch energy
4.62 MeV
Energy spread (minimum, rms)
[email protected]
0.32%
RF gun geometry.
What are the electric and magnetic RF focusing?
Electric RF focusing region
Magnetic RF focusing region
Cu
T=
78K
T=78K
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
Other injector parameters
Acceleration field frequency
1300 MHz
Frequency of magnetic focusing TE mode
Acceleration peak field at the cavity axis
50 MV/m
Axis peak induction of TE mode (optimized)
0.3 Tesla
Launch phase of bunch centre (optimized)
55º
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
Laser spot radius at the photocathode
(optimized)
Depth of photocathode Cu stem in back cavity
wall (optimized to create optimal RF focusing)
1.5 mm
3788 MHz
2 mm
Ratio of Peak Induction on the surface and on
the axis (RPI)
0.358
External quality factor of input coupler (Qext)
3.79·105
Unloaded Quality factor of TE mode assuming
the Qo for 1300 MHz
0.85·108
Input power
/ Average beam current assuming the Qext
320 kW
/ 70 mA
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 TCu=78K
3.63 W
Dissipated 1300 MHz power at Cu pipe of input
coupler assuming the Qext and TCu=78K
9.4 W
Dissipated 1300 MHz power at cavity Nb wall
(assuming unloaded quality factor Qo=1010, 2K)
12.13 W
Transversal normalized emittance of bunch
(thermal emittance is not taken into account)
0.62
π mm mrad
5W
Full emittance: thermal emittance of Cs2Te
photocathode (0.64 mm) is taken into account
0.89
π mm mrad
Dissipated 1300 MHz power at photocathode
Cu stem assuming TCu=78K
Surface peak field at the photocathode
32.8 MV/m
Axis coordinate of emittance minimum
disposition from the cathode
0.85 m
RF fields in the cavity /SLANS cod
The vectors of TE and TM fields are ortogonal
F=3788 MHz
E
F=1300 MHz
Peak fields
50 MV/m
axis
BTE
0.3 T
axis
BTM
0.128 T
surface
BTE
0.108 T
surface
BTM+BTE
0.132 T
surface
High order TE modes selection
for low Ratio of Peak Induction (RPI) at the surface and at the axis
TE011
TE021
F=2572.5 MHz
F, MHz
RPI
2572.5
0.539
3787.8
0.358
3899.7
0.819
3947.2
0.863
F=3899.7 MHz
F=3787.8 MHz
Pipe cut off
TE frequency
5226 MHz
F=3947.2 MHz
Emittance dependence from TE field phase
 n   av  A  sin 2  TE  o 
Set examples
n – transversal normalized rms emittance
av- average emittance
A – emittance amplitude
φTE – 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 TM=50º at maximum bunch
energy)
Set examples
1
2
εav, mm
0.805
0.712
Aε, mm
0.212
0.08
BTE, T
0.28
0.3
R, mm
1.0
1.5
Parameter scanning for emittance minimization /ASTRA cod
TE induction (BTE), laser spot size (R), launch phase (TM)
0.32
0.30
BTE,T
+ +0.7
Sensitivity
for Dn=5%:
DBTE=0.03T
DR=0.6 mm
DTM=10º
0.28
0.26
Optimum
φTM=46.3º
BTE=0.29 T
R=1.5 mm
εmin=0.7mm
1.0
1.25
1.5
R,mm
1.75
0.32
Emittance amplitude, mm
0.30
+
BTE, T
Average emittance, mm
+
0.28
0.26
1.0
1.25
1.5
R, mm
1.75
Launch phase scanning
φTM
Extreme values
Average emittance, mm
Emittance amplitude, mm
Energy, MeV
Energy spread, KeV
0.62
55º
0!
60º
4.62
50º
15
42º
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
Emittance compensation instances:
without any RF focusing, with only electric RF focusing, with only
magnetic RF focusing, with sum - electric and magnetic RF focusing
Without any RF
focusing
Electric RF
focusing only
Magnetic RF
focusing only
Electric and
magnetic RF
focusing
εn, p mm mrad
3.66
1.49
1.28
0.62
(εn2+ εth2)1/2
3.76
1.72
1.44
0.89
R (laser), mm
2
2
1.5
1.5
φTM, deg
49.4º
46.3º
49.4º
55º
Cathode depth,
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
Optimized
settings &
performances
mm
 th  0.43  R [p mm mrad] - Cs2Te photocathode thermal normalized emittance [K.Floettmann studed]
1 cell superconducting RF gun (“DROSSEL”)
with electric and magnetic RF focusing
Optimized performances
TE021
Bunch transv. norm.emitt., mm
0.51÷0.52
Emitt. minimum disposition, m
0.265
Average Energy, MeV
Launch phase of 1300 MHz
Laser spot radius, mm
25.0º
1.5
BTE (peak,, axis), T
0.300
BTE (peak, surface), T
0.168
RPI
Emittance TE compensation
2.26
0.56
BTM (surface), mT
0.123
| BTM+ BTE|, mT
0.173
3.5 cell superconducting RF gun
with electric and magnetic RF focusing
TE021
Bunch transv. norm.emitt., mm
0.78÷0.98
Emitt. minimum disposition, m
4.25
Average Energy, MeV
8.82
Launch phase of TM 1300 MHz
Laser spot radius, mm
74.6º
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
Conclusions
•
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•
•
•
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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.
Acknowledgments
The author would like to thank
Dietmar Janssen (FZR),
Klauss Floettmann (DESY),
Victor Petrov (BINP)
for helping in the work.