Transcript Slides - Agenda INFN

BIMODAL TWO-FREQUENCY HALF-CELL RF GUN
S.V. Kuzikov1, Y. Jiang2 , A.A. Vikharev1, J.L. Hirshfield2,3
1Institute
of Applied Physics, 46 Ulyanov St., Nizhny Novgorod, 603950, Russia
2Yale University, New Haven, CT, USA
3Omega-P, Inc., New Haven, CT, USA
Outline
1. Motivation to use multi-frequency multi-mode RF gun
2. Two-frequency, bimodal, 0.5 cell RF gun (MUFFIN) based on TM010 -TM011 f+2f cavity
Optimization of parameters
Feeding system
3. Feeding RF source for a gun controlled by laser of the same gun
4. Conclusion
Why multi-frequency RF gun?
1. In a cavity excited by several harmonically related modes simultaneously electrons are
accelerated by a field which is a sum of partial mode contributions (Eacc~NEn). The
necessary RF power to provide this gradient is by factor N less in comparison with a single
mode gun1.
Application of several harmonically related modes in RF injection gun cell promises
less loss and less input power in comparison with traditional single mode gun of the
same bunch energy, length, charge, and emittance.
2. In a multi-frequency cavity there are effects of exposure time reduction and a so-called
“anode-cathode” phenomenon2. In a cavity surface cathode fields (fields pointed out so
that electron autoemission is possible) and anode fields (such fields prohibit electron
emission) are not equal each to other. So bunch acceleraration might be at higher field
than breakdown threshold.
Anode-cathode principle allows obtaining faster acceleration (higher acceleration
gradient) and less dark current in comparison with a single mode gun.
3. Several modes of different frequencies and different space structures allows to optimize
time-space structure of fields by means of manipulations with amplitudes and phases of
these modes.
Optimizations of field structure promises to obtain better emittances in comparing to
classical design.
[1] Kuzikov S.V., et al. A Multi-Mode RF Photocathode Gun, Proceedings of International Particle Accelerator
Conference 2011, San Sebastian, Spain, (2011), pp. 1135-1137.
[2] S.V. Kuzikov, et al. Asymmetric Bimodal Accelerator Cavity for Raising rf Breakdown Thresholds, Phys.
Rev. Lett. 104, 214801 (2010).
MUFFIN RF gun based on TM010-TM011 f+2f cavity
Cathode wall
Anode wall
TM010 eigen mode (E-field)
f=1.3 GHz,
Q=2104 (normal copper conductivity)
TM011 eigen mode (E-field)
f=2.6 GHz,
Q=2104 (normal copper conductivity)
1
1
Normalized mode field, arb. un.
0.8
0.6
0.4
0.2
0
-0.2
-0.4
2
-0.6
-0.8
0
2
4
6
8
10
12
14
distance, cm
Field distributions of modes at axis of the cavity: 1 – TM010 mode, 2 – TM011 mode
Cathode fields are responsible for autoelectron emission and fast breakdown.
In a multi-frequency system anode fields could be much higher than cathode
fields which one must keep less than breakdown threshold.
E (t )  a1 ( z )  cos(1  t   )  a2 ( z )  cos(2  t   
2
a
  2 ), 2  0.25,   270 ,  2  180
1
a1
Cathode-like fields
Anode-like fields
Anode-like fields
Cathode-like fields
Time dependence of electric field at cathode wall
at r=0.
Time dependence of electric field at anode
wall (at point on surface where field reaches
maximum)
Cavity profile
ASTRA simulation of dPz/dz (i.e., a force experienced by an electron), (at left); and energy
gain along the axis (at right).
In this example the beam energy at the exit is 5.2 MeV with optimized injection.
dPz/dz is about 40 MV/m at the cathode surface, and rises up to 85 MV/m, with the peak
cathode-like surface field only 64 MV/m.
High gradient allows to use half cell design instead of 1,5 cell. Cooling of simple half cell
could be more effective.
Half-cell RF gun setup. The cavity profile and the compensation coils are shown. The electric
fields of TM010 (green) and TM011(blue) modes and the magnetic field (purple) of the
compensation coil along the axis are plotted.
Transverse beam size of electron bunch along the axis, and bunch cross section at the
entrance of linac. The initial beam size is 1.35 mm and the final beam size at 1.2 m
downstream is about 0.65 mm. The dashed line indicates the cavity profile.
The transverse and longitudinal emittance evolution along the axis of MUFFIN with initial
beam size 1.35 mm, pulse length 3 ps, and bunch charge 0.5 nC
Transverse and longitudinal phase space diagrams at entrance to the boost linac
Cell Number
Operating Parameters
Laser pulse length (ps)
Laser Spot Size σx,y (mm)
Bunch Charge (nC)
Compensation Magnetic Field (T)
Beam Dynamics
Beam Energy (MeV)
εx,n(π mrad mm)
εy,n(π mrad mm)
εz, n(π keV mm)
Energy spread (%)
st
1 Harmonic
Frequency (GHz)
Q Factor
Mode Amplitude (MV/m)
Power (MW)
nd
2 Harmonic
Frequency (GHz)
Q Factor
Mode Amplitude (MV/m)
Power (MW)
Total Power (MW)
Anode-Cathode Effect
Peak Anode-like Field (MV/m)
Peak Cathode Field (MV/m)
MUFFIN
MUFFIN
MUFFIN
FNPL Gun
0.5
0.5
0.5
1.5
EXFEL
Gun
1.5
3
1.35
0.5
0.36
3
1.35
0.5
0.47
20
0.75
1.0
0.35
3
1.35
0.5
0.129
20
0.75
1.0
0.163
5.16
2.79
2.80
14.0
1.46
7.64
3.03
3.06
10.0
0.73
4.91
2.45
2.48
16.3
2.23
5.03
6.58
6.58
12.35
1.57
5.10
2.71
2.66
20.60
0.62
80
4.85
1300.05
22007.9
120
10.92
80
4.85
1300.0
23974
40
2.83
1299.6
26304.6
40
2.92
20
1.70
6.55
2600.22
20376.1
30
3.82
14.7
20
1.70
6.55
2.83
2.92
120.8
64.0
181.2
96.0
120.8
64.0
49.63
49.63
47.06
47.06
Beam dynamics parameters, including beam energy, transverse emittance, longitudinal
emittance, and energy spread, versus the peak electric field of the fundamental frequency
component.
The field strength of the second harmonic component is one quarter of that of fundamental.
Laser pulse length is 3 ps and bunch charge is 0.5 nC.
Feeding system of bimodal cavity
TE10
Excitation of the TM010 mode at frequency f
Reflection and transmission of TE10 mode
TE11
Excitation of the TM011 mode at frequency 2f
Reflection of TE11 mode
Electron gun fed by RF source controlled by laser to be responsible for photoemission
high-power RF
at =N0
bunches at =0
photoinjector
What is better?
1) RF source – master, laser – slave;
2) RF source – slave, laser – master.
laser radiation
pulsed at =0
phase-locked RF
oscillator
The 2nd case promises smaller jitter,
because laser pulse duration is much
shorter than RF period.
laser
perfect conductor
1
0.9
GaAs
photoconducting layer
0.8
R
θ
d
L
|R|2, arb. units
0.7
0.6
0.5
0.4
0.3
laser light
0.2
0.1
0
10
10
microwave radiation
11
10
12
10
13
10
14
10
15
16
10
Ne, cm
10
17
10
18
10
19
10
20
10
-3
Irradiation of a semiconductor (GaAs or CVD diamond) by laser with photon energy equal to band
gap causes appearance of a thin photoconductive layer. RF properties depends on Ne (laser light
energy).
Schematic of a phase-locked electron maser. Electric fields of high-Q (1) and low-Q
(2) eigenmodes in case of Ohmic losses modulation*.
k " / k0  ( s / 2Q)(1  cos Lt )
s – amplitude of losses modulation
Example: Gyrotron with diamond switch,  =193 nm, f=30 GHz, T=33 psec,
r=2.9 mm, s=1 mm, a=3 mm, =1-10 psec, L=10 , Wlas=100 nJ.
Qmax=25(L/ )2=2500 (at free oscillations)
Ne=1.81015 cm-3 , 1-R2=20%
Qmin=490 (with insertion loss caused by photoconductivity )
“S.V. Kuzikov, A.V. Savilov, Parametric Phase Locking in an Electron RF Oscillator, Phys. Rev. Lett. 110, 174801
(2013).
Simulation of phase and frequency locking
=-0.25, s=0.3
RF-wave frequency (up) and phase (bottom) at various phases of the initial rf noise in
case of the phase-locked operation
metallic electrodes
skin depth
laser spot
GaAs
b
a
Klystron-type scheme of phase locked RF source
(any existing klystron could be used with input RF
signal produced by laser modulated current)
Active trigger producing modulated current
laser light
laser light
GaAs
laser light
GaAs
Rsh
R(t)
GaAs
U0
L
electron beam
l
Equivalent scheme
TM010 modulation cavity
Simulation of modulation and parameter optimizations
0.6
16
10
E
1
14
0.5
8
2
0.3
10
6
8
4
6
0.2
average losses, W
0.4
E-field, kV/cm
current, Amperes
12
Ploss
4
2
0.1
3
2
Plaser
0
0
0
2
4
6
8
10
time,sec
Modulation current: 1 – Rsh=0, 2 – Rsh=10 kOhm,
3 – Rsh=100 kOhm
(f=1 GHz, Wlas=10 nJ, U0=5 kV, Rc=25 mm, n=30)
0
0
10
20
30
40
50
60
70
80
90
100
shunt DC resistance, kOhm
Produced RF electric field amplitude
(red) and average Ohmic power loss in
GaAs (blue).
(f=f0=1 GHz, Wlas=10 nJ, U0=5 kV,
Q=1000, =100 mks, rep. rate=100 Hz,
l=100 mm, a=b=2 mm, Rc=25 mm, n=30)
Rrad
 1kOhm
Q
Conclusion
1. The TM010-TM011 f+2f MUFFIN has high acceleration gradient. This allows
using 0.5-cell design instead of classical 1.5 cell design.
2. The MUFFIN gun in comparison with reference single-mode gun provides
optionally:
- two times smaller transverse emittances at regime of 3 ps bunch in case of
similar cathode field,
- better transverse and longitudinal emittances, and two times smaller energy
spread at regime of 3 ps bunch in case of higher cathode field,
- smaller transverse emittances at regime of 20 ps bunch in case of similar
cathode field.
3. Fast semiconductor switch, controlled by ~GHz repetition rate laser, allows
either to make rf amplifier phased with bunches or to obtain phase locking
of existing rf oscillator.