1. Electric Field Amplitude (MV/m)
The RF System of the Probe Beam Linac in CTF3
Franck Peauger, Alban Mosnier, Daniel Bogard, Gilles Dispau, Wilfrid Farabolini, Patrick Girardot, Jean Luc Jannin, CEA/DSM/DAPNIA, Gif-sur-Yvette Cedex, France
Alexej Grudiev, Gerard Mcmonagle, Jean Mourier, CERN, Geneva, Switzerland
Probe Beam Linac Parameters
Schematic layout of CTF3
The probe beam linac aims at simulating the main beam of CLIC in order to measure precisely the performances of the 30 GHz accelerating structures
Abstract: The probe beam Linac in CTF3 will simulate the main beam of CLIC in order to measure precisely the performances of the 30 GHz accelerating structures.
The poster shows a layout of the probe beam linac and its RF system. Three important points are described in details: the use of a RF pulse compression cavity to increase the RF power, the development of a high power phase shifter to control the bunch length and transient simulations of the accelerating field in the RF structure to determine the best RF pulse shape to be provided.
Introduction: The Compact Linear Collider (CLIC) is a study of a linear multi TeV collider. The test facility CTF3 will demonstrate the RF power generation scheme. This 30 GHz high power RF is exacted from a drive beam, running parallel to the main beam.
The probe beam Linac in CTF3 will simulate the main beam in order to measure precisely the performances of the CLIC 30 GHz accelerating structures.
The RF power of the probe beam linac is produced by a single 45 MW klystron working at 3 GHz. A pulse compression system increased the power up to 90 MW and a distibution network divides the power into four arms.
Schematic layout of the probe beam linac (CALIFES)
Parameters
Specifications
Observations
Energy
200 Mev
Avoid beam disruption in high RF fields
Norm. rms emittance
< 20 π mm mrad
Fit in 30 GHz structure acceptance
Energy spread
<  2 %
Measurement resolution
Bunch charge
Bunch spacing
0.2 nC
0.33 ns
~ CLIC Parameters
Number of bunches
1 – 64
Measure 30 GHz structure transients
Rms bunchlength
< 0.75 ps
Acceleration with 30 GHz
0.33 ns
3 GHz
Bunch length < 0.75 ps
DAPNIA / IN2P3 / CERN Collaboration
30 GHz
33 ps
RF Gun
Layout of the RF System
Power phase shifter
TE10□
E Field
H Field
RF Generator GHz
Slow Phase Shifter 360 °
Fast Phase Shifter 180 °
Solid State Pre-amplifier
The Power Phase Shifter is a 3 GHz scaling of the one studied at SLAC at 11.4 GHz and consists of a circular waveguide operating on the TE01 mode and two wrap-around mode converter
The circular waveguide has an area with an expanded diameter, inducing a reduction of the waveguide length.
The length of this widened guide can be varied to produce a phase variation.
PFN (25 cells)
320 kV – 360 A
7.6 µs – 5 Hz
300 W – 3 GHz
TE01○
 0.25% ripple during 5.5 µs
45 MW – 3 GHz Klystron
DC Power Supply + -
Thyratron
21.5kV
Pulse Transformer
SF6
1:15
45 MW – 5.5 µs
BOC Cavity
Klystron Gallery
Beam Tunnel
Parameters
Specifications
Peak RF Power
25 MW
Sensitivity
1° / mm
Maximum course
200 mm
Precision
0.5 °
Stability
0.1 °
Bandwidth |S11|
< dB
90 MW
90 MW – 1.4 µs
4.5 dB Splitter W
Electric field simulation (HFSS)
The RF components operates mainly under a vacuum of 10-9 mbar. Losses in rectangular waveguides are 0.02 dB/m (WR284 in Copper)
4.5 dB Splitter
Circ
3 dB Splitter
SF6
Ampl
Phase adjustment by length of waveguides
Power Phase Shifter F
Taking into account the losses in RF components, a total power of approximately 70 MW is delivered to the linac, W
F = - 90°
F = 0 °
F = 0 °
Electric field superposition for two positions
7 MW
15 MW
25 MW
22 MW TW
Acceleration TW
Acceleration
SW Gun
TW bunching
bellows
3D mechanical model
Surface = 1535 x 400 mm, Heigth = 508 mm
Weight = 270 kg
2D mechanical drawing
Pulse compressor cavity
Transient calculation in traveling wave section
E Field
A specific code, based on the coupled resonator model has been developed to study the transient effects in the traveling wave sections
Kn-1 Kn
H Field
The Pulse compressor system is based on a high Q0 storage cavity working on a “whispering gallery” mode TM10, 1, 1
Q0 = (calculated with HFSS)
f0 = GHz
β = Qx / Q0 = 6
n-1 n
n+1
The LIL section is a quasi constant gradient structure, composed of 9 constant impedance families linked by 4 linearly tapered transition cells
Vn-1 Vn Vn+1 Yn
Lkn-1 Lkn Lkn+1 Vg Vr
Good directivity provided by λg/4 multi-hole coupler
Equivalent circuit
differential equation relative to cell n
Φg(t)
ic(t) 1
Forward wave Vg
vg(t)
vc(t) G C L
Backward wave Vr = Vc - Vg
Differential equation
For a phase step of 57°, and a phase variation until 180° over the total pulse length,
Filling time
Reflection coefficient
Electric Field Amplitude (MV/m)
Approximations :
Sinusoidal voltage
A flat top pulse is obtained :
- Voltage amplification factor = 1.45
- Power amplification factor = 2.1
- Vg varies slowly compared to
Time (s)
Cell number
- High Q0 factor