RF for Linacs Erk Jensen – CERN AB/RF - CERN Accelerator School

RF for Linacs
Erk Jensen – CERN AB/RF
Outline
�� RF Power Sources (solid state, tetrodes, klystrons, ...)
�� RF Pulse Compression (principle, flat pulses, BOC)
�� Accelerating Structures (Brillouin diagram, HOMs, beam
loading, SC accelerating structures, HDS)
E. Jensen, RF for Linacs 1

CW/Average power [kW]
10000
1000
100
10
1
0.1
Transistors
SSPAs (x32)
Grid tubes
IOT
Klystrons
CCTWT
RF Power sources
Typical ranges (commercially available)
10 100 1000 f [MHz] 10000
E. Jensen, RF for Linacs 2

MOSFET
VDMOS
Vertical Doubly Diffused
www.essderc2002.deis.unibo.it/ESSDERC_web/Session_D18/D18_4.pdf
RF power transistors
LDMOS
Lateral Doubly Diffused
E. Jensen, RF for Linacs 3

LEIR SSPA, 1 kW, 0.2 – 50 MHz
MRF151G
E. Jensen, RF for Linacs 4

DU1029UK
Soleil Booster SSPA, 40 kW, 352 MHz
http://accelconf.web.cern.ch/AccelConf/e04/PAPERS/THPKF031.PDF
147 modules
E. Jensen, RF for Linacs 5

potential
control
grid
Ug1
Ug2
Ra
cathode anode (plate)
4CX250B
(Eimac/CPI),
< 500 MHz, 600 W
(Anode removed)
screen
grid
Ia=0
Ia max
Tetrode
+Ua
Ua
RS 1084 CJ (ex Siemens, now Thales),
< 30 MHz, 75 kW
YL1520 (ex Philips, now Richardson),
< 260 MHz, 25 kW
E. Jensen, RF for Linacs 6

Classes of operation:
A: large DC bias current, η < 50 %
AB: small DC bias current, η ≈50 … 60 %
B: 0 bias current, halfwave, η < 78 %
C: current only with large modulation, η ≈90 %
Tetrode operation (AB1)
operating line
Ia(t)
This example: Thales TH 681
E. Jensen, RF for Linacs 7

High power tetrode amplifier
CERN Linac3: 100 MHz, 350 kW
50 kW Driver: TH345, Final: RS 2054 SK
CERN PS: 13-20 MHz, 30 kW
Driver: solid state 400 W, Final: RS 1084 CJSC
E. Jensen, RF for Linacs 8

-V 0
t
Cathode
velocity
modulation
drift
RF in RF out
Klystron principle
density
modulation
z
Collector
E. Jensen, RF for Linacs 9

Klystrons
CERN CTF3 (LIL):
3 GHz, 45 MW,
4.5 μs, 50 Hz, η 45 %
CERN LHC:
400 MHz, 300 kW,
CW, η 62 %
E. Jensen, RF for Linacs 10

1
4
2
3
RF Pulse Compression
6
4
2
Input pulse
0
0 2000 4000 6000 8000
360
180
Input phase
0
0 2000 4000 6000 8000
6
4
2
“SLED” output
pulse
0
0 2000 4000 6000 8000
SLED: SLAC Energy Doubler LIPS: LEP Injector Power Saver
E. Jensen, RF for Linacs 11

6
4
2
3
2
1
Standard “SLED”
Pulse
0
0 2000 4000 6000 8000
0
0 2000 4000 6000 8000
Flat pulse
Flat output pulses
200
100
200
100
0
0
RF phase
modulation
5000 6000
Old
New
5000 6000
RF phase
modulation
3
2
1
0
0 2000 4000 6000 8000
3
2
1
Flat pulse
0
0 2000 4000 6000 8000
Distorted pulse
-10 kHz
E. Jensen, RF for Linacs 12

BOC „Barrel Open Cavity“
Pulse compressor
E. Jensen, RF for Linacs 13

Electric field, logarithmic scale
BOC
2.99848 GHz,
S11: -12.9 dB
Magnetic field
E. Jensen, RF for Linacs 14

Travelling Wave Structure
SiC wedges
st
31 disc
m 1.10
Coupler cell
length:
(2 waveguide ports) total
dimple tuning holes
E. Jensen, RF for Linacs 15

Input coupler
Travelling wave structure
4-cell travelling wave structure
¼ of geometry shown
shown: Re {Poynting vector}
(power density)
Output coupler
E. Jensen, RF for Linacs 16

Iris loaded waveguide
1 cm
30 GHz structure (CLIC)
11.4 GHz structure (NLC)
E. Jensen, RF for Linacs 17

ω
Pass band
Higher order modes
(HOM)
Brillouin diagram
synchronous
interaction
speed of light line,
ω = k /c
Stop band or band gap
π 2 π
k d = phase advance per cell
E. Jensen, RF for Linacs 18

ω d/c
2π
π
π/2
0
synchronous
Brillouin
Diagram
speed of light line,
ω = β /c
π/2 k d
π
π
2π/3
π/2
0
E. Jensen, RF for Linacs 19

12
11
10
9
8
7
6
5
4
3
2
Brillouin diagram TM 0n modes
iris variation (parameter a ):
blue solid: 1st cell (17 mm)
red dashed: 32nd cell (13.3 mm)
speed of light line
0 30 60 90 120 150 180
Brillouin diagram
12
11
10
9
8
7
6
5
4
3
Brillouin diagram dipole modes
iris variation (parameter a ):
blue solid: 1st cell (17 mm)
red dashed: 32nd cell (13.3 mm)
2
0 30 60 90 120 150 180
E. Jensen, RF for Linacs 20

E r
H r
Fundamental accelerating
mode (TM 0n )
Higher order modes
E v
1 st dipole mode Higher order
longitudinal mode (TM 0n )
E. Jensen, RF for Linacs 21

“Choke mode cavity ”
E. Jensen, RF for Linacs 22

TDS: Tapered Damped Structure
accelerating mode: cut-off in waveguide
shown |E|, color coding logarithmic
Dipole mode: Poynting vector towards SiC load
SiC load
E. Jensen, RF for Linacs 23

Slotted iris structure
E. Jensen, RF for Linacs 24

OP-:4024
#1DGRAPH
ORDINATE: WAKET
COMPONENT: X
FIXED COORDINATES:
DIM...........MESHLINE
X 11
Y 1
ABSCISSA: GEOMWAKE
[BASE OF WAKET]
REFERENCE COORDINATE: S
VARY..........MESHLINE
FROM 0
TO 2994
HOM damping at work
FRAME: 1 01/06/00 - 17:06:38 VERSION[V4.024] SICA32.DRD
transverse wake for 3 cells.
offset 10 mm, 2.5 mm
σ
4.00
2.00
0.000E+00
-2.00
-4.00
SI: A+3,10,4, B-5,CELL-6,15
A: 1.700E+01 MM, B: 3.703E+01 MM.
FOR REF., E.J., JUNE 2000
X COMPONENT OF WAKE POTENTIAL IN V [INDIRECT CALC.]
3 GHz SICA structure: Transverse wake suppression
W [V/pC]
t
0.000E+00 0.500 1.00 1.50 2.00 2.50
s [m]
E. Jensen, RF for Linacs 25

SICA 3 GHz structures
lined up waiting for installation mounted on the girder in CTF3
E. Jensen, RF for Linacs 26

Beam Loading
E. Jensen, RF for Linacs 27

Methods of
beam loading
compensation:
Beam loading compensation
Ramped RF pulse
Staggered timing
I.V. Syrachev, T.Higo:
“Numerical investigation of transient
beam loading compensation in JLC
X-band main linac”, KEK Report 96-8
E. Jensen, RF for Linacs 28

Superconducting RF
10.2 MV/ per cavity
E. Jensen, RF for Linacs 29

115 MHz split-ring cavity, 172.5 MHz β = 0.19 “lollipop” cavity
345 MHz β = 0.4 spoke cavity
SC cavities for small β
β = 0.021 fork cavity
β = 0.03 fork cavity
57.5 MHz cavities:
β= 0.06 QWR
(quarter wave resonator)
E. Jensen, RF for Linacs 30

25 -35 MV/m
TESLA/ILC SC cavities
E. Jensen, RF for Linacs 31

Surface field [MV/m]
10000
1000
100
10
Surface field breakdown limits
Kilpatrick, 1957
Wang&Loew '88, SLAC-pub-4647
Wang&Loew '97, SLAC-pub-7684
CLIC design
c e E
f in GHz, E c in MV/m
1
0.01 0.1 1 10
Frequency [GHz]
100
Normal conducting, on Cu
24.
67
f
=
4.
25
−
Ec
E. Jensen, RF for Linacs 32

Pulse length dependence
6
1
−
τ
E. Jensen, RF for Linacs 33

CLIC HDS structure
30 GHz, phase advance per cell: 70°,
Cell length: 1.94 mm,
Smallest iris diameter: 3 mm,
accelerating gradient 150 MV/m,
Max surface field 380 MV/m,
max. ΔT 56 K,
Optimized for Mo iris, CuZr cavities
E. Jensen, RF for Linacs 34

E r
HDS: Optimizing the cavity contour
Field distribution in the optimized HDS cell geometry. The contour optimization is done to prevent hot spots.
H r
E. Jensen, RF for Linacs 35

HDS: Cu prototype
1 cm
E. Jensen, RF for Linacs 36