Transcript X-band Overview and Applications

Overview of NLC X-Band Technology
and
Applications Including a Compact XFEL
Chris Adolphsen
SLAC
X-Band (11.4 GHz) RF Technology
Chose X-Band technology as an evolutionary next step for the Next Linear
Collider (NLC) from the SLAC Linac S-Band (2.86 GHz) technology.
In general want higher rf frequency because:
- Less rf energy per pulse is required, so fewer/smaller rf components.
- Higher gradients achievable, so shorter linacs (with reasonable efficiencies)
Offsetting these advantages are the requirements of:
- High power (100’s MW) HV pulses with fast rise times (100’s ns).
- High surface gradients in the klystrons, waveguide transport system and
accelerator structures.
- Tight alignment tolerances due to stronger wakefields (much less an issue
for light sources where the bunch charge is low, and bunch length short).
SLAC Linac RF Unit
(One of 240, 50 GeV Beam)
NLC/GLC Linac RF Unit
(One of ~ 2000 at 500 GeV cms, One of ~ 4000 at 1 TeV cm)
Next Linear Collider Test Accelerator (NLCTA)
In 1993, construction began
using first generation X-Band
components.
In 1997, demonstrated 17%
beam loading compensation
in four, 1.8 m structures at
~ 40 MV/m.
In 1998-99, added second
klystron to each linac rf
station.
In 2000-10, used for high
gradient studies and other
programs.
Eight-Pack Project:
Second Generation
RF Unit Test
Phase I: Generate RF
Power and Transport
to Loads
RF Component Performance
Solid State Induction Modulator
Eight-Pack Modulator
76 Cores
Three-Turn Secondary
> 1500 Hours of Operation
Waveforms When Driving Four 50 MW
Klystrons at 400 kV, 300 A Each
Next Generation Induction Modulator:
The ‘Two-Pack’
Features
- 6.5 kV IGBTs with in-line
multi-turn 1:10 transformer.
- Industrialized cast casings.
- Improved oil cooling.
- Improved HV feed through.
2-Pack Layout
(never built)
Bechtel-LLNL-SLAC 20 kV Test Stack
A Hybrid 2-Pack Modulator (15 core stack
driving a conventional 1:10 transformer) was
built and is still being used – also built a
version to drive a single ‘5045’ S-Band Tube
RF Component Performance
X-Band Klystrons
‘XL4’ – Have built at least 15, now producing 12 GHz ‘XL5’ versions
PPM Klystron Performance
(75 MW, 1.6 ms, 120/150 Hz, 55% Efficiency Required)
KEK/Toshiba
Four tubes tested at
75 MW with
1.6 ms pulses at
50 Hz (modulator limited).
Efficiency = 53-56%.
SLAC
Two tubes tested at
75 MW with
1.6 ms pulses at
120 Hz.
Efficiency = 53-54%.
• At 75 MW, iris surface field ~ 70 MV/m,
lower than in 3% vg structures, but higher
than sustainable (~ 50 MV/m) in
waveguide with comparable vg (~ 20%)
as the klystron TW output structures.
• May require multi-beam klystron
approach for stable > 50 MW, 1.6 ms
operation.
14 mm
KEK PPM2
Output
Structure
Power – Normal Pulse
Klystron Tear Events
Reflected
Transmitted
Power – Tear Event
• Character of events suggest they
originate in output cavity – visual
inspection inconclusive so far.
Time (100 ns / div)
For RF Unit Test, Four 50 MW Solenoid-Focused Klystrons
Installed in the Eight-Pack Modulator
(In Place of Two 75 MW PPM Klystrons)
RF Component Performance
First Generation RF Pulse Compression
(SLED II) at NLCTA
TE02
TE01
TE02
For NLC/GLC, Use Dual
Moded Delay Line to Reduce
Delay Line Length in Half
TE01
Also Use Over-Height Planar Waveguide
to Lower Surface Fields
Design for < 50 MV/m for 400 ns pulses
Example: Power Splitter
Dual-Moded SLED-II Performance
(475 MW, 400 ns Pulses Required)
Output Power
(Gain = 3.1, Goal = 3.25)
Combined Klystron Power
RF Component Performance
NLC Accelerator Structure Requirements
Convert rf energy to beam energy efficiently.
Short-range transverse wakefields small to limit linac emittance
growth: iris radius limited to 17% of rf wavelength (i.e. a/l = 17%).
Long-range wakefields suppressed so bunch train effectively acts as
a single bunch.
Dipole mode power coupled out for use as guide for centering the
beam in the structure.
Operate reliably at the design gradient and pulse length.
NLC/GLC Structures were developed by a FNAL/KEK/SLAC collaboration –
CERN/KEK/SLAC now developing 11%-13% a/l structures that run at ~ 100 MV/m
High Gradient Structure Development
Since 1999:
-
Tested about 40 structures with over
30,000 hours of high power operation at
NLCTA.
-
Improved structure preparation procedures -
includes various heat treatments and
avoidance of high rf surface currents.
-
Found lower input power structures to be
more robust against rf breakdown induced
damage.
-
Developed NLC-Ready ‘H60’ design with
required wakefield suppression features.
50 cm ‘T53’ Structure
H60 Structure Cells and Coupler
Assembly
High Power
Output Coupler
Cells with Slots
for Dipole Mode
Damping
Port for Extracting
Dipole Mode Power
Wakefield Damping and
Detuning
Dipole Mode Density
Wakefield Amplitude (V/pC/m/mm)
Ohmic Loss Only
Frequency (GHz)
Measurements
Detuning Only
Time of
Next Bunch
Damping and Detuning
Time After Bunch (ns)
Breakdown Rate History (Goal < 0.1/hr)
Four H60 Structures at 65 MV/m
t = 600 hr
Breakdown Rate at 60 Hz (#/hr)
with 400 ns Pulses
t = 400 hr
-1
10
-1
10
-2
10
-2
0
200
400
600
800
1000
10
0
500
t = 400 hr
t = 400 hr
-1
-1
10
10
-2
10
1000
-2
0
200
400
600
Hours of Operation
800
10
0
200
400
600
800
Hours of Operation
RF Unit Test in
2003-2004
Powered Eight H60
accelerator structures in
NLCTA for 1500 hours at 65
MV/m with 400 ns long
pulses at 60 Hz and
accelerated beam
From
Eight-Pack
From
Station 2
3 dB
From
Station 1
3 dB
Beam
3 dB
3 dB
3 dB
RF Component Efficiencies
Efficiency (%)
Design
Achieved
Comment
Two-Pack
Modulator
70
60
Use Integrated Transformer
(> 70% Expected)
PPM Klystron
55
53-56
~ 60% Possible
SLED II
81
78
Improve Flanges/Fab/Assembly
Wave Guide
Transport
92
77
Use Design Layout / Reduce
High-Loss WG
Structures
31
29
Use Round Cell Shape
High Power (Multi-MW) X-Band
Applications
• Energy Linearizer
– Single Structure: in use at LCLS, planned for BNL, PSI,
Fermi/Trieste and SPARX/Fascati
• Deflecting Cavity for Bunch Length Measurements
• CERN Linear Collider Structure Development
• 100’s of MeV to Many GeV Linacs
– LLNL 250 MeV linac for gamma-ray production
– SLAC 600 MeV energy ‘dither’ linacs for LCLS II
– LANL 6-20 GeV linac for an XFEL source to probe proton-matter
interactions
– SPARX 1-2 GeV X-Band linac for their FEL
– SLAC study of a 6 GeV Linac for a Compact XFEL (CXFEL) source
• See my Thursday talk in WG4 for more details
CERN/CLIC X-band Test-Stand
(Under Construction)
Directional coupler
Klystron XL5
Circular pumping port
Mode convertors
RF Valve
High voltage
modulator
Circular waveguide
F=50 mm
SLED Pulse
compressor
CERN - CEA – PSI – SLAC
High Voltage Modulator
SLAC XL5
Klystron
Development of a new solid
state modulator by
SCANDINOVA
Magnet and Ion
pump power
supplies
(1.8 x 3 x 2 m3)
Magnet
Specification :
High Voltage :
450 kV
Current :
335 A
Flat pulse length:
1.5 µs
Pulse length at 50%:
2.3 µs
Repetition rate:
50 Hz
HV ripple:
0.25 %
Pulse to pulse stability:
0.1 %
Oil tank with
pulse transfomer
Tuning circuit
Oil pump and
filter
L1
L2
L3
HVPS
CLIC WS
10/2009
1000VDC
Solid State Switch
modules
SOLID STATE
SWITCH
KM
Schirm BE-RF 31
1000V
PULSE
TRANSFORMER
450kV
Water cooling
circuit
KLYSTRON
Compact X-Ray (1.5 Å) FEL
Parameter
symbol
LCLS
CXFEL
unit
Bunch Charge
Q
250
250
pC
Electron Energy
E
14
6
GeV
gex,y
0.4-0.6
0.4-0.5
µm
Ipk
3.0
3.0
kA
0.01
0.02
%
Undulator Period
sE/E
lu
3
1.5
cm
Und. Parameter
K
3.5
1.9
Mean Und. Beta
β
30
8
m
Sat. Length
Lsat
60
30
m
Sat. Power
Psat
30
10
GW
FWHM Pulse Length
DT
80
80
fs
Photons/Pulse
Ng
2
0.7
1012
Emittance
Peak Current
Energy Spread
X-band Linac Driven Compact X-ray FEL
Linac-1
250 MeV
S
BC1
X
Linac-2
2.5 GeV
BC2
Linac-3
6 GeV
X
X
rf
gun
LCLS-like injector
L ~ 50 m
Undulator
L = 40 m
undulator
X-band main linac+BC2
G ~ 70 MV/m, L ~ 150 m
250 pC, gex,y  0.4 mm
• Use LCLS injector beam distribution and H60
structure (a/l=0.18) after BC1
• LiTrack simulates longitudinal dynamics with wake
and obtains 3 kA “uniform” distribution
• Similar results for T53 structure (a/l=0.13) with
200 pC charge
Operation Parameters
Units
CXFEL
NLC
GeV
6
250
Bunch Charge
nC
0.25
1.2
RF Pulse Width*
ns
150
400
Linac Pulse Rate
Hz
120
120
Beam Bunch Length
μm
7
110
Final Beam Energy
* Allows ~ 50-70 ns multibunch operation
Layout of Linac RF Unit
50 MW XL4
100 MW
1.5 us
400 kV
12 m
480 MW
150 ns
Nine T53 Structures (a/l = 13%) or Six H60 Structures (a/l = 18%)
NLC RF
Component
Costs
Cost (k$) per Item
(2232 RF Units)
LLRF
26.1
Modulator
83.7
Klystron
56.6
TWT
13.3
SLED-II
242.3
Structures
21.5
For the 6 GeV CXFEL, assume the cost per item
will be 4 times higher. For 70 MV/m operation:
Units
Total RF units
T53
H60
18
24
X-Band Linac Length
m
122
108
Total Accelerator Length
m
192
178
X-Band Linac Cost
M$
56
62
Gradient Optimization
1.4
Relative 6GeV ML Cost
1.35
H60VG3R
T53VG3R
1.3
1.25
1.2
1.15
1.1
1.05
1
50
60
70
80
Gradient (MV/m)
90
100
Assuming 1) Tunnel cost 25 k$/m, AC power + cooling power 2.5 $/Watt
2) Modulator efficiency 70%, Klystron efficiency 55%.
Breakdown Rate at 150 ns 120 Hz (#/hr)
Structure Breakdown Rates with 150 ns Pulses
10
10
10
10
10
10
1)
2)
3)
0
H60VG3R
T53VG3R
-1
At 70 MV/m,
Expect Rate
Less Than
0.01/hr at
120 Hz
-2
-3
-4
-5
60
65
70
Gradient (MV/m)
75
80
H60VG3R scaled at 0.2/hr for 65 MV/m,400 ns, 60Hz
T53VG3R scaled at 1/hr for 70 MV/m, 480 ns, 60 Hz
Assuming BDR ~ G26, ~ PW6
Single Bunch Wake Tolerances
• In both Linac-2 and Linac-3, short-range transverse wakefields in H60
are not a major issue in that:
- An injection jitter equal to the beam size yields a 1% emittance
growth in Linac-2 and .003% growth in Linac-3
- Random misalignments of 1 mm rms, assuming 50 structures in
each linac, yields an emittance growth of 1% in Linac-2, 0.1% in
Linac-3.
• With the T53 structure, the jitter and misalignment tolerances are
about three times smaller for the same emittance growth.
• The wake effect is weak mainly because the bunches are very short.
X-Band Revival
•
The 15 year, ~ 100 M$ development of X-band technology for a linear
collider produced a suite of robust, high power components.
•
With the low bunch charge being considered for future XFELs, X-band
technology affords a low cost, compact means of generating multi-GeV,
low emittance bunches.
– Gradients of 70-100 MV/m possible vs ~ 25 MV/m at S-Band and
~ 35 MV/m at C-Band
•
The number of XFELs is likely to continue to grow (e.g., normal
conducting linacs being considered in Korea and China).
•
To expand X-band use, need to have components industrialized and a
small demonstration accelerator built, such as the 150 MeV C-band linac
at Spring-8 in Japan where they have done light source studies.