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Vector Modulation of High
Power RF
Y. Kang
J. Wilson, M. McCarthy, M. Champion
and RF Group
Spallation Neutron Source
Oak Ridge National Laboratory
LLRF05 Workshop, CERN
10-13 October, 2005
Y. Kang
1
Accelerator Systems Division/SNS/ORNL
High Power RF Vector Modulation
•
For savings in construction and installation of a charged particle accelerator
– Fanning out a higher power amplifier output to many cavities with individual
amplitude and phase controls is less expensive than using an amplifier/cavity.
– Applicable to all types of particle accelerations; cab be more effective for SRF ion
accelerators
•
Independent controls of amplitude and phase in high power RF transmission
– Use two high power phase shifters with a hybrid junction (or two)
– Well known principle not used for high power
– Development in HPRF hardware (and LLRF control interface)
•
Concept sought for possible application to the SNS linac; no time to implement
•
Many new accelerator projects may benefit employing the design
•
Phase shifters may be constructed using:
– Ferrimagnetic materials
• Control orthogonal magnetic field bias in ferrite (or YIG) material to change
permeability
– Ferroelectric materials (high frequency)
• Control voltage bias on electro-optic material to change permittivity
– PIN or Varacter diodes (lower power, short pulse)
Y. Kang
2
Accelerator Systems Division/SNS/ORNL
HP Vector Modulator Development
and Related Work
• ORNL, FNAL, CERN, and other institutions are now working on
development of VMs
• HPSL 2005 Workshop, May 22-24, Naperville, IL
– Y. Kang, “ High Power RF Distribution and Control using Ferrite Phase
Shifters”
– I. Terechkine, “High Power Phase Shifter for Application in the RF
Distribution System of Superconducting Proton Linac”
– D. Valuch, “A Fast Phase and Amplitude Modulator for the SPL”
– D. Sun, “325 MHz IQ Modulator for the Front End of Fermilab Proton
Driver”
• More
– V. P. Yakovlev, “Fast X-Band Phase Shifter,” Advanced Accelerator
Concepts: 11th Workshop, 2004
– Y. Kang, “ Fast Ferrite Waveguide Phase Shifter,” PAC2001
Y. Kang
3
Accelerator Systems Division/SNS/ORNL
Baseline: 26 mA
2.5 MeV
RFQ
86.8 MeV
185.6 MeV
DTL
1
2
3
CCL
4
1
5
6
2
185.6 MeV
3
1
2
3
4
4
5
6
7
391.4 MeV
SCL,  = 0.61
1
2
3
4
5
6
to SCL
SCL,  = 0.81
7
8
9
10
11
1
from
CCL
8
9
10
391.4MeV
1 GeV
SCL,  = 0.81
1
2
3
4
5
6
7
8
9
10
11
12
1.4MW
11
402.5 MHz, 2.5 MW klystron
SNS Linac RF
Y. Kang
12
805 MHz, 5 MW klystron
13
805 MHz, 0.55 MW klystron
14
Modulator p.s.
SNS Linac
HPRF
Systems
Accelerator
Systems Division/SNS/ORNL
4
Comparison of Two Configurations
Cavities
One Klystron/
One Cavity
Klystrons
PS
RF Signals &
Controllers
Cavities
Fanning out
One Klystron
Vector
Modulators
Klystron
PS
Y. Kang
5
Accelerator Systems Division/SNS/ORNL
Cost Savings ?
• Example: a system similar to SNS 805 MHz SRF linac
–
–
–
–
–
–
–
25-40 mA beam current (8% duty)
Eacc ~ 10 ~ 16 MV/m
Qext ~ 7 x 105
±1% amplitude, ±1 phase
-mode superconducting Nb cavities will need ~200-600 kW/m
Klystron power spec: 550-600 kW/cavity
Klystron power supply (converter modulator) already fanned out
to drive many klystrons
• Fan out configuration
– Can use klystrons with ~10 – 50 times higher RF power output
– Savings in construction and installation: klystrons, waveguides,
labor and buildings
– Extra cost for the vector modulators and control components
Y. Kang
6
Accelerator Systems Division/SNS/ORNL
Linac RF Cost for a 805 MHz System
(non-official estimate for a linac with100 cavities)
One/one
Fan out
(1:20)
Savings
Quantity
Unit Price
($k)
Total ($k)
Quantity
Unit Price
($k)
Total ($k)
($k)
100
150
15,000
5
700
3,500
11,500
5
700
3,500
5
700
3,500
0
Circulator +
Loads
100
50
5,000
100
50
5,000
0
RF Controls
100
105
10,500
100
135
13,500
-3,000
Waveguide
100
46
4,600
5
250
1,250
3,350
Gallery
40,000
0.20
8,000
5,000
0.20
1,000
7,000
Labor for
WG/Klystron
10,000
0.10
1,000
2,000
0.10
200
800
27,950
19,650
Klystron
Transmitter +
Power
Supply
Subtotal ($)
47,600
Other items
Y. Kang
Can be more
7
Accelerator Systems Division/SNS/ORNL
Vector Modulation
Low Level RF Control
Driver
Amplifier
Driver
Amplifier
1
RF input
V1
Hybrid
1
Hybrid
2
2
RF ouput
V2
Matched
Load
Matched
Load
 1 2 

2 
 1  2   j 
Vout,1 (1 , 2 )  V0 sin 
e
 2 
 1  2 

2 
 1   2   j 
Vout, 2 (1 ,  2 )  V0 cos
e
 2 
Y. Kang
8
Accelerator Systems Division/SNS/ORNL
Vector Modulators
• Transmissive
Vo
1
Hybrid
0/90
• Reflective
Hybrid
180/90
 1 2 

2 
 1  2   j
Vout (1 , 2 )  Vocos
e
 2 
2
90-degree
hybrid
180-degree
hybrid
– Standingwave is formed
– Reflected wave must be trapped before the
RF generator (klystron): circulator
Y. Kang
9
Accelerator Systems Division/SNS/ORNL
VM Output Amplitude and Phase vs. 1 and 2


2(rads)
Phase
2(rads)
Amplitude
-
-
-
Mm
Y. Kang
1(rads)

-
Mp
10
1(rads)

Accelerator Systems Division/SNS/ORNL
VM with Ferrite Phase Shifters
•
Phase shifter uses ferrimagnetic material (ferrite, YIG)
– Magnetic bias field is orthogonal to the RF magnetic field in the material
– Magnetic field bias (usually high current, Hb ~ 10-50 kA/m) can change
the permeability of the magnetic material
– Waveguide type (FNAL and others) and coaxial type (ORNL) being
demonstrated
•
Design optimization:
– High power handling
– low RF loss
– Dimensions
• Waveguide design may be too bulky for SRF accelerator
frequencies (especially < 1000 MHz)
– LLRF Control
– Fast response time
– Reliability
– Cost
Y. Kang
11
Accelerator Systems Division/SNS/ORNL
Waveguide Vector Modulator (FNAL)
Output
Magnetic Field
Magnetic Field
Short
Short
Input
Y. Kang
12
Accelerator Systems Division/SNS/ORNL
Square Coaxial Phase Shifter Measurement
(ORNL)
Operating Frequency vs. Bias Current
of a Phase Shifter (10” active length)
600
550
B
Frequency (MHz)
500
450
400
350
300
250
200
150
100
0
5
10
15
20
25
Bias Field (103 A/m)
Y. Kang
13
Accelerator Systems Division/SNS/ORNL
30
805 MHz Vector Modulator Construction
• Prototype construction and
measurement
– Square coaxial TEM
transmission line design
– For 402.5 MHz operation
– 100-300 kW peak power
– 10 kW average power
– 10” active length
Y. Kang
14
Accelerator Systems Division/SNS/ORNL
Amplitude and Phase vs. Bias Fields
The lookup table
21
0.61
0.550.67
220
0.73
19
18
0.95
0.96
200
0.85
0.79
3
Bias Field 2 (10 Amps/m)
20
210
17
190
0.89
16
180
0.85
170
15
150
0.91160
14
140 0.93
13
0.91
13
14
0.73
0.89
0.79
0.55
0.61
0.67
15
16
17
18
19
20
21
3
Bias Field 1 (10 Amps/m)
Y. Kang
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Accelerator Systems Division/SNS/ORNL
VM RF Control (preliminary)
LLRF
Set
Amplitude
& Phase
Converter
RF from Klystron
Detector
Feedforward
+
+
Feedback
Compensation
Driver 1
Phase
Shifter 1
X
Hybrid
Driver 2
Adaptive Feedforward +
Feedback
Phase
Shifter 2
HPRF Modulator
To Cavity
Y. Kang
16
Accelerator Systems Division/SNS/ORNL
Control Response Consideration
B
R
L
Good conductor
•
Bandwidth limitation due to conductive housing:
– Skin depth causes control field loss through the phase shifter housing => =1/(fµ)1/2
Ex) for copper wall t==1mm, f=4.2kHz
•
Magnetic bias field control :
– Time constant of solenoid circuit => R=L
Ex) for solenoid L=10 µH, R=1: -3dB frequency = 15.9 kHz, Time constant =L/R=10 µsec
•
Time constant may be reduced:
– by control loop gain of the detector/driver
– by putting a zero in loop to cancel pole
– The conductor loss also be minimized by properly slitting or laminating the housing for
elimination of Eddy current
Y. Kang
17
Accelerator Systems Division/SNS/ORNL
System Design with VMs
Amplitude/Phase Variable Range
•
Accelerators RF cavities
– SNS SCL like configuration uses only few cavity designs that match to few beam
beta’s
– Variable ranges of phase and amplitude have to be greater
•
Phase range requirement
– Broader range is always desirable – some wants full 360-deg phase scanning for
flexibility – expensive
– If accelerator operates with any disabled (and detuned) cavity, a greater phase
tuning range is needed at a cavity to compensate the phase slippage
– With the knowledge, the right cavity phases can be predetermined for each case
- the range can be smaller
•
Amplitude range requirement (...)
– all adjoining cavities will require all predetermined field distribution
– To control the beam energy, the klystron power can be controlled
•
Use additional slow phase shifters between the cavities
– A slower inexpensive phase shifter, either ferrite or motorized mechanical stub
types can be used in each cavity for sustained phase settings
Y. Kang
18
Accelerator Systems Division/SNS/ORNL
VM RF Control Consideration
•
The steady state characteristics of the phase shifters and the vector modulator can
be measured and a lookup table can be provided
•
Current (or voltage) drivers selected and transfer functions characterized
•
LLRF development - adaptive feedforward with feedback control needed like in many
other systems
•
Frequency responses of phase shifters, bias circuits, and current/voltage drivers
•
Other important factors:
– Accelerator beam specification and control system requirements
– Pulsed or CW
– Temperature regulation
– Power supply regulation
•
Dynamic Range/Slew Rate/Linearity/Noise
– Driver amplifier/power supply performance
– Control system performance
•
Optimization of bias circuits: Slow high current supply + Fast lower current supply
Y. Kang
19
Accelerator Systems Division/SNS/ORNL
Summary
•
VM using YIG ferrite material
– 402.5 MHz square coaxial TEM phase shifter design prototyped for
• Size and Integration
• Manufacturing cost
• Cooling
– Low power bench measurements performed
– High power testing being prepared
• Housing and solenoid designs optimized
• Power supplies/audio amplifiers
– High power RF measurement and test to be completed
• First goal is to demonstrate 100-300 kW pulsed system
• Will be modified for higher power operation (> 500 kW)
• SNS RFTF has been equipped and readied for the testing
•
LLRF Control
– Initial high power testing will have only simplest feedforward
– Preliminary design and bench testing of the VM LLRF
– Full LLRF controls to be demonstrated with cavity load
– Needed for HPRF improvement
Y. Kang
20
Accelerator Systems Division/SNS/ORNL