RF Sources and Combiners
Emma Wooldridge
Outline
• Very quick RF power overview
• RF power sources
– Triodes
– Tetrodes
– Klystrons
– IOTs (Inductive Output Tubes)
• RF power combining
– Waveguides
– Combining cavity
28th-29th April 2004
Joint Accelerator Workshop
Coax or Waveguide
E Field
B Field
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Standing and travelling waves
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Triodes
• Invented in the early
1900s
• Differs from a diode due
to a grid added between
the cathode and the plate
• By applying an RF
voltage across the grid it
is possible to obtain
bunches of electrons
instead of a continuous
stream of electrons
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Joint Accelerator Workshop
Tetrodes
• This is the same as a
triode but with an added
screen grid
• This grid protects the
anode and control grid
from being linked due to
capacitance.
• The screen grid is at
ground so most electrons
go past and continue to
hit the anode
28th-29th April 2004
Joint Accelerator Workshop
IOTs (Inductive Output Tubes)
• First designed by
Haeff in the late
1930s
• Electrons are
bunched as in a
tetrode
• The power is obtained
from the electron
beam as it passes the
cavity
28th-29th April 2004
Joint Accelerator Workshop
Klystrons
• Invented in the late
1930s by the Varian
brothers
• Extract energy in a
similar way to an IOT
• Produces electron
bunches by velocity
modulation
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Joint Accelerator Workshop
RF Power combining – Why?
•It is not always possible to get the perfect
power source for each job
•
•
•
•
Klystrons
Expensive
Long lead time
High power
Higher gain
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•
•
•
•
IOTs
Comparatively cheap
Shorter lead time
Lower power
Lower gain
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Methods of Combining
Magic Tee
Combining Cavity
– Large Structure.
– Failure of 1 input
results in loss of
symmetry and failure
of the system.
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– Small/Compact.
– Failure of 1 or more
components gives
graceful degradation
of power.
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Magic Tees
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Combining Cavity
• Magic Tees are the standard for RF power
combining
• H. Bohlen of CPI has modelled a
combining cavity in 1D using 6 inputs
• Thales have built a working model cavity
using 5 inputs
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Joint Accelerator Workshop
Modelling the cavity
• Modelled in CST
Microwave Studio.
– With either: 2,3 or 4 IOTs.
– At 1.3 GHz.
• 1st step - modelling the
cavity at the required
frequency.
• 2nd step – Optimising the
IOT inputs into the cavity
by varying their width and
length within the cavity.
28th-29th April 2004
Joint Accelerator Workshop
Results
• Efficiency of all models between 65%99%.
• Efficiency of 65% only seen in models with
3 inputs
• Due to the analysis not converging to a
single result
• Reflection minimised to less 1% for most
models
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Joint Accelerator Workshop
4 IOT plot
• For the 2 and 4 IOT models the output for each IOT is
identical.
Simulataneous Port Excitation
4 IOTs at 1.5 GHz
1.00E+01
0.00E+00
Normalised DFT Magnitude / dB
0.00E+00
5.00E-01
1.00E+00
1.50E+00
2.00E+00
2.50E+00
-1.00E+01
-2.00E+01
Port 1
-3.00E+01
-4.00E+01
Reflected
power 1%
-5.00E+01
-6.00E+01
-7.00E+01
-8.00E+01
Frequency / GHz
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Joint Accelerator Workshop
Output
Power
396%
Port 2
Port 3
Port 4
Port 5
3 IOT plot
• Due to constraints on the way the ports are defined
the asymmetry of the 3 IOT model led to it not being
Simulataneous Port Excitation
optimised fully.
3 IOTs at 1.5 GHz
1.00E+01
0.00E+00
Normalised DFT Magnitude / dB
0.00E+00
5.00E-01
1.00E+00
1.50E+00
2.00E+00
2.50E+00
-1.00E+01
-2.00E+01
Port 1
-3.00E+01
-4.00E+01
-5.00E+01
-6.00E+01
Reflected
power 52%
Reflected
power 12%
-7.00E+01
-8.00E+01
Frequency / GHz
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Output
Power
208%
Port 2
Port 3
Port 4
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Technical Demonstrator
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Specifications
•
•
•
•
•
Main body is made of aluminium
Couplers are made from brass
Inner cavity radius 88.5mm
Outer cavity radius to be 150mm
Both input and output couplers can move
in and out of the cavity by ±10mm
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Joint Accelerator Workshop
28th-29th April 2004
Joint Accelerator Workshop
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Joint Accelerator Workshop
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Joint Accelerator Workshop
28th-29th April 2004
Joint Accelerator Workshop
Measurements
• Symmetric placing of
couplers to compare
against Microwave Studio
results
• Asymmetric positioning of
couplers
• Rotate the couplers
• Alter the length of coupler
inside the cavity
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Joint Accelerator Workshop
Conclusions
• Power from RF sources can be combined
efficiently
• The combining cavity could be a new way
of doing this
• Modelling suggests that an efficient IOT
combining cavity can be created
• Prototype will be ready this week
• Measurements to be taken in the next few
weeks
28th-29th April 2004
Joint Accelerator Workshop
Any Questions?