Transcript Magnetron_Stabilisation

Stabilised Magnetrons
Presentation to mark Professor Richard Carter’s
contributions to Vacuum Electronics
Delivered by Amos Dexter
with thanks for contributions from Dr Imran Tahir
and of course Richard
CarterFest 14th July 2010
Rough Investigation
Extract magnetron
Saw open
Look inside
Operation now plain to see ?
CarterFest 14th July 2010
The Carter Video Lectures
CarterFest 14th July 2010
Magnetron Operation
Magnetic Field into Page
Anode forms slow wave structure
cathode (negative volts)
sub synchronous zone
spoke
vane
anode (earth)
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Trajectories and Charge Density
Litton 4J50 - rigid rotating field model
ANODE
VANE
SPOKE
MAGNETIC
FIELD INTO
PAGE
Electrons which gain
energy from the RF
field return to the
cathode, those which
lose move to the
anode
ELECTRIC FIELD
LINES RF + DC
SUBSYNCHRONOUS
ZONE
CATHODE
CarterFest 14th July 2010
Carter the Educator
CarterFest 14th July 2010
Magnetron Instabilities
Moding
• Large frequency jump (several MHz at S band)
• Low output, reduced efficiency, increased voltage.
Multipactor
• Reduction in efficiency
• Arc precursor
Gauss Discontinuities
• Anode currents where the magnetron does not operate
• Depend on magnetic field and heater power
Twinning
• Small frequency jump ( < 1 MHz at S band)
• Efficiency and output often acceptable at both frequencies
• No good for Radar or Accelerators
• Depends on magnetic field, heater power, cathode condition
CarterFest 14th July 2010
Twinning
CarterFest 14th July 2010
Gauss Line Discontinuities
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Frequency Stabilisation with Phase Lock Loop (PPL)
Frequency (GHz)
2.452
2.450
2.448
2.446
2.444
2.442
Pushing Curve
2.440
Water
Load
2.438
2.436
100
150
200
250
300
350
400
3 Stub
Tuner
Loop
Coupler
Anode Current (mA)
Compare frequency to
reference and adjust anode
current with PI controller
(loop filter) to prevent
frequency drifting.
Low Pass Filter
8 kHz cut-off
Frequency
Divider / N
Divider
/R
Phase - Freq
Detector &
Charge Pump
ADF 4113
MicroController
10 MHz
TCXO
1ppm
High Voltage
Transformer
Power supply
325 V DC with
5% 100 Hz ripple
Loop
Filter
40kHz Chopper
Pulse Width
Modulator
SG 2525
1.5 kW Power Supply
CarterFest 14th July 2010
Spectral Improvement
0
Amplitude(dBm)
-20
-40
-60
-80
2.430
-20
-40
-60
-80
2.440
2.450
2.460
2.470
2.43
2.44
2.45
2.46
Frequency(GHz)
Frequency (GHz)
National (Panasonic) M137, 1.2kW CW “cooker” magnetron,
full heater power, 5% ripple at 100Hz on dc supply
As left but 4.2W heater
0
Amplitude (dBm)
Amplitude (dbm)
0
-20
Bandwidth ~ 200 kHz
(depends on comparison
frequency and loop filter)
-40
-60
-80
2.43
2.44
2.45
2.46
2.47
Frequency (GHz)
With frequency stabilisation
CarterFest 14th July 2010
2.47
Heater Power Dependence of Magnetron
Pushing Curves
At heater powers
of about 18W to 21W
two frequencies are
possible at the same
anode current.
2.4525
Frequency (GHz)
2.4520
Pushing curves can
only be measured in
this range with a
stabilised magnetron
hence we had a world
first.
National (Panasonic) M137, 1.2kW CW “cooker” magnetron,
2.4530
800 kHz
2.4515
Cooker
operation
2.4510
fc(36W)
fc(33W)
fc(30W)
fc(27W)
fc(25W)
fc(21W)
fc(18W)
fc(16W)
2.4505
2.4500
2.4495
2.4490
160
Twining
200
240
280
320
Anode Current
CarterFest 14th July 2010
360
400
Twinning
National (Panasonic) M137, 1.2kW CW “cooker” magnetron,
We believe the lower frequency
corresponds to a state where the subsynchronous zone is not space charge
limited.
If a pulsed magnetron is operated in
the lower frequency state (having less
associated noise) then if too many
electrons are released from the
cathode during the pulse then the
magnetron twins.
2.4525
Frequency (GHz)
At one particular cathode
temperature there are two
possible frequencies for the same
anode current.
2.4530
2.4520
2.4515
Heater Power
2.4510
21 W
2.4505
18 W
2.4500
2.4495
2.4490
160
200
240
280
320
Anode Current
Ball and Carter only studied pulsed magnetrons driven from modulators
They observed that the anode current for “ twinned” pulses, start identically but diverges early for low
currents and later for high currents.
They observed dependence on anode current, cathode coating, heater power and magnetic field.
Direct comparison is difficult between the CW magnetron and the pulsed magnetron as the modulator
current and voltage change when twinning occurs.
CarterFest 14th July 2010
360
The Magnetron Reflection Amplifier
• Linacs require accurate phase control
• Phase control requires an amplifier
Cavity
Magnetron
• Magnetrons can be operated as reflection amplifiers
Load
Circulator
Compared to Klystrons, in general Magnetrons
Injection
Source
- are smaller
- more efficient
- can use permanent magnets
- utilise lower d.c. voltage but higher current
- are easier to manufacture
Consequently they are much cheaper to
purchase and operate
CarterFest 14th July 2010
Reflection Amplifier Controllability
1.
Phase of output follows the phase of the input signal
2.
3.
Phase shift through magnetron depends on difference between input frequency and the
magnetrons natural frequency
Output power has minimal dependence on input signal power
4.
Phase shift through magnetron depends on input signal power
5.
There is a time constant associated with the output phase following the input phase
Anode
Voltage
10kW
915MH
z 30kW
20kW
916MHz
40kW
12.0 kV
3.00A
11.5 kV
2.92A
Magnetron frequency and output vary
together as a consequence of
1.
Varying the magnetic field
2.
Varying the anode current (pushing)
3.
Varying the reflected power (pulling)
0o
Arcing
Power
supply
11.0 kV
load line
towards
magnetron
Moding
2.85A
900 W
800 W
700 W
2.78A
10.5 kV
2
270o
VSWR
3 4 6
90o
2.70A
10.0 kV
1
2
3
Anode Current Amps
4
5
Magnetic
field coil
current
+5MHz
+2.5MHz
-5MHz
-2.5MHz
+0MHz
180o
CarterFest 14th July 2010
Solution of Adler’s Equation
The phase of injection locked oscillators is determined by
Like
Vinj o
d

sin   o  i
dt
VRF 2 Q L
o
inj

Vinj /RF
d
 A  const
dt
for small  hence phase
stabilises to a constant offset
= oscillation angular frequency without injection
= injection angular frequency
= phase shift between injection input and oscillator output
= equivalent circuit voltage for injection signal / RF output
Adler’s equation predicts that :if o = i then  → 0
if o close to i then  → a fixed value (i.e. when sin  < 1 then locking occurs)
if o far from i then  → no locking unless Vinj is large
Steady state
sin   2 Q L
PRF o   i
Pinj
o
If the natural frequency of the magnetron is fluctuating then the phase  will be fluctuating.
Advancing or retarding the injection signal allows low frequency jitter to be cancelled and the
magnetron phase or the cavity phase to be maintained with respect to a reference signal.
CarterFest 14th July 2010
Power Needed for Injection Locking
Adler steady
state solution
sin   2 Q L
PRF o   i
Pinj
o
  i o 
Minimum power

Pinj  4 PRF Q 2L 

requirement for locking
 o 
Minimum power given
when sin  = 1
2
PRF is output power
QL refers to the loaded magnetron.
For 2.45 GHz cooker magnetron
(fi –fo) due to ripple ~ 2 MHz
(fi –fo) due to temperature
fluctuation > 5 MHz
Frequency (GHz)
2.452
2.450
2.448
2.446
2.444
2.442
2.438
2.436
100
Plock
 4 Q2
Poutput
2
Panasonic
Pushing Curve
2.440
2
 f i f o 
2  2.455  2.450 

  4 100  
  0.166
f
2
.
450


 o 
150
200
250
300
350
400
Anode Current (mA)
This is big hence must reduce fi – fo ( can do this dynamically using the pushing curve)
CarterFest 14th July 2010
Experiments at Lancaster
Tahir I., Dexter A.C and Carter R.G. “Noise
Performance of Frequency and Phase Locked
CW Magnetrons operated as current
controlled oscillators”, IEEE Trans. Elec. Dev,
vol 52, no 9, 2005, pp2096-2130
Phase shift
keying the
magnetron
Tahir I., Dexter A.C and Carter R.G.,
“Frequency and Phase Modulation
Performance on an Injection-Locked CW
Magnetron”, IEEE Trans. Elec. Dev, vol. 53,
no 7, 2006, pp1721-1729
0 dBm
RBW = 100Hz
Span = 100 kHz
Centre = 2.44998488 GHz
Locked
spectral
output
-50 dBm
Lancaster has successfully
demonstrated the injection
locking of a cooker magnetron
with as little as -40 dB injection
power by fine control the anode
current to compensate shifts in
the natural frequency of the
magnetron.
-100 dBm
-50 kHz
+50 kHz
CarterFest 14th July 2010
Frequency Shift Keying the Magnetron
Input to pin diode
Output from
double balanced
mixer after mixing
with 3rd frequency
CarterFest 14th July 2010
Long pulse proton driver solution for SPL?
Phasor
diagram
output of
magnetron
1
Permits fast full range phase and amplitude control
output of
magnetron
2
Cavity
combiner /
magic tee
Advanced
Modulator
Advanced
Modulator
440 kW
Magnetron
Fast
magnetron
tune by
varying output
current
440 kW
Load
~ -30 dB
needed for
440 W
locking
Magnetron
440 W
LLRF
440 kW Magnetron design is less demanding than 880 kW design
reducing cost per kW, and increasing lifetime and reliability.
CarterFest 14th July 2010
Fast
magnetron
tune by
varying output
current
Magnetrons for Proton Drivers
The Carter solution for IFMIF
“Conceptual Design of a 1 MW 175MHz CW Magnetron”,
IVEC 2008
Diacrode
Magnetron
Anode Voltage
14 kV
60 kV
Anode Current
103 A
20 A
Efficiency
71%
90%
Gain
13 dB
~ 30 dB
Drive Power
50 kW
~ 1 kW
Cooling
Anode
Anode and Cathode
Electromagnet
No
Yes ( ~ 1.5 kW)
CarterFest 14th July 2010