Jitter Reduction Project
Linac Modulator HVPS Upgrade
Initial Test Results
Minh Nguyen
April 25, 2013
Scope of modulator 24-8 upgrade
• Installs a low power, high voltage inverter power supply (HVPS) in
parallel to the existing high power, resonant charger to provide fine
PFN voltage regulation
• Develops a PFN voltage regulator board to provide voltage
regulation for both de-Q’ing resonant charger and the new HVPS
charger
• Installs and modifies other components to improve the overall
beam voltage stability
– Tail clipper: To minimize PFN voltage variations due to random thyratron
recovery and to protect the klystron from high PIV
– Negative bias grid 2: To promote thyratron consistent recovery
– Thyratron grid drive circuit: To minimize turn-on time jitter
• Changes to the existing modulator control will be minimal and
oblivious to the MKSU control system. No additional LOTO
procedures are required
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Modulator HVPS Upgrade AIP
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Modulator upgrade circuit (in red lines)
Modulator Output: 360 kV, 420 A , 151 MW peak, 91 kW Ave. @ 120 Hz
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Modulator HVPS Upgrade AIP
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Upgrade HV components
120Vac input
HVPS inhibit
monitor
FB/Error
monitor
Tail Clipper Assembly
HVPS control
De-Q SCR trigger
PFN voltage
feedback
VVS reference
PFN Voltage Regulator Box
SCRTD with -75Vdc Bias
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Installed upgrade components
Ross Divider
TDK Charging Resistors
TDK Reverse Voltage
Protection Diode
Power Transformer 3-Phase
600Vac In, 208/120Vac Out
Tail Clipper
PFN Voltage Regulator
Box (Outside of Cab 3)
TDK 50kV HVPS
Circuit Breaker 208Vac / 20A
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Upgraded modulator tests
with PFN voltage regulated at 42kV
De-Q’ing
regulation
Resonant charging
voltage (5000/1 ratio)
Thyratron
fire
Regulated
PFN voltage
Zoom –in
HVPS charging voltage
(5000/1 ratio)
HVPS charging
current
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Modulator HVPS Upgrade AIP
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Klystron backswing voltage
before and after tail clipper installation
Tail Clipper
Current
10A/div
Voltage scale: 50kV/div
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Typical PFN and BV stability
with de-Q’ing regulation only
Modulator ran at 42kV PFN, 340kV BV and 120Hz
Measurement setup: 1000 statistical samples, one-time slot, both scopes are triggered at the same time
Typical scope rms noise is ~ 1% of vertical setting per division
Zoom-in PFN and BV voltages are generated by LeCroy DA1855A in comparator mode
•
•
•
•
PFN voltage
(5000/1 ratio)
Zoom-in
Beam voltage
(5000/1 ratio)
Zoom-in
PFN voltage
PFN stability (rms) = 988uV/8.26V = 120 ppm
MNN
BV stability (rms) = 7.71mV/62.72V = 123 ppm
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Typical PFN and BV stability
with both de-Q’ing and TDK HVPS regulation
PFN stability (rms) = 131uV/8.29V = 16 ppm
MNN
BV stability (rms) = 2.58mV/63.24V = 41 ppm
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Short term (2-minute intervals)
PFN and BV stability (rms)
60
50
Stability - ppm
40
30
PFN Stability
BV Stability
20
10
0
1
2
3
4
5
6
Interval
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20-minute period
PFN and BV stability (rms)
80
70
Stability (ppm)
60
50
PFN Stability
40
BV Stability
30
20
10
0
1
5
10
15
20
Time (minutes)
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Modulator stability is improved
by HVPS charging upgrade
140
120
Stability -ppm
100
80
PFN Stability
BV Stability
60
PFN-deQ only
BV-deQ only
40
20
0
1
2
3
4
5
6
Interval
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Some remaining challenges
•
•
•
•
TDK-Lambda HVPS fails to start up reliably – Latched off on load fault
PFN voltage random fluctuations
Beam voltage random fluctuations and drift
Relation between Thyratron unstable recovery and random fluctuations
PFN voltage
MNN
Klystron beam voltage
Modulator HVPS Upgrade AIP
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