Linac 6575 Modulator
PFN Charging Power Supply Upgrade
Minh Nguyen
December 5, 2012
Present L1S (21-1) modulator
• Several modifications have been made since
May 2011 to stabilize beam voltage
– Added a tail clipper
– Added negative bias on thyratron control grid
– Modified grid drive circuit
– Tuned de-Q’ing feedback signal
• Pulse-to-pulse stability not including 120Hz
hump is ~ 80 ppm (rms)
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AIP Goal
• To stabilize beam voltage amplitude and time
as much as we possibly can on existing Linac
modulators that include
– Improving PFN voltage regulation to minimize
amplitude jitter
– Improving Thyratron grid drive circuit to minimize
time jitter
• Upgrades one modulator (24-8) to
demonstrate the stability improvement and
reliability of the upgraded components
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Motivation
• Existing 6575 L-C resonant charging system cannot meet
stability requirements of < 100 ppm (rms) for critical LCLS
stations
– The system relies on a single-shot voltage regulation for each main pulse.
There is no control feedback to develop fine corrections of PFN voltage
– De-Q’ing regulation performance is dependent on several factors, such as AC
line voltage fluctuations, PFN charging slopes, accuracy of the de-Q’ing phaseadvanced signal compensation, and thyratron operating conditions
• Direct PFN charging systems using multiple HV invertertype power supplies for voltage regulation much better
than 100 ppm have been successfully utilized at SACLA.
However, they are designed for low power and PRF (35kW,
60Hz). This charging system would be fairly complex and
really expensive to be adopted by SLAC LCLS (91kW, 120Hz)
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Upgrade approach
• A hybrid scheme in which a low power, high voltage,
inverter power supply is added in parallel to the existing
high power, resonant charger to provide fine PFN voltage
regulation. The coarse, resonant-charging level will be at
about 99.5% of the target level
• Implementation is low cost and relatively simple
• Changes to the existing modulator will be minimal and
oblivious to the MKSU control system. No additional
modulator LOTO is required for the new 50kV power supply
• Installs 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 grid bias PS: to minimize time jitter on the control grid
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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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Preliminary tests with TDK 40kV, 500W
power supply at low PRF
Resonant
charging
voltage
(40kV)
TDK PS
charging
voltage
Target PFN
voltage before
Thyratron firing
Pulse-to-pulse stability: ~ 40 ppm (rms)
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Semi-custom TDK-Lambda HVPS
• Peak output power
2.5 kJ/sec
• Output voltage
50 kV
• Output current
100 mA (to charge
700nF from 49.5 to 50kV in < 4ms)
• Current rate of rise (0-100%)
500 A/sec
• Switching frequency
40 kHz
• AC input voltage
208 Vac, 3-phase
• Efficiency
85%
• Rack-mount chassis
19”x 17”x 7”
• The power supply is protected against open circuits, short
circuits, overloads and arcs.
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Theoretical PFN voltage stability and
regulation range
• Power supply rated: 50kV, 2.5kJ/sec peak, 40kHz switching freq.
• Peak charge current:
– Ipk = 2 (Ppk /Vrated) = 2(2.5kJ/50kV) = 100mA
• Voltage variations:
– ∆V = Ipk x 0.5Tsw /Cload = 100mA x 12.5µs/0.7µF = 1.78V
• Pulse to pulse repeatability:
– 1.78V / 50kV = 36 ppm
• Charging voltage-time ratio:
– Vchg/Tchg = Ipk /Cload = 100mA /0.7µF = 143 V/ms
• For 4ms charge time (120Hz operation)  Vchg = 572V
• PFN voltage regulation range = 572V / 50kV = 1.1%
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Open modulator cabinet
Modulator Front
Step Start Resistors
De-Qing Chassis #2
600VAC Circuit Breaker
Capacitor Discharge Switch
Filter Capacitors
De-spiking Coil
Contactors
Charging Diode
Full Wave Bridge Rectifier
Pulse Forming Network
De-Qing Chassis #1
Anode Reactor
Power Supply
Thyratron
AC Line Filter Networks
Keep Alive Power Supply
Power Transformer (T20)
Charging Transformer
Cabinet 3
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Cabinet 2
Cabinet 1
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Schedule Update
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Comments and Suggestions
• Comments and suggestions ?
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Tail clipper
•
The thyratron normally latched on at the end
of the main pulse and does not recover for >
200µs later. However, due to changes in gas
pressure, sometimes it recovers much sooner
which results in higher inverse voltage than
normal
•
A tail clipper (HV diodes and thyrite connected
in parallel with the pulse transformer primary)
clamps the peak inverse voltage to a normal
level, which reduces PFN voltage variations
when the thyratron recovers early
100kV PIV
Clipper Current
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Negative bias on thyratron grid
•
•
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Thyratron grid voltage waveform
Negative biased control grid improved
BV time jitter
•
Pulse-to-pulse jitter: ~ 2 ns
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Grid drive circuit
•
•
Existing grid drive circuit was designed to be used in conjunction with the thyratron that had pretrigger electrode, which required it to be triggered in advance of the control grid. However, it was no
longer used
Removing the filter for the time delay and using non-inductive series resistor reduced grid trigger risetime from ~ 160 to 40 ns
Risetime after
modification
Existing
grid trigger
risetime
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de-Q’ing divider signal compensation
•
•
•
•
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PFN voltage is regulated by de-Q the
charging transformer.
Tight regulation requires high accuracy
of the de-Q’ing divider signal
Due to regulation system delay,
amplitude variations in resonant
charging voltage produce PFN voltage
error (∆Vpfn)
The error is minimized if the sensing
signal to regulate is accurately
compensated - advanced in phase an
equal amount to the system delay
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