linac 6575 modulator pfn charging power supply upgrade
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
Linac 6575 Modulator PFN Charging Power Supply Upgrade
Minh NguyenDecember 5, 2012
Modulator HVPS Upgrade AIP 2
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)
MNN
Modulator HVPS Upgrade AIP 3
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
MNN
Modulator HVPS Upgrade AIP 4
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 phase-advanced signal compensation, and thyratron operating conditions
• Direct PFN charging systems using multiple HV inverter-type 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)
MNN
Modulator HVPS Upgrade AIP 5
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
MNN
Modulator upgrade circuit (in red lines)Modulator Output: 360 kV, 420 A , 151 MW peak, 91 kW Ave. @ 120 Hz
MNN 6Modulator HVPS Upgrade AIP
Modulator HVPS Upgrade AIP 7
Preliminary tests with TDK 40kV, 500W power supply at low PRF
MNN
Resonant charging voltage (40kV)
TDK PS charging voltage
Pulse-to-pulse stability: ~ 40 ppm (rms)
Target PFN voltage before Thyratron firing
Modulator HVPS Upgrade AIP 8
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.
MNN
Modulator HVPS Upgrade AIP 9
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%
MNN
Modulator HVPS Upgrade AIP 10
Open modulator cabinet
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De-Qing Chassis #2
Capacitor Discharge Switch
De-spiking Coil
Charging Diode
Pulse Forming Network
Anode Reactor
Thyratron
Keep Alive Power Supply
Charging Transformer
Step Start Resistors
600VAC Circuit Breaker
Filter Capacitors
Contactors
Full Wave Bridge Rectifier
De-Qing Chassis #1
Power Supply
AC Line Filter Networks
Power Transformer (T20)
Cabinet 1Cabinet 2Cabinet 3
Modulator Front
Modulator HVPS Upgrade AIP 11
Schedule Update
MNN
Modulator HVPS Upgrade AIP 12
Comments and Suggestions
• Comments and suggestions ?
MNN
Modulator HVPS Upgrade AIP 13
Tail clipper
MNN
• 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
Clipper Current
100kV PIV
Modulator HVPS Upgrade AIP 14
Negative bias on thyratron grid
MNN
• Thyratron grid voltage waveform• Negative biased control grid improved
BV time jitter • Pulse-to-pulse jitter: ~ 2 ns
Modulator HVPS Upgrade AIP 15
Grid drive circuit
MNN
• Existing grid drive circuit was designed to be used in conjunction with the thyratron that had pre-trigger 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 rise-time from ~ 160 to 40 ns
Existing grid trigger risetime
Risetime after modification
Modulator HVPS Upgrade AIP 16
de-Q’ing divider signal compensation
MNN
• 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