Linac 6575 Modulator PFN Charging Power Supply Upgrade Minh Nguyen December 5, 2012.

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)

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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 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)

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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

MNN 6Modulator HVPS Upgrade AIP


Preliminary tests with TDK 40kV, 500W power supply at low PRF

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Resonant charging voltage (40kV)

TDK PS charging voltage

Pulse-to-pulse stability: ~ 40 ppm (rms)

Target PFN voltage before Thyratron firing


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

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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


Schedule Update

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Comments and Suggestions

• Comments and suggestions ?

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Tail clipper

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• 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


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


Grid drive circuit

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• 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


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

Linac 6575 Modulator PFN Charging Power Supply Upgrade Minh Nguyen December 5, 2012.

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