Linac 6575 Modulator PFN Charging Power Supply Upgrade

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

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) 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 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) Modulator HVPS Upgrade AIP

5. Upgrade approach • Ahybrid 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 Modulator HVPS Upgrade AIP

6. Modulator upgrade circuit (in red lines) Modulator Output: 360 kV, 420 A , 151 MW peak, 91 kW Ave. @ 120 Hz Modulator HVPS Upgrade AIP

7. Preliminary tests with TDK 40kV, 500W power supply at low PRF TDK PS charging voltage Resonant charging voltage (40kV) Target PFN voltage before Thyratron firing Pulse-to-pulse stability: ~ 40 ppm (rms) 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. 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% Modulator HVPS Upgrade AIP

10. Open modulator cabinet Modulator Front 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) De-Qing Chassis #2 Capacitor Discharge Switch De-spiking Coil Charging Diode Pulse Forming Network Anode Reactor Thyratron Keep Alive Power Supply Charging Transformer Cabinet 3 Cabinet 2 Cabinet 1 Modulator HVPS Upgrade AIP

11. Schedule Update Modulator HVPS Upgrade AIP

12. Comments and Suggestions • Comments and suggestions ? Modulator HVPS Upgrade AIP

13. 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 Modulator HVPS Upgrade AIP

14. Negative bias on thyratron grid • 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 • 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 seriesresistor reduced grid trigger rise-time from ~ 160 to 40 ns Risetime after modification Existing grid trigger risetime Modulator HVPS Upgrade AIP

16. de-Q’ing divider signal compensation • 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 Modulator HVPS Upgrade AIP