Carlos A. Martins ESS – Accelerator Division - RF Electrical Power Systems June 2 th, 2014 Design, construction and measurement challenges to enhance pulse flat-top quality in long pulsed high power modulators

Main topologies of long pulse klystron modulators Non modularPartly modularEntirely modular -Pulse transformer based (ver. 1-monolythic PT) -Direct switch -Resonant polyphase -Pulse transformer based (ver. 2-multi prim. winding) -Pulse transformer based (ver. 3- stacked converters) -Marx generator ESS topology (Stacked Multi-Level, SML)

Sources of errors and noise – Power system -Differential mode noise is generated in the system, BUT: it’s easy to predict (by calculations and/or simulations); can be filtered by usual techniques (power stacks interleaving, passive filters); -Common mode noise is also generated AND: Depends on parasitic elements (stray inductances; stray capacitances); Very hard to predict by calculations; Can be quantified by simulations, but the results are very dependent on the quality of the models used; Common mode noise can mix with the differential mode noise, generating random noise. General characteristics: -Low voltage at primary stage (before HV transformer), therefore switching at high currents -> stray inductances are a concern (v = L*dI/dt); -High voltage at secondary side (after HV transformer), therefore switching at high voltages -> stray capacitances are a concern (i= C*dV/dt);

Sources of errors and noise – Control system Pulse – to – pulse reproducibility affected by “random noise” (either power converter noise and/or measurement noise) ; Feed forward compensation techniques on the LLRF might be difficult to implement Flat top ripple, pulse N Flat top ripple, pulse N+1

Mitigation techniques  Design level: Understanding the physical layout of the system with respect to EMC:  Low inductive conductors (busbars) in the low voltage stage with short distances;  Differential cabling (out cable and return cable as close as possible);  Differential measurements with shielded twisted pairs; Filtering in the power system side (differential mode is easy; common mode noise difficult and requires experimental tuning); Filtering in the measurement signals;  Exploitation level: EMC oriented approach; Good EMC grounding mesh; Shielded cables and cable trays;  In long pulsed high power modulators, the only viable option to mitigate the effect of voltage ripple is by reducing its amplitude: not by feed forward compensation.

Typical achievable performance of a klystron modulator in practice SNS modulators: ~ 0.5% pk-to-pk; XFEL DESY modulators:~ 0.3% pk-to-pk; Linac4 modulators:design was for ~ 0.1%, never measured in practice; ESS modulators:goal is ~ 0.2% pk-to-pk (challenging) How to measure this voltage ripple ? (0.2% in top of 115 kV) - We cannot compensate what we don’t see…; - We cannot judge on what we can’t see…; The accuracy of the High Voltage probe should be at least 5 times better over a wide frequency band of around 150 kHz.

High Voltage measurement techniques Uncompensated resistive divider parasitic Compensated resistive divider Effect of compensation: -Reduces the distortion at high frequencies; -Limits the bandwidth

High Voltage measurement techniques – Proximity effect Totally open HV divider Partly open HV divider h D Proximity effect negligible if: D > 2 * h Proximity effect negligible with D << 2 * h D h

High Voltage measurement techniques – Proximity effect Reduced scale modulator prototype Installation of a Northstar HV Divider in the oil tank Still, some space is required in the oil tank

High Voltage measurement techniques – Cable back termination HV Divider Termination box Measurement cable - All these 3 elements impact on the quality of the measurement - The entire measurement chain should be designed, integrated and calibrated by the same experts

High Voltage measurement techniques – Pulse response of HV divider Courtesy Northstar Pulse response of: a)- non compensated HV probe; b)- compensated HV probe; a) b)

High Voltage measurement techniques Courtesy CERN outside oil tank inside oil tank 0.4% 0.11% CONCLUSIONS - Measurement precisions above 0.1%, at high frequencies, very difficult to achieve in practice; - ESS modulators cannot guarantee a voltage ripple, pk-to-pk, below %