Conference Agenda

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Session Overview
Solid State Modulators
Tuesday, 21/June/2022:
10:00am - 12:00pm

Session Chair: James Randall Cooper, Cooper Consulting Services, Inc.
Location: 301B

Oral Session

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10:00am - 10:20am
Solid State Modulators: 1

Design and Analysis of a 24 kV PCB-Bus for the Low Impedance Interconnect of a Multiphase PEBB-based Converter

J. Stewart

Virginia Tech - Center for Power Electronics Systems, United States of America

In this work, the design and analysis of a medium voltage (MV) printed circuit board (PCB) -based planar power bus rated for 24 kV with distributed capacitor daughtercards is presented. This bus provides the low impedance interconnect between an external power supply input and two phase legs of a 24 kV/2 MW power electronics building block (PEBB) -based multilevel modular converter (MMC). The bus serves as a motherboard for capacitor daughtercards which are rated for 9 µF 3 kV each. Electric field (E-field) analysis and parasitic extraction was performed via finite element analysis (FEA) using COMSOL Multiphysics and Ansys Electromagnetics for board- and system-level integration. Insulation performance for each component and the assembly was verified through partial discharge (PD) analysis using an Omicron MPD600. The motherboard and capacitor daughtercards partial discharge inception voltage (PDIV) were tested on a layer-to-layer basis, in addition to the full assembly to ensure the system was PD free under normal operation.

A single PEBB is a converter in which the power stage and all ancillary circuitry required to operate independently are contained in a single structure. PEBBs may be stacked in a series and/or parallel fashion to achieve a higher voltage and/or current rating respectively. The 24 kV MMC includes four PEBBs in the upper arm and four PEBBs in the lower arm of each of the two phase legs. Each PEBB also has an independent forced air cooling system which is referenced to earth ground (GND). Due to the system’s complexity, the boards layer stackup and connector design were critical when fully integrated into the system.

This 24 kV bus was implemented using a 22-layer stackup with a staggered offset between conductor edges for field grading. A layer of FR4 with a controlled thickness was used between the outermost conductive layer and surface pads. This allowed high fields to be contained with the solid dielectric to avoid corona along the surface of the PCB. Additionally, guard pads were implemented to reduce the E-field intensity in air near terminals. These guard pads are placed directly below device terminals and connected to the same potential, at some height within the bus so the field intensity is reduced.

Eight capacitor daughtercards rated at 9 µF 3 kV each were mounted to the motherboard creating a 1.13 µF 24 kV capacitor bank. The daughtercards were constructed using a series-parallel array of 1.5 kV commercial-off-the-shelf (COTS) capacitors. Using this method, the target capacitance for our specific application was achievable. It was beneficial to avoid bulky high voltage can capacitors which require their cans to referenced somewhere near the voltage of their terminals. Additionally, the array of parallel film capacitors provided a much lower impedance which is desirable for such an interconnect.

The final manuscript will provide background on the 24 kV PEBB-based converter at the heart of this work. Detailed analysis for and design of the 24 kV PCB bus and the 3 kV capacitor daughtercards will be presented. Test methods and more extensive results will also be presented.

10:20am - 10:40am
Solid State Modulators: 2

A 30-kV Solid-State Impedance-Matched Marx Generator: Practical Considerations on Impedance Matching

T. Huiskamp, J. van Oorschot, M. Azizi

Eindhoven University of Technology, The Netherlands

Based on the topology of the Impedance-Matched Marx Generator (IMG) presented in 2017 by researchers from Sandia (and others) [1], we created a solid-state IMG using MOSFET switches [2]. The advantage of using the IMG topology is that by using transmission lines to transmit the pulses from the Marx stages the rise time of the pulses can be maintained at the output waveform (when carefully impedance-matched). By designing the Marx stages very compactly and using fast semiconductor components, adjustable pulses with rise times of just several nanoseconds are feasible with this topology. Since we require such fast rising pulses for transient plasma generation, the solid-state IMG is ideally suited for our purpose. In this contribution we present the development of a 30-kV version of the solid-state IMG. It utilizes 12 stages of gate-boosted [3] and series-connected 1200V SiC MOSFETs and achieves several ns rise time at 30-kV output voltage. Specifically, we also focus on the practical considerations on impedance matching with a modified, much longer version of the IMG to investigate practical considerations on impedance matching. We will show that if the matching criteria are not observed, severe distortion of the waveform is possible and that for the fastest pulses we need the best matching possible.

[1] W. A. Stygar et al., “Impedance-matched Marx generators,” Phys. Rev. Accel. Beams, vol. 20, 040402 (2017)

[2] T. Huiskamp and J. J. van Oorschot, “Fast Pulsed Power Generation with a Solid-State Impedance-Matched Marx Generator: Concept, Design and First Implementation”, IEEE T. Plasma Sci., vol. 47, 4350 - 4360 (2019)

[3] M Azizi, J. J. van Oorschot, T Huiskamp, “Ultrafast Switching of SiC MOSFETs for High-Voltage Pulsed-Power Circuits”, IEEE Transactions on Plasma Science, vol. 48, 4262 - 4272 (2020)

10:40am - 11:20am
Solid State Modulators: 3

The Stacked Multi-Level Klystron Modulators for the ESS Linac

C. Martins1, M. Collins2, M. Kalafatic1, L. Yury1

1European Spallation Source ERIC; 2Lund University Faculty of Engineering - LTH, IEA Division

The European Spallation Source (ESS) Linac will require by its completion a total of 33 klystron modulators. They are based on the novel Stacked Multi-Level topology and are rated at 115kV/4x25A; 3.5ms/14Hz, therefore capable of powering 4 klystrons rated at 1.6MWpk, 704MHz in parallel. Besides complying with the ESS requirements in terms of pulse quality (i.e. rise times0..99% < 120µs; flat-top droop <1%; ripple <0.2%pk-pk), they also comply with relevant power quality standards on the electrical grid (current THD below 5%, unitary power factor, flicker free operation), thanks to the adoption of Active Front End (AFE) rectifiers in conjunction with constant power regulated DC/DC capacitor chargers. These features allow their direct connection to the AC line without the need for external line compensators or filters.

The topology is modular and based on the association in series of 6 identical HV modules, each rated for 20kVpk and formed by a HVHF transformer, a HV diode rectifier bridge and a HV LC low-pass filter. These modules are placed in an oil tank and are driven by a 1kV/15kHz H-bridge inverter, which in turn is fed from a capacitor bank charged by the aforementioned AFE+DC/DC chargers.

The modulators have a footprint of 1.6m x 4m and a weight of 11.5 ton (including oil). The mechanical layout was designed in order to facilitate access to each component for repairing. In particular, the low voltage and high voltage cabinets can be assembled and repaired independently, with the first one directly pluggable into the top of the second one. The high voltage cabinet comprises the complete oil tank assembly and it can be easily extracted from each side of the modulator by using the built in sliding rail system, facilitating access for maintenance purposes and interchangeability of the HV modules.

In a first part of this contribution, an overview of the different blocks and functionalities of the power conversion structure will be addressed. In a second part, the main lessons learned during the design, construction and validation of both the reduced scale prototype and the full scale series units will be presented, together with the implemented corrective actions and their effectiveness. This will include issues related to the design of the HV modules like field control, effect of stray inductances and parasitic capacitances, integration into the oil tank, the reliability of the insulated oil and the impact of their handling procedures, the common mode noise effects and their mitigation, eddy currents mitigation etc. In the third part, experimental results obtained with the first 660kVA rated series modulator powering a HV resistive dummy load and klystron load will be presented and discussed.

A total of 18 series modulators have been delivered to ESS for the completion of phase I of the Linac construction, allowing an average beam power in the target of 2MW. For future power upgrades of the Linac to 5MW, additional 16 units of similar type will be required.

11:20am - 11:40am
Solid State Modulators: 4

Evaluation of Klystron Modulator Performance in Interleaved Pulsing Schemes for the ESS Neutrino Super Beam Project

M. Collins1,2, C. Martins1,2, M. Eshraqi2, B. Gålnander2

1Lund University; 2European Spallation Source ERIC

It has been proposed that the relatively low duty cycle of the European Spallation Source (ESS) linac allows acceleration of additional H- ion pulses interleaved with the baseline proton pulses, representing a unique opportunity to construct a neutrino super beam (ESSnuSB) facility of unparalleled luminosity. Coupled with a distant Cherenkov detector, it is believed that evidence of CP violations in leptons could be obtained, representing a significant step towards understanding the matter/antimatter asymmetry.

In this paper, several such interleaved pulsing schemes are considered from the perspective of the klystron modulators and the RF power system in investigating the possibility to realize the ESSnuSB. Conserving the required output RF energy, these pulsing schemes vary in terms of 1) number of added H- ion pulses per baseline cycle, 2) pulse amplitude and 3) pulse length. Each prospective pulsing scheme offers unique advantages while differently impacting klystron modulator performance. Whereas the ESS linac baseline design requires 33 klystron modulators (rated for pulse amplitude 115kV/4x25A, pulse length 3.5ms and pulse repetition rate 14Hz; each modulator powering 4 parallel klystrons rated 1.6MWpk at 704MHz), the proposed upgrade requires doubling the baseline linac average output power and thus either doubling the capacity of existing modulators or the procurement of additional modulator systems.

In order to evaluate and compare the merit of these solutions from a system perspective, a mathematical framework connecting the attributes of the proposed pulsing schemes to the power transfer curves of the klystrons and subsequently to the performance of the klystron modulators is developed. In particular, the impact on modulator-to-beam efficiency, modulator average input power quality, modulator output pulse flat top ripple, total upgrade cost, total operational cost (assuming a life time of 25 years), and required additional system size is assessed.

Two particularly promising interleaved pulsing schemes are evaluated in circuit simulation. It is demonstrated that an upgrade of the existing modulators utilizing the selected pulsing schemes maintains the baseline modulator-to-beam efficiency (>90%) keeping the input power quality and output pulse quality performance intact while representing a cost-effective solution for the linac upgrade to implement the proposed ESSnuSB project.

11:40am - 12:00pm
Solid State Modulators: 5

Analysis of the Triggering Instants of the Solid-State Switches of the Pulsed Power Sources for Achieving Optimal Projectile Velocity in a Multistage Induction Coilgun

R. Ram, J. T. Meledath

Indian Institute of Science, Bangalore, India

A multistage induction coilgun works on the principle of electromagnetic induction between an array of drive coils, which are wound on a long insulating barrel of appropriate length, and an electrically conducting projectile (or armature) placed inside the barrel. Previously charged high voltage capacitor banks are sequentially discharged into the drive coils through high voltage solid-state switches leading to the generation and flow of high magnitude impulse currents through the drive coils. Time-varying magnetic flux thus produced by the pulsed currents through the drive coils interact with the projectile inside and induce a resultant eddy current in it. The electromagnetic thrust (F) exerted on the projectile is a product of the excitation current through the drive coil (ic), induced current on the projectile (ip), and the mutual inductance gradient (dMcp/dx, i.e., the change in mutual inductance between the drive coil and the projectile as the projectile moves along the barrel). The higher the electromagnetic thrust is available from each stage, the higher is the launch velocity (vp) of the projectile that can be achieved after each stage. A higher dMcp/dx leads to a higher F. Now, dMcp/dx essentially depends on the radii of the drive coil and the projectile, and the distance between the midplanes of the drive coil and the projectile, i.e., the triggering instant of the pulsed power source w.r.t. the position of the projectile inside each drive coil. For a particular multistage induction coilgun design, where the radii of the drive coil and the projectile are fixed, the appropriate instant of synchronization of triggering of the stages, i.e., the optimum triggering instant of the solid-state switch of the pulsed power source w.r.t. the position of the projectile inside each drive coil, plays a vital role in achieving a higher vp of the projectile. The several stages of a multistage induction coilgun can typically be energized in two ways: (1) positive-positive (PP), where the direction of the excitation current in the subsequent drive coils is kept the same, and (2) positive-negative (PN), where the direction of the excitation current in the subsequent drive coils is reversed alternatively. The optimum triggering instant for the projectile inside each drive coil of the subsequent stages changes accordingly in these two launching configurations. This paper investigates and compares the difference in triggering instants for the projectile inside each drive coil in PP and PN launcher configurations. The analysis is carried out with a four-stage induction coilgun. The input electrical energy to each stage is kept constant. The triggering instant for the projectile inside each drive coil is then set such that the projectile position is anywhere from 0 mm to 18 mm with a linear step of 1.6 mm. The triggering position inside each drive coil is measured w.r.t. the rear end of the corresponding drive coil. The analysis presented in this paper will help better understand the operation of a multistage induction coilgun.

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