MARX GENERATOR FOR THE NEW HRR PULSE POWER SUPPLY M.J. Barnes and L. Redondo (Lisbon Superior Engineering Institute, Portugal) 13/05/2014CLIC RF Breakdown Meeting1

Some highlights: PhD in Electric and Computing Engineering, from the Technical University of Lisbon, Portugal; Master degree in Nuclear Physics, Faculty of Sciences from the University of Lisbon; Coordinator Professor, Lisbon Engineering Superior Institute; Currently supervising 4 PhD students and 6 Masters students; Elected member of the IEEE Nuclear and Plasma Sciences Society NPSS, Standing Technical Committee for Pulsed Power Science and Technology, PPS&T, from 2011 to 2016; Five Technology and Science Portuguese Foundation grants, totalling €157k (Sept till March 2014): main goal of this project was to develop a solid‐state modulator with energy recovery for the CERN ISOLDE facility; Luis Redondo, Fernando A. Silva, in Muhammad Rashid et al, editors: Power Electronics Handbook 3ed, 2010, Butterworth‐Hinemann Publishing, Elsevier, ISBN # , chapter 26, pp 669‐710; Considerable experience/expertise in Power Electronics and Marx Generators; Co‐founder, in 30 November 2011, of the company Energy Pulse Systems, which develops, assembles and sells solid‐state modulators for various (normally industrial) applications. Luís Redondo 13/05/2014CLIC RF Breakdown Meeting2

Present HRR System  Reliability issues: occasional failure of Behlke switch. Probably due to turning off high current following a BD [trigger to switch-on is increased in duration for 3 µs from the instant of a BD – but a turn-off command can have been sent ≤200 ns before the BD …..].  Limitations – no active pull down at present (23 µs fall time-constant  250 ns to 99%: =0.74); system could be modified to include active pull- down, but same reliability issues – so better to explore other possibilities (e.g. Marx Generator) Sample voltage without BD (right) and measured current following BD at 12 kV (left) The measured voltage rise-time is less than 55 ns (10% - 90%) and the voltage reduces below 1% of the applied voltage within 100 µs. The measured current has a 2 µs "flat top" of ~120A and a rise time of 14 ns (10% - 90%). The estimated inductance, based on the 14 ns rise-time, is approximately 320 nH. 13/05/2014CLIC RF Breakdown Meeting3

Principle of Marx Generator (1) A Marx generator is an electrical circuit first described by Erwin Otto Marx in Its purpose is to generate a high-voltage pulse from a low-voltage DC supply. The circuit generates a high-voltage pulse by charging a number of capacitors in parallel, then subsequently connecting them in series. This is illustrated below for a 5 stage Marx. 1a) All the odd numbered MOSFETs/IGBTs (i.e. M 1, M 3, M 5, …) are off. 1b) The capacitors (C 1, C 2, … C 5 ) are charged in parallel, from Vdc, by turning on all the even numbered MOSFETs/IGBTs (i.e. M 2, M 4, M 6, …) [Vmarx ≈ 0 V]: 13/05/2014CLIC RF Breakdown Meeting4 Stored energy:

Principle of Marx Generator (2) The circuit generates a high-voltage pulse by charging a number of capacitors in parallel, then subsequently connecting them in series. This is illustrated below for a 5 stage Marx. 2a) Capacitors C 1, C 2, … C 5 have been charged to V dc in step (1b). All the even numbered MOSFETs/IGBTs (i.e. M 2, M 4, M 6, …) are then turned off. 2b) All the odd numbered MOSFETs/IGBTs (i.e. M 1, M 3, M 5, …) are then turned on, to connect the capacitors in series. V MARX ≈ 5V dc 13/05/2014CLIC RF Breakdown Meeting5 Load voltage:

Example of Each Stage The following circuit has been implemented, by Luis Redondo, using MOSFETs (in each charge stage [M2] and pulse stage [M1] two parallel MOSFETs are used). Note: modular design so that, in case of failure of a component, a card can be replaced. 13/05/2014CLIC RF Breakdown Meeting6

Commercial unit: EPULSUS-PM1-10 Typical 10 kV / 62.5 A pulse waveform on a 160 Ω resistor: 26 μs width pulse and 100 Hz repetition rate. Commercial unit characteristics: Standard galvanised steel enclosure, 800x600x400 mm, 80 kg; Mains input V cable supplied; Output cable; Output Ethernet plug for optional control available; BNC for monitoring the output voltage pulse available; Touch screen for programming output voltage, frequency and pulse width, and for monitoring ; Safety interlocks and reset condition after power on Overcurrent protection; Series 2.2 Ω resistance for increasing overall output stability and short-circuit protection. 13/05/2014CLIC RF Breakdown Meeting7

Example Waveforms. Waveforms from: 1 kHz, IGBT based, modulator into a 250 pF load, using a 10 kV/180 A (3.5 kW, single phase) commercial modulator at Energy Pulse Systems. 13/05/2014CLIC RF Breakdown Meeting8

Estimate…. The estimated budget for a modulator, for the CLIC RF tests, is between 5000 € and 6000 €: to be confirmed when specifications are agreed upon. For this project the modulators should be supplied via Energy Pulse Systems, as materials and human capability are not available in the institute (only available for small prototypes and concept validation). With the specifications agreed and material ordered, in principle a (CE marked) modulator would be delivered in 1-2 months. Suggestion: Mike (et al.?) visit Luis, in Lisbon, for 1 day. 13/05/2014CLIC RF Breakdown Meeting9

Questions…. 13/05/2014CLIC RF Breakdown Meeting10 a)Maximum capacitance to be driven ? b)10 kV flattop ? c)In the case of no BD, 1 kHz rep-rate,? d)Importance of rise-fall time (given E 30 ) ? [with such a strong dependency on field strength (e.g = 0.74, = 0.55 and =0.04), the rise and fall times might not have a significant effect…. [But, given the same strong dependency upon E, it is important to avoid overshoot (e.g =4.3 and =1.35)]]; e)Acceptable voltage droop during flattop (capacitive load) ?; f)Required “squareness” of current pulse following a breakdown ? g)Requirements for pulse flattop duration and flatness (e.g. dark current measurements?); h)Others ?? For RF BD group: From RF BD group…