An Asse Ssment on Klystron Modulator Topologies for the ESS Project

AN ASSESSMENT ON KLYSTRON MODULATOR TOPOLOGIES FOR THE ESS PROJECT

C. A. Martins(*), K. Rathsman(**) (*) - Laval University, Dept. of Electrical and Computers Engineering, Québec, QC, Canada (**) - European Spallation Source (ESS), Accelerator Division, PO Box 176, Lund, Sweden

Abstract complex. This will translate directly on reliability requirements for the different LINAC equipment, like The European Spallation Source (ESS) is a joint klystron modulators. Redundant operation schemes will European project. ESS will be a world-leading centre for be defined at the levels of cavities, klystrons and materials research and life science with neutrons and will modulators. The maximum allowed beam losses (1W/m) host the World’s most powerful neutron source. A is another demanding parameter, particularly for the low superconducting LINAC will be required to accelerate energy sections. proton beams at energy levels up-to 2.5 GeV. About 200 The LINAC main parameters and layout are resumed in klystrons will be required which will be supplied Table 1 and Figure 1, respectively. individually by pulsed modulators of the solid state type. The nominal parameters for the initial test stand prototype Table 1. Key parameters of the ESS Linac are -115kV/21A, 3.5ms/14Hz. Particle species protons The state of the art of solid state pulsed klystron Proton energy range 1 to 2.5 GeV modulators suited for long pulse applications (>100 µs) is Pulse frequency range 10 to 20 Hz reported. This will include a descriptive study of the Pulse length range 0.8 to 3.5 ms different solutions envisaged which are based on the Beam power, nominal 5 MW following topologies: a)- Monolithic pulse transformer Beam on target availability > 95 % based; b)- Direct switch; c)- Marx generator; d)- Beam loss ~ 1 W/m Interleaved multi-level converters association; e)- Multi- level resonant converters association. These topologies will be qualitatively assessed on the context of the ESS project taking items like reliability, maintainability, reparability, efficiency, and safety as comparative criteria. Figure 1. Sections of the ESS Linac

I. THE EUROPEAN SPALLATION SOURCE PROJECT III. MAIN PARAMETERS OF THE ESS KLYSTRON MODULATORS The European Spallations Source (ESS) [0] is a joint European project, presently formed by 17 European The main electrical parameters of the ESS klystron member states, intended to be an international facility modulators are reported in Table 2. It is estimated that a dedicated to applied scientific research in several fields total quantity of 200 units will be required for the whole like chemistry, nano and energy technology, LINAC. environmental engineering, foodstuff, bioscience, The rise/fall times will have an impact on the energy pharmaceuticals, IT, materials and engineering science, lost before and after the flat-top (cavities are started to be archaeology where neutron beams will be used to probe filled only once the voltage pulse is flat within the various materials. Due to their unique properties neutrons required precision) and shall therefore not exceed 10% of are gentle probes that can penetrate deep into materials the pulse length. without causing damage, providing detailed information Droop and low frequency oscillations can be easily on crystallographic structure, atomic and molecular compensated by the LLRF in closed feedback loop. dynamics and magnetic properties. However, in order to reduce the amount of energy dissipated in the klystron collector this value shall be within 1% of the voltage pulse amplitude. II. THE ESS SUPERCONDUCTING LINAC High Frequency ripple (above 1 kHz) cannot be compensated by the LLRF due to its relatively low A high current superconducting proton LINAC, with an regulation bandwidth and is therefore specified to be approximate length of 450 m, will be used to accelerate lower than 0.02% (200 ppm). Feedfoward compensation beams of protons which will be projected into a fixed techniques at the LLRF are avoided since the target, generating neutrons from the interaction. The reproducibility of HF ripple is very difficult to quantify in LINAC [0] delivers 5 MW of power to the target at 2500 such high voltage, high power and fast power supply MeV, with a nominal current of 50 mA. The reliability is systems. Indeed, such ripple depends a lot on the topology one of the most challenging parameters to achieve, with a itself, on the high voltage measurement noise and on the target value of at least 95% for the whole physics jitter introduced by the solid state switches and their driving electronics. The maximum energy that can be deposited in a klystron arc by the modulator is 10J, considering a typical arc voltage of 50V. This will allow for a safety margin with respect to the maximal energy specified by the klystron manufacturers (20J). Depending on the topology, some klystron modulators may generate a temporary reverse (positive) voltage on the klystron cathode at the end of the pulse (voltage swing due to demagnetisation of pulse transformers). The Figure 2. Monolithic Pulse Transformer based topology maximum repetitive reverse peak voltage shall not exceed 10% of the nominal pulse voltage amplitude (typical Principle value allowed by klystron manufacturers is 20%). A medium voltage capacitor bank is charged through a capacitor charger prior to the pulse forming. The output Table 2. Klystron modulator’s nominal parameters HV pulse is formed by switching on the HV solid state Pulse width (50% amplitude) 3.5 ms switch assembly which connects the capacitor bank to a Flat-top duration 3.2 ms pulse transformer (PT). Rise/fall times (0..99% / 100..10%) <250 µs The PT rises the voltage up-to the level required by the Droop or oscillations on flat-top (low <1 % klystron and its secondary winding (output) is directly freq.,< 1kHz) connected between the klystron cathode and body (ground HF ripple at flat-top (> 1 kHz) <0.02 % potential). Repetition rate 14 Hz A passive LC resonant bouncer system compensates for Pulse voltage 115 kV the voltage droop, both due to the main capacitor bank Pulse current 21 A discharge over time and to the PT internal voltage droop Pulse power 2.42 MW across the primary winding. Average power 120 kW A damping circuit is used to demagnetize the Efficiency >90% transformer at the end of each pulse, by dissipating the Maximum energy in case of arc 10J magnetization energy into a resistor. A non-linear voltage clamping system is usually required in order to guarantee Maximum reverse cathode voltage 10% that the maximum reverse voltage remains below a specified limit even in such cases where the main switch IV. SOLID STATE KLYSTRON is open, following a klystron arc event. MODULATOR TOPOLOGIES Advantages The powering strategy for the ESS LINAC klystrons The power circuit is simple and reliable. was settled on the basis of two main principles: 1)- All electronic active devices are at a medium-voltage klystrons are of the pulsed type (no mode anode terminal), level (transformer primary side) and placed inside an air therefore less expensive and more reliable; 2)- only solid insulated cabinet, therefore with easy access for state pulsed modulators, with integrated arc protection maintenance and repair. Only the pulse transformer, the capability excluding HV tube crowbars, were to be output voltage and current sensors (all passive devices) considered. need to be placed inside an oil tank. The first four topologies reported herein were derived In case of arc, the dI/dt is limited by the leakage from a literature overview of the state of the art for long inductance of the pulse transformer, which gives time pulse applications. The last one (E) is a new proposal enough to open the solid state switch (may be rather slow: based on a modified version of the SNS klystron ~10 µs total switch off-time) without excessive modulators’ topology [0]. overcurrent and arc energy. Since the droop compensation system is based on a passive LC resonant circuit, high-frequency (HF) voltage A. Monolithic Pulse Transformer based ripple on the flat-top is inexistent (no HF switch mode The simplified schematic of the monolithic pulse power devices are used within the pulse forming loop). transformer based topology [0] – Reass, W.A.; Doss, J.D.; Gribble, R.F.; “A 1 megawatt polyphase boost converter- Hard points / limitations modulator for klystron pulse application”, IEEE Conf. on The design and construction of the pulse transformer Pulsed Power Plasma Science, 17-22 June 2001;][0][0] is are complex tasks due to the simultaneous presence of the presented in Figure 2. following three key factors,:- high voltage insulation (insulating materials, isolation distances); fast pulses (core materials, leakage inductances); The correct management of these aspects requires specific engineering expertise and a long record of practical experience. Manufacturing of such devices at an industrial scale is single source. The fully controllable HV solid state switch, being a master piece in the system (their reliability is a major topology or it could be based on a fast switch mode power concern particularly for arc protection), are difficult to supply (active bouncer) with closed loop feedback. find at an industrial level (association of several power semiconductors in series is still not a standard technique). Even so, at least 3 industrial companies specialised in the Figure 3. Direct Switch topology field and capable of producing such assemblies exist worldwide. Advantages As the pulse length and the average power requirements Since no pulse transformers are needed, very fast increase, the pulse transformers will become considerably rise/fall times are possible. or prohibitively bulky and expensive. Furthermore, It is easily adaptable to a very large range of pulse rise/fall times will increase and may lead to a lengths and pulse repetition rate requirements. considerable impact on the global modulator + klystron No reverse voltage is generated on the klystron. The HF efficiency. ripple depends solely on the droop compensation The LC resonant bouncer volume and cost (particularly technique adopted (might be zero if a passive resonant those of the bouncer inductor) will become bulky and bouncer is used, for instance). costly as the pulse length and average power increases, Since the majority of power parts are in oil, a compact with still poor performance (open loop compensation solution can be obtained. system). Note that in an optimal design [0], the oscillation period of the bouncer shall be around 6 times higher than Hard points / limitations All power components are in oil (longer time access for repair, large quantities of oil is required). the pulse length (=60⁰; : compensation angle) and the The reliability in arc protection is dependent on the reliability of the HV direct switch. The effectiveness of the droop compensation system bouncer peak current shall be twice the pulse current has still to be proven in large average power and amplitude, taken at the primary side. repetition rate applications. Finally, the reverse voltage on the klystron to High voltage (up-to ~100kV) IGBT assembly demagnetize the pulse transformer may be a strong technology for these levels of power ratings is very limitation to the duty cycle increase. Indeed, for a specific at an industrial scale and single source. complete reset of the core flux, the forward V.s value of a pulse shall be equal to the reverse V.s. As the forward V.s increases (either by increasing the pulse voltage C. Marx generator amplitude or the pulse length or both) the reverse V.s has The schematic of the solid state Marx topology [0][0] is to increase identically. Hence, the reverse voltage, the shown in Figure 4. reverse time or both will have to increase, but still the maximum reverse voltage is limited to 10% and the maximum reverse time is limited by the pulse repetition Principle rate (off-time between pulses). An active bouncer system A number of N capacitor banks are pre-charged in (fast switch mode power supply) could be an alternative parallel from a single medium voltage capacitor charger that would avoid such constraints. (typically ~10 kV). During this state, all switches T1’..TN’ are closed and all switches T1..TN are open. In order to generate the HV pulse, switches T1’..TN’ are B. Direct Switch open and simultaneously switches T1..TN are closed. All capacitor banks are thus connected in series during the The Direct switch topology is depicted in Figure 3 [0]. pulse forming period; therefore the output voltage is N x VC; (N: number of marx cells, VC: Voltage across each Principle capacitor). Inductors for dI/dt limitation in case of arcing A capacitor bank is charged directly at the klystron are added within each cell, in series with the discharging voltage level by a HV capacitor charger power supply. To switches T1..TN. form the pulse, the HV capacitor bank is connected At each cell, DC/DC converters are used to power the directly to the klystron by closing the HV solid state driver electronics. These DC/DC converters are fed from direct switch. Such switch is formed by a series small capacitors connected in the same way as the main connection of medium-voltage power semiconductor capacitor banks (therefore forming a mini-marx devices like IGBT’s. generator) and following the same charge/discharge At the end of the pulse, the HV direct switch is opened cycles [0]. again and the charging process restarts. The same switch A droop compensation system (not shown) may be also stops the pulse in case of klystron arcing. To that either formed by a single fast DC/DC switch mode power purpose, a small inductance is connected in series which supply (vernier regulator) located in series with cell #1 on limits the dI/dt in case of arcing, therefore limiting the the low potential side; or formed by N fast DC/DC current increase during the switch-off delay time. regulators (PWM choppers) distributed within the power A voltage droop compensation system (not shown) shall circuit, one located per cell. In the first case, a global be accounted for. Such system could be based on the regulation of the output voltage is performed whereas in same LC resonant bouncer circuit as in the former the second case individual regulation of the voltage within each cell is obtained. transformer is still required.

Figure 5. Multi-level topology based on the association Figure 4. Marx generator topology of interleaved DC/DC sub-converters

Advantages Principle Since no pulse transformers are used, very fast rise/fall A special 3-phase transformer, with multiple (N) times are possible. secondary winding systems, feeds N isolated capacitor A compact mechanical layout can be achieved, chargers. Each capacitor charger feeds one capacitor particularly if “field shaping elements” are used [0]; bank, which in turn supplies one DC/DC sub-converter, Since the design is oil free, no oil maintenance and either of one quadrant (in black) or two quadrants (in security issues need to be accounted for. black & red). A very high efficiency (>95%) can be achieved. The outputs of the N sub-converters are connected in The topology and the mechanical layout are modular series for voltage multiplication. Therefore, by using any with easy access to components for repair. A full cell can technique of interleaved control (PSM or multi-level be exchanged completely as a single rack. Redundant PWM) a variable DC voltage is obtained at the end of the spare cells may be added to increase availability. series connected sub-converters, although with high HF The energy stored in the system is segmented in N harmonics. A common passive filter extracts such HF independent capacitor banks. This will limit the damage harmonics. in case of an internal or external (short-circuit) failures in The obtained filtered voltage is then applied to the a given capacitor bank, since its stored energy is divided primary winding of a pulse transformer which rises up the by N with respect to the single capacitor bank solutions. voltage till the value required by the klystron.

Hard points / limitations Advantages Air insulation at voltage levels of 100kV and above, Active demagnetisation of the pulse transformer is with such short insulation distances required for possible by using some two-quadrant DC/DC sub- compactness of the system, constitute an additional converters out of the total N (typically 10 to 15% would engineering challenge and may have a strong impact on be enough), however requiring additional switchnig long term reliability, which will be affected by air and devices (in red in Figure 5). cleanness conditions inside the cabinets. Active droop compensation is intrinsic to the topology, Fast and intense electric fields constitute a major therefore requiring no additional sub-systems. The concern in the reliability of sensitive electronic DC/DC sub-converters are regulated through PWM or components, particularly those used in the IGBT drivers. PSM in closed loop feedback during the whole duration Advanced modelling using Finite Element Analysis of the pulse, including the demagnetisation period at the (FEA) and shielding techniques are required. end. The long term reliability of the solid state switches Active klystron arc extinction is also possible by under such circumstances is still to be proven. applying a controlled reverse (positive) voltage to the klystron whenever such an arc is detected. In case of arcing, all discharging switches (T1..TN) shall open. If one fails and is kept closed, the amount of energy Partial modularity is obtained which will increase the stored in the associated cell capacitor will be dissipated availability. Automatic detection of module failures and into the arc. Reliability in protecting the klystron in case subsequent reconfiguration are possible. of arcing is therefore divided by the number of cells, N. All active power electronics (at primary side of the The active droop compensation system may generate pulse transformer) is housed in air insulated cabinets, significant HF ripple, well above a few hundreds ppm therefore with easy access for repair and diagnosis. Only level. Furthermore, depending on the controls strategy the pulse transformer is within a HV oil tank. implemented, the pattern of such ripple may be randomly variable (reproducibility concern), caused by asymmetries Hard points / limitations and bad synchronisation between the different PWM The HF voltage ripple at the pulse flat-top may be choppers (one existing per cell). considerably high with respect to the 200 ppm limit, depending on the interleaving control technique adopted and on possible dissemetries existing between modules. D. Interleaved Multi–level Sub-converters Thermal cycling of semiconductors, operating under hard-switching conditions, may have an impact on their A topology based on the association of several lifetime. To overcome this effect, either the interleaved DC/DC sub-converters in series at a medium semiconductor are oversized or the number of modules is voltage level is shown in Figure 5 [0]. A pulse increased. other passive devices are in an oil tank. This will facilitate Two special transformers are required: the 3-phase access for repair and minimizes the quantity of oil multisecondary-winding & HV pulse transformer. required. All other inconveniences related to the usage of a pulse Semiconductor switches and drivers are of standard transformer (stated in point Monolithic Pulse Transformer commercial types. Several industrial sources exist based) apply. worldwide. The HF transformers operate in AC mode which is the natural way of operation for such devices. On opposition E. Multi-level Resonant Sub-converters to pulse transformers, no intrinsic limitations exist on A new topology based on the association of several their design with regards to the pulse length. No resonant sub-converters in parallel/series configuration, demagnetisation circuits are needed. The design and the on a multi-level structure, is shown in Figure 6. construction of this type of transformers seem easier to carry on than pulse transformers. Like in Interleaved Multi–level Sub-converters, the flat- top voltage (droop) is regulated in closed loop, by measuring the output voltage and by adjusting in consequence the H-bridge control signals dynamically, during the whole pulse duration. In case of klystron arcing, the resonant circuits will be “automatically De-Quewed” as their load resistance will be virtually zero. Therefore, even without disabling the H- bridge control signals, an intrinsic voltage shut-down with “self-current” limitation is obtained. Figure 6. Multi-level topology based on the association The topology and the mechanical layout are entirely of multiple resonant DC/AC/DC sub-converters modular.

Principle Hard points / limitations A set of N H-bridges are fed from a common DC-link The construction of the HF transformers is still a bus at medium voltage (~ 1 to 2kV). This DC-link bus is challenge:- mechanical stress due to pulsed operation; formed by several capacitor banks connected in parallel. high frequency (20 kHz) design with high voltage Several capacitor chargers may be also connected in insulation (100 kV and above at highest point); high parallel on a one to one basis with respect to the capacitor average power (tens kW). banks. Although not mandatory on a functional point of Due to the poor power factor of the resonant circuits, view, this last principle will improve the modularity of the the H-bridges shall handle a significant amount of system. reactive power and must be therefore oversized. Each H-Bridge supplies a circuit formed by:- a LC Due to the existence of the intermediate resonant stages, parallel resonant circuit, a HF step-up transformer, a which operate in AC mode and require an additional diode rectification bridge and an output HF filter. rectification and filtering stages, long rise times (typically The outputs of the different circuits herein referred are in the order of 100µs) are obtained. connected in series; the total output voltage is therefore Assuring “soft-switching” of the IGBT’s in all multiplied by N. operating points might be complex, particularly during The resonant circuits are excited by the H-bridges with pulse “build-up” (low current). Reliable interlocking a frequency above the resonance one. A voltage circuitry that detects and prevents missing soft switching amplification factor of up-to 3 times, exclusively procured conditions is mandatory. by the resonant circuits, is easily achievable in practice. A HF ripple content on the flat-top is to be expected. Its The HF transformers can additionally multiply the voltage real value depends not only on the theoretical design by a factor of 10. Typically, a number of 5 to 6 modules design and on the output filters, but mainly on practical would be enough for a 100kV and above application. aspects like control accuracy (jitter) of IGBT’s, symmetry Soft switching techniques of the H-bridge IGBT’s on assembling and component tolerances between should be implemented to reduce switching losses. They modules. could be considerably high otherwise at switching The energy stored in the modulator circuit (HF frequencies typically in the order of 20kHz, with a transformer leakage inductances/leakage capacitances; consequent negative impact on thermal cycling fatigue of output filter devices, etc.), which potentially can be IGBT’s. dissipated into a klystron arc, may be significant and considerably higher than the maximum 10J allowed. Advantages Adding a special resistive dump circuitry in series with All active power electronic components are located at the output HV line might be necessary to overcome this the transformer primary side (medium voltage level). Like drawback. the topologies Monolithic Pulse Transformer based and A klystron modulator system rated for 100kV, 20A, Interleaved Multi–level Sub-converters, the majority of 1ms/50Hz, composed by 5 modules according to Figure power electronics are installed in standard air insulated 6, was simulated using SABERDesigner software tool. cabinets, whereas only the HF transformers, diodes and The results are reported in Figure 7, Figure 8 and Figure 9. Figure 7 – Output voltage pulse shape of a multi-level resonant klystron modulator. The rise-time obtained was in the order of 170µs. Overshoot and voltage droop are less than 1%. The HF ripple obtained (ideal control of IGBT’s; no unbalances between modules) was in the order of 800ppm (a factor of 4 higher than the specified limit). The reduction of the HF ripple under 100ppm and of the rise time down-to 100µs (0..99%) was recently obtained by modifying the control loop and the configuration of the output filters. Figure 8 shows the soft-switching conditions obtained (ZVS for the leading leg and ZCS for the lagging leg). Finally, Figure 9 shows the condition of a klystron arc simulation (short-circuit applied at the modulator’ output at instant t=0). No modifications have been implemented on the control loop nor on the H-bridge control signals during the time period shown. Although the converter is left running “normally” after the occurrence of such arc, Figure 8 – Internal waveforms of one resonant circuit the output current “is self-limited” to about 3 times the module. Note: ZVS on leading leg (with parallel capacitors nominal value. This illustrates well the De-Qweing effect across IGBT’s); ZCS on lagging leg. described above. a)- voltage (VH) and current (ILR)at the output of one H-bridge, VCR: voltage across the resonant capacitor; b)- Voltage (VQ1) and current (IQ1) of the same H-bridge’s top/leading leg IGBT; c)- Voltage (VQ2) and current (IQ2) of the same H-bridge’s bottom/lagging leg IGBT.

Figure 9 – Klystron arc simulation: VLF: Sum of all voltages across the output filter inductances; ILF: Klystron arc current. V. CONCLUSIONS challenging operation parameters. Five klystron modulator topologies based on solid state power electronics and covering the current state of the art VI. REFERENCES in the field were presented and discussed comparatively, on the context of the ESS project. The main particularities [0] – Mezei, F.; Tindemans, P.; Bongardt, K.; “The 5MW of such modulators reside on the long pulses required (3.5 LP ESS best price-performance”, February 2008; ms), considerable average power (120 kW), compactness [0] – Eshraqi, M.; et Al.; “Conceptual Design of the ESS and high reliability. Linac”, Proc. of International Particle Accelerators Although the resonant multi-level topology seems Conference, IPAC’2010, 23-28 May 2010, Kyoto, Japan; interesting in terms of accessibility, reliability, [0] – Reass, W.A.; Doss, J.D.; Gribble, R.F.; “A 1 modularity, adoption of standard and multi-source megawatt polyphase boost converter-modulator for components, self-limitation of arc current, its practical klystron pulse application”, IEEE Conf. on Pulsed Power implementation and validation requires a considerable Plasma Science, 17-22 June 2001; amount of effort and time and could hardly be an option. [0] – Pfeffer, H. et al.; “A Long Pulse Modulator For The Marx generator topology is compact, oil free and Reduced Size And Cost”, 21st Int. Power Modulator modular. However, due to the large amount of Symposium, 27-30 June 1994; semiconductors present at HV potential and submitted to fast pulsed electric fields, its long term reliability and cost [0] – Martins, C.A.; Bordry, F.; Simonet, G.; "A Solid effectiveness is still to be proven on an industrial scale. State 100kV Long Pulse Generator for Klystrons Power All other topologies mentioned in this paper exist on an Supply", 13th European Conference on Power industrial scale and have been used in other similar Electronics and Applications, EPE '09, 8-10 Sept. 2009; applications. However, their functional characteristics [0] – Wagner, R. et al.; “The Bouncer Modulators at (rise/fall times; HF voltage ripple; reverse voltage on the DESY”, IET European Pulsed Power Conference, CERN, klystron) and their ease of operation (accessibility; Geneva, 21-25 Sept. 2009; expected reliability; oil quantity required) differ [0] – Kempkes, M.; et al.; “A klystron power system for considerably between them. the ISIS front end test stand”, IEEE Pulsed Power Considering the significant amount of klystron Conference, PPC '09, June 28 -July 2 2009; modulators required (~200 units), costs, accessibility [0] – Leyh, G.E.; “The ILC Marx Modulator (Mean Time to Repair) and reliability (Mean Time Development Program at SLAC”, IEEE Pulsed Power Between Failures) are key parameters that shall be Conference, PPC ’05, 13-17 June 2005; quantified or at least evaluated deeply. For instance, if the MTBF is 50.000 hours per unit (1 [0] – Burkhart, C.; et al.; “ILC Marx modulator failure in every 5.7 years of continuous operation), and development program status”, IEEE Pulsed Power the MTTR is 2 hours, we obtain a global Machine Conference, PPC '09, June 28 -July 2 2009; Availability of 99.2% due only to klystron modulators. [0]- Alex, J.; et al.; “A New Prototype Modulator for the The construction, testing and evaluation of prototypes, European XFEL Project in Pulse Step Modulator before series production, are therefore mandatory Technology”, Proc. of Int. Particle Accelerators intermediate steps, which are required to assess the Conference, PAC ’09, 4-8 May 2009, Vancouver, Canada; compliance of the whole infrastructure with such