A pulse generator with the following characteristics is presented: the voltage ranges in the interval 0.1–6 kV, the maximum delivered current is 150 A, the pulse length ranges within the interval 100–300 ns, the rise time and the decay times are, respectively, 10 and 25 ns on 50 Ω resistive load and the repetition rate is higher than 1 MHz. The circuit has a source capacitor of 10 nF charged at the needed voltage, the capacitor feeds the load through a parallel of two fast and high voltage solid state switches. The nanosecond rise time and the square fashion of the pulse have been accomplished arranging all the components in cylindrical symmetry. A bipolar pulse is obtained coupling two circuits with opposite polarity

I. Boscolo

S. La Torre

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REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 70, NUMBER 3 MARCH 1999A 6 kV–150 A, 8 ns rise time pulse generator for excitationof ferroelectric cathodesI. Boscolo and S. La TorreUniversity and INFN, via Celoria 16, 20133 Milano, Italy~Received 17 March 1998; accepted for publication 9 November 1998!A pulse generator with the following characteristics is presented: the voltage ranges in the interval0.1–6 kV, the maximum delivered current is 150 A, the pulse length ranges within the interval100–300 ns, the rise time and the decay times are, respectively, 10 and 25 ns on 50V resistive loadand the repetition rate is higher than 1 MHz. The circuit has a source capacitor of 10 nF charged atthe needed voltage, the capacitor feeds the load through a parallel of two fast and high voltage solidstate switches. The nanosecond rise time and the square fashion of the pulse have beenaccomplished arranging all the components in cylindrical symmetry. A bipolar pulse is obtainedcoupling two circuits with opposite polarity. ©1999 American Institute of Physics.@S0034-6748~99!02502-2#csa-totrotr crgoice-hroseeisecitorionhethetor-ibleingonntalighighis aesrkfeedFast polarization switching of ferroelectric ceramiused as electron emitters1 require fast voltage pulse genertors of some kV and 100 A. We have built such a generafor our experimental system devoted to studies on elecemission from ferroelectric ceramics. The pulse generaacts upon a ceramic disk, which behaves as a nonlineapacitor of typically 500 pF. The current necessary for chaing this capacitor at 2 kV in 20 ns results inI c5CV/t5100 A. The capacitor is placed parallel with a resistorsome tens of ohms in order to drain off the charge whremained in the capacitor after the emission~so theexcitation–emission cycle can restart!. The parallel resistorabsorbs a currenti 5V/R, i.e., some tens of amperes. Bcause of the capacitor–resistor parallel the feeding pulserto provide more than 100 A.The pulse generator for the electric excitation of ferelectric cathodes must provide the three types of pulsquare positive, square negative, and bipolar.The electric scheme of a pulser with the abov1850034-6748/99/70(3)/1857/3/$15.00rnora--fhas-s:-mentioned features of high current, high voltage, short rtime, and duration has to be based on the idea of a capacharging and its discharging for the required time durat~Fig. 1!. The charging phase can be relatively slow, while tdischarging must be as fast as possible. In addition,square fashion of the pulse requires such a value of the sage capacitor that the emitted charge per pulse is negligcompared with the stored one. The repetition rate rangfrom several kHz up to the MHz are required dependingthe utilization.The controlled switches, which gate the flux of curreinto the external load, are the most difficult technologicitems of the system, due to the strict requirements of hcurrent and nanosecond reaction time and relatively hrepetition rate. We have chosen solid state switches, thatparallel of five lines of ten field effect transistors in seri~see Fig. 2!, instead of gas switches, as thyratron and~lessfrequently used! cold cathode tubes, and triggered spagaps, because solid state switches are much easier toa-c-FIG. 1. Schematic of the circuit. The high voltage cpacitors are 10 kV low inductance capacitors. The 16Vcarbon film resistor at the output is for switches protetion against possible short circuits.7 © 1999 American Institute of PhysicsraoiledactShraestritieenhethinpeengrltfoucgalre-ingonthebendedpF:theurrentis aion.on1858 Rev. Sci. Instrum., Vol. 70, No. 3, March 1999 I. Boscolo and S. La Torreand drive in comparison with the latter ones.2 In addition,these gas switches have finite lifetime and unstable chateristics contrary to solid state switches. Our chains~MOS!POWER fets are triggered synchronously by the traing edge of the gate pulse because each of them is couplthe drive circuit. The coupling is inductive in order to setgalvanic isolation between the metal–oxide–semicondufield effect transistor~MOSFETs! and the drive circuit~seeFig. 2!.The model STP5N90 POWER MOSFET of the SGThomson catalog have been chosen for our switches. Toperate in avalanche mode in the pulser. Their main chateristics are: Vdrain-source5900 V, I drain520 A in pulsedmode and fast shutting. A very careful assembling of thtransistors, that is a compact mounting so as to avoid sinductances at the maximum extent, led to switches wvery short recovery time and low jitter, as is summarizedTable I. These characteristics are quite better than thosconventional high voltage gas switches. The recovery tima switch is determined by the turnoff of each MOSFET aby the relaxation time of the network connecting all tMOSFETs. However, the recovery time is so short thatrepetition rate is determined by the time required for coolthe whole switch. The generator has been tested for a retion rate around 25 kHz. It is possible to obtain a sequenctrains having a frequency of a couple of MHz, but occurriat around the millisecond rate.In Fig. 1 the electrical scheme of the pulse generatoshown. In order to exploit the extremely rapid rates of voage and current possible with our switches, the criteriathe circuit design are: maximum compactness, low indtance leads and components, cylindrical and/or striplineTABLE I. Switches specifications.Max. operating voltage 6 kV single pulseMax. operating voltage 63 kV bipolar pulseMax. peak current 150 A pulse width<200 nsTurn-on rise time 8 ns on 50V and 3 kVTurn-off rise time 25 ns on 50V and 3 kVRecovery time <1 msInternal resistance '4 VFIG. 2. Scheme of one of the two switches built for the pulser and ofdriver circuit.c-f-toor-eyc-eayhnofofdegti-ofis-r-e-ometry at the maximum possible extent. Two 6 kV externgenerators feed the 10 nF source capacitor through a 1 MVresistor. This current limiting resistor can be reduced inlation with an enhancement of the current of the feedpower supply. This would allow an increase of the repetitirate of the pulser. The source capacitor is connected toload by two switches set in parallel. The switches canFIG. 3. Voltage pulse wave form, with a resistive load of 50V: the rise timeis 8 ns, while the decay time is 25 ns. The pulse duration can be extefrom some tens of nanoseconds up to 500 ns.FIG. 4. Pulse wave form with an antiferroelectric sample in site of 100the rise and decay time are expanded for convenience. In image~b! thepolarization switching current is reported, upper signal, in order to showcorrespondence between its peaks and the pulse deformations. The ccharging the capacitor is only 20 A because the polarization switchingrelatively slow process for the kind of ceramics useful for electron emisse, olaannidhdthohaonetraluinaoshtrnntugly-abinmrighinsar-deapeby at 60f theofre-edthesedionso-theticsur-., J.hioug1859Rev. Sci. Instrum., Vol. 70, No. 3, March 1999 I. Boscolo and S. La Torregated either by an optical pulse running in an optical fiberby a single pulse manually launched, or by a rectangupulse generator. A matching resistor couples pulserswitches. A particular care has been addressed to contions from the driving pulser to the switches in order to avoany delay between them.Figures 3–6 show the output wave forms probed witresistive voltage divider placed near the load and recordea 300 MHz bandwidth oscilloscope. The 8 ns rise time ofpulse on a resistive load is mostly due to the shutting timethe switches, 5 ns, plus some nanoseconds due to the cing time of stray capacitances, which~in our circuit! can beestimated some tens of picofarad. The 25-ns-decay timeresistive load of 50V comes from the opening time of thswitches, 10–15 ns, and the discharging time of the scapacitance through the 50V. Adding a capacitor in paralleto a 100V resistor, the rise and decay times increase becaof the increasing of the capacitor charging and dischargtimes.The test has been carried out with lead titanate zirconlanthanum doped ceramics, called PLZT, having comption 8/65/35 and 4/95/5, where the numbers refer to lantnum ~in relation to lead!, zirconium, and titanium relativeatom percentage. The 8/65/35 ceramic is in ferroelecphase at room temperature, while the 4/95/5 one is in aferroelectric phase, but it undergoes a transition from aferro to ferroelectric phase under the action of a high enoelectric field.3 These ferroelectric ceramics are commonused for emission.1 The cathodes are disks of 16 mm diameter and 1 mm thickness, coated by a uniform metallic filmthe back surface and by a metallic grating interconnectedan external ring at the front surface. These disks wereserted in a vacuum diode gap~a turbopump sets the vacuuat 1025 mbar). The disk holder and the connections aFIG. 5. Pulse wave form with a ferroelectric sample in site of 400 pF. Tcurrent signal of the switching polarization current is reported as in prevfigure.rrdec-aonefrg-aysegtei-a-icti-i-hty-every compact and have a cylindrical symmetry. When a hvoltage is applied through the ferroelectric disk, the domaof the material lineup and in turn induce the so-called polization switching current.4From Fig. 4 we see that with an antiferroelectric cathoof an initial capacitance of about 100 pF, the pulse shremains substantially the same, the rise time increasesfactor around 1.5, and the decay time increased to abouns. This latter depends on the fact that the capacitance oferroelectric disk is increased of a factor 4–5 by the actionthe electric field. It could be reduced lowering the loadsistor. In fact, halving the load resistor to 50V, the decaytime is almost halved as well.An 8/65/35 ferroelectric disk of about 400 pF producan output pulse again substantially similar to the one ofpure resistive load, but the rise and the decay times increaup to about 20 and 100 ns, respectively. Some deformatare introduced by the polarization switching current.For completeness the bipolar signal with the relative plarization switching current is reported in Fig. 6.The pulse generator has been successfully used inlast two years to run emission experiments. Its characterisof rise and decay time, of pulse length, and voltage and crent amplitude have been stable. The company AEDI~AEDI,via De Gasperi 19, 20020 Lainate, Milano! placed the itemin production.1H. Riege, NIM Phys. Res. A340, 80 ~1994!; G. Benedek, I. Boscolo, JHanderek, and H. Riege, J. Appl. Phys.81, 1396~1997!; G. I. Rosenman,O. V. Malyshkina, and Y. L. Chepelev, Ferroelectrics110, 99 ~1990!,these are some references among many papers.2W. Moch, Jr. and W. H. Holt, J. Appl. Phys.50, 2740~1978!.3M. E. Lines and A. M. Glass,Principles and Applications of Ferroelectricand Related Materials~Clarendon, Oxford, 1977!.4G. Benedek, I. Boscolo, J. Handerek, A. Moscatelli, and A. ScuratiAppl. Phys.83, 2766~1998!.esFIG. 6. A bipolar voltage signal with the relative polarization switchincurrent.

A 6 kV–150 A, 8 ns rise time pulse generator for excitation of ferroelectric cathodes

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A 6 kV–150 A, 8 ns rise time pulse generator for excitation of ferroelectric cathodes

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