Saturn is a dual-purpose accelerator. It can be operated as a large-area
flash x-ray source for simulation testing or as a Z-pinch driver especially for
K-line x-ray production. In the first mode, the accelerator is fitted with
three concentric-ring 2-MV electron diodes, while in the Z-pinch mode the
current of all the modules is combined via a post-hole convolute arrangement
and driven through a cylindrical array of very fine wires. We present here a
point design for a new Saturn class driver based on a number of linear
inductive voltage adders connected in parallel. A technology recently
implemented at the Institute of High Current Electronics in Tomsk (Russia) is
being utilized[1].
  In the present design we eliminate Marx generators and pulse-forming
networks. Each inductive voltage adder cavity is directly fed by a number of
fast 100-kV small-size capacitors arranged in a circular array around each
accelerating gap. The number of capacitors connected in parallel to each cavity
defines the total maximum current. By selecting low inductance switches,
voltage pulses as short as 30-50-ns FWHM can be directly achieved.Comment: 3 pages, 4 figures. This paper is submitted for the 20th Linear
  Accelerator Conference LINAC2000, Monterey, C

Long, F. W.

Mazarakis, M. G.

Spielman, R. B.

Struve, K. W.

English

arXiv

Saturn is a dual-purpose accelerator. It can be operated as a large-area flash x-ray source for simulation testing or as a Z-pinch driver especially for K-line x-ray production. In the first mode, the accelerator is fitted with three concentric-ring 2-MV electron diodes, while in the Z-pinch mode the current of all the modules is combined via a post-hole convolute arrangement and driven through a cylindrical array of very fine wires. We present here a point design for a new Saturn class driver based on a number of linear inductive voltage adders connected in parallel. A technology recently implemented at the Institute of High Current Electronics in Tomsk (Russia) is being utilized. In the present design we eliminate Marx generators and pulse-forming networks. Each inductive voltage adder cavity is directly fed by a number of fast 100-kV small-size capacitors arranged in a circular array around each accelerating gap. The number of capacitors connected in parallel to each cavity defines the total maximum current. By selecting low inductance switches, voltage pulses as short as 30-50-ns FWHM can be directly achieved. The voltage of each stage is low (100-200 kv). Many stages are required to achieve multi-megavolt accelerator output. However, since the length of each stage is very short (4-10 cm), accelerating gradients of higher than 1 MV/m can easily be obtained. The proposed new driver will be capable of delivering pulses of 15-MA, 36-TW, 1.2-MJ to the diode load, with a peak voltage of {minus}2.2 MV and FWHM of 40-ns. And although its performance will exceed the presently utilized driver, its size and cost could be much smaller ({approximately}1/3). In addition, no liquid dielectrics like oil or deionized water will be required. Even elimination of ferromagnetic material (by using air-core cavities) is a possibility

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Abstracts(qfdozo~~1A NEW LINEAR INDUCTIVE VOLTAGE ADDER DRIVERFOR THE SATURN ACCELERATORM. G. Mazarakis, R. B. Spielman, K. W. Struve, F. W. LongSandia National Laboratories, Albuquerque, NM 87185, USASaturn is a dual-purpose accelerator. It can be operatedas a large-area flash x-ray source for simulation testing oras a Z-pinch driver especially for K-line x-ray production.In the first mode, the accelerator is fitted with threeconcentric-ring 2-MV electron diodes, while in the Z-pinch mode the current of all the modules is combined viaa post-hole convolute arrangement and driven through acylindrical array of very fine wires. We present here apoint design for a new Saturn class driver based on anumber of linear inductive voltage adders connected inparallel, A technology recently implemented at theInstitute of High Cument Electronics in Tomsk (Russia) isbeing utilized[l].In the present design we eliminate Marx generators andpulse-forming networks. Each inductive voltage addercavity is directly fed by a number of fast 100-kV small-size capacitors arranged in a circular array around eachaccelerating gap. The number of capacitors connected inparallel to each cavity defines the total maximum current.By selecting low inductance switches, voltage pulses asshort as 30-50-ns FWHM can be directly achieved.The voltage of each stage is low (100-200 kv). Manystages are required to achieve multi-megavolt acceleratoroutput. However, since the length of each stage is veryshort (4-10 cm), accelerating gradients of higher than 1MV/m can easily be obtained. The proposed new driverwill be capable of delivering pulses of 15-MA, 36-TW,L2-MJ to the diode load, with a peak voltage of -2.2 MVand FWHM of 40-ns. And although its performance willexceed the presently utilized driver, its size and cost couldbe much smaller (-1/3). In addition, no liquid dielectricslike oil or deionized water will be required. Evenelimination of ferromagnetic material (by using air-corecavities) is a possibility.1 INTRODUCTIONSaturn is a pulsed power accelerator presently inoperation at Sandia[2]. It represents a modification of theold PBFA 1[3] accelerator used for ion fusion research. Itis named Saturn because of its unique multiple ring diodedesign which is utilized while operating as an x-raybremsstrahlung source. As an x-ray source Saturn candeliver to the three-ring e-beam diode a maximum energyof 750 kJ with a peak power of 32 TW providing an x-rayexposure capability of 5 x 10’2 radsksec over a 500cm2area. As such it is the highest power electrical driver forbremsstrahlung production in the world. As a z-pinchdriver Saturn can deliver up to 8 MA to a wire or gas-puffload. A 700-kJ total x-;ay output was obtained fromaluminum or tungsten wire pinches, and a 60-70-kJ K-lineradiation from aluminum2 PRESENT ACCELERATORCONFIGURATIONWJiNFig. 1 Schematic representation of the present SaturnconfigurationFigure 1 is a schematic representation of the Saturnconfiguration presently in effect. It utilizes conventionalpulse power architecture. The driver starts with 36 Marxgenerators as the principal energy source. Then ensues acascade of pulse compression stages to convert themicrosecond FWHM Marx output to the 50-ns final pulsethat powers the electron diode or the z-pinch load. Thepower flow follows the following stages: Themicrosecond pulses are stored in 36 water intermediatestore capacitors. From there through 36 triggered gasswitches they charge 36 pulse-forming lines whichthrough 36 self-breaking water switches launch the shortpulses into 36 triplate water transmission lines andimpedance matching transformers. Finally, the short(now -50 ns) pulse is applied to the diode through awater-vacuum insulating ring interface. The 36 verticaltriplate transmission lines are connected to threehorizontal triplate disks in a water convolute section. Inthe vacuum section the power is split into three parts andfeeds three conical-triplate magneticrdly insulatedtransmission lines (MITL) that power the three separaterings of the e-diode. A final pulse compression of 25:1 isachieved. To accomplish all thatj the device requiressubstantial dimensions. The overall diameter is 30 m andthe height 5 m. The 36 Marxes are immersed in a250,000-gallon oil-filled annular tank while the rest of thedevice is in 250,000 gallons of deionized water. In our. -- .- - .- .—.—. .-..> . .. ... ), . . . . . . . . . . . . . . . . . ,,~ - ~= .,-DISCLAIMERThis report was.prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.I(proposed design, utilizing the fast-pulser technology[5],the entire Saturn device will be smaller than 17 m indiameter and will be only 5 m in height. In addition, nowater and oil tanks are required. Except for the 5-m high,4-m diameter cylindrical central section which is invacuum, the entire accelerator is in the air and readilyserviceable.The three diodes are concentric and, hence, havedifferent power requirements. The imerrnost dioderequires less power than the outside one to achieve thesame x-ray area average output. The diode design is suchthat a power partition with a ratio of 3:21 from outsidering to center ring is required. The bottom triplate MITLfeeds the largest outer diode ring while the top MITL isconnected with the innermost diode ring.3 NEW SATURN DESIGN BASED ON OURFAST PULSER ARCHITECTUREIn our new design we utilize the revolutionarytechnology of the fast pulser[4]. We select 24 modules of-700 W, 2.2 MV each to produce a total current of theorder of 14 MA The novelty of the design is that each ofthe 24 modules is a self-magnetically-insulated voltageadder. No liquid or solid insulator is utilized between theinner cathode electrode and the outer anode cylinder. Acoaxial geometry is adopted. The stages of each modulevoltage adder are of relatively low voltage (100 kV).Therefore, in principle a 20-stage structure would beenough to obtain a 2 MV output. However, because weadopted a design where the output impedance is equal tothe characteristic impedance of the pulser[4], the peakvoltage of each stage will not exceed 55 kV. Hence, 40stages per module will be necessary. The prime powerper stage is a circular array of 24, 30-kA, maximum-current fast capacitors, The inductance and capacitance ofeach capacitor is 25 nH and 11 nF respectively, which—provide a very fast, ~LC =17 ns, time constant for thesystem. Hence, no further pulse power compression isrequired. The power-pulse FWHM from its onset has therequired width to be applied directly to the diode.The capacitor arrays are switched to the acceleratinggaps through very low inductance, -- 1 nH, externallytriggered switches. Two candidates are presentlyconsidered and being evaluated. The firstj developed inRussia[5], is a ring switch that has a fraction of l-nHinductance switching large currents up to -1 MA throughmultiple conducting channels. It can do that at a low 100-kV voltage. The second is being developed in Sandia[6].It is a low inductance (-50 pHJ high-gainphotoconductive semiconductor switch (PCSS) that canswitch up to 250-kV, 7-kA currents in a parallel array ofsix, 2-inch wide GaAs wafers. We believe thephotoconducting switch can be fhrther improved toincrease the current per unit width by at least one order ofmagnitude for the same size. The Russian switch hasalready reached maturity, and the design and hardware arereadily available. Those switches make it possible toswitch at low, 100-kV voltage and still produce verynarrow 50-ns pulses dmectly from the capacitors withoutthe need of cumbersome and expensive pulsecompression.The fastest capacitors available to date are the compactMaxwell Laboratories S type capacitors model 31165 [7].They are very small (58x150x274 mm2) and very fast(L=25 nH and C=40 IIF). These capacitors could easilybe modified to the faster ones (L=25 nH, C= 11 @considered in the present point design. Although thecapacitor dimensions could be further reduced or specialsemi-circular geometries could be developed, for thepurpose of our point design the dimensions of thepresently available off-the-shelf 31165 Maxwellcapacitors were assumed.Fig. 2 Cross-sectional view of the new designThe accelerator gaps are magnetically insulated withMetglas.m A very small Metglasm cross-sectional area(10 cm2) is needed because of the modes~ 2.4x10-3voltsecs of each gap. Therefore, the dimensions of eachmodule are as follows: The overall diameter is 2.2 m andthe length 2.4 m (Fig. 2). The cathode electrode is conicalstarting with a diameter of 1.17 m and terminating at theoutput end with a 1.08 m diameter cylinder. The anodeelectrode is a 1.20-m imer diameter cylinder. A 4-mcoaxial MITL vacuum transmission line connects eachmodule to the respective conical triplate MJ.TL of eachdiode ring. The coaxial MITL is longer than theminimum required (2 m) in order to provide easyaccessibility and servicing of the accelerator. Throughoutthe power flow from the voltage adders to the diode ringsself-magnetic insulation and impedance matching wasrigorously implemented. The power partition ratio of3:2:1 for the diode rings was retained since the samediode as the one presently operating was assumed to beutilizedOne of the advantages of the proposed design is that thediode insulating stack of the central section is eliminated.The only insulator left is the short plastic ring insulatingeach of the -60-kV accelerating gaps of the modules. Atotal of 24 modules are required to provide the 15 MAcurrent to the diode. To maintain the proper power ratio,4,8, and 12 modules are connected respectively to thetop, middle and bottom conical triplate MITL’s. Aminimum anode-cathode gap of 1 cm was maintainedthroughout the entire power flow transport.Table 1: Comparison of the two designs’ performanceNew Point DesignLevel Zsource Zload V1oad I loadOhms Ohms MvTop 0.91 0.99 2.5 2.5Middle 0.45 0.38 2.2 5.7Bottom 0.30 0.33 2.4 7.4kEIHEEmPresent Accelerator PerformanceTable 1 summarizes the peak pulse power parameter ofthe new point design and compares it with the presentaccelerator performance. The design was doneanalytically and verified numerically utilizing the circuitdesign code SCREAMER[8].Figure 3 provides the voltage current and power for theouter-diode ring connected to the bottom MITL.86 -42:0-21 I 1 I Io 50 100 150 2065time(m)Fig. 3 Outer diode ring (bottom MITL) waveforms.Similar pulse waveforms are applied to the other twodiode rings. Figure 4 is a three dimensional rendition ofthe entire accelerator.Fig, 4 A 3 D visualization of the new Saturn point designThe same design can deliver to a Z-pinch load amaximum total-cument of 15 MA. In this case only 2conical MITLs will be utilized with each connected to 12voltage-adder modules. Details of this application anddesign optimization will be presented in futurepublication.5 SUMMARYWe have developed a point design for an alternativepulsed power driver for the Saturn accelerator. It has thesame or higher power and energy output as the onepresently in operation. However, its size is appreciablyreduced from the present 30-m diameter to 17 m. Theoverall height remains the same and of the order of 5 m.The expensive large amounts of oil and deionized waterare eliminated together with the requirement for a centralwater-vacuum interface-insulating stack. A first cutestimate of the cost suggests a relatively modest value of-$10 M. A cheaper device of -$5 M could be built if theMetglassm cores are substituted with air core equivalents.B~ause of the very short pulse 40-ns FWHM, thedimensions of an air core device will not be appreciablylarger than our Metglassw point design[1][2][3][4][5][6][7][8]6 REFERENCESB.M. Kovalchuk, and A. Kim “High Power Driver forZ-pinch Loads.” Private Communication, 1998.D.D. Bloomquist, R.W. Stimett, D.H. McDaniel,J.R.Lee, A.W.Sharpe, J.A.Halbleib, L.G. Schlitt.P.W. Spence, and P. Corcoran, “Saturn, A Large AreaX-Ray Simulation Accelerator,” Proc. 6* IEEEPulsed Power Conference, Arlington, VA, 1987,p.310.T.H. Martin, J.P. VanDevender, G.W.Barr, S.A.Golstein, R.A. White, and J.F. Seamen, “IEEETransactions on Nuclear Science,” NS-28, No. 3,3368 (1981).M.G. Mazarakis, and R.B. Spielman, “A Compact,High-Voltage E-Beam Pulser,” 12* IEEE PulsedPower Conference, Monterey, CA, July 1999.B. M. Kovalchuk, “Multi-Gap Spark Switches,” Proc.11* IEEE Pulsed Power Conference, 1997, p. 59.F.J. Zutavem, G.M. Loubriel, et al., “Properties ofHigh Gain Gas Switches for Pulsed PowerApplications,” Proc. 11* IEEE Pulsed PowerConference, 1997, p. 959.Maxwell Laboratories, Inc., “Capacitor SelectionGuide” Maxwell Laboratories, Inc. San Diego, CA,Bulletin No. MLB-2112B.M. L. Kiefer, K. L. Fugelso, K. W. Strove, M. M.Wldner, “SCREAMER, A Pulsed Power DesignTool, User’s Guide for Version 2.0.” Sandia NationrdLaboratory, Albuquerque, N.M., August 1995Sandiais a mukiprogramlaboratoryoperatedbySandiaCorporation,a LockheedMartinCompany,for the UnitedStatesDepartmentof EnergyunderContractDE-AC04-94AL85000.

A New Linear Inductive Voltage Adder Driver for the Saturn Accelerator

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