A 3.2 MeV injector has been designed and built for the Darht II Project at
Los Alamos Lab. The installation of the complete injector system is nearing
completion at this time. The requirements for the injector are to produce a 3.2
MeV, 2000 ampere electron pulse with a flattop width of at least 2-microseconds
and emittance of less than 0.15 p cm-rad normalized. A large high voltage
column has been built and installed. The column is vertically oriented, is 4.4
meters long, 1.2 meters in diameter, and weights 5700 kilograms. A novel method
of construction has been employed which utilizes bonded mycalex insulating
rings. This paper will describe the design, construction, and testing completed
during construction. Mechanical aspects of the design will be emphasized.Comment: 3 pages, 4 figures, Linac 200

Elliott, B.

Eylon, S.

Henestroza, E.

Peters, C.

Yu, S.

English

arXiv

A 3.2 MeV injector has been designed and built for the DARHT II Project at Los Alamos Lab. The installation of the complete injector system is nearing completion at this time. The requirements for the injector are to produce a 3.2 MeV, 2000-ampere electron pulse with a flattop width of at least 2-microseconds and emittance of less than 0.15 pi cm-rad normalized. A large high voltage column has been built and installed. The column is vertically oriented, is 4.4 meters long, 1.2 meters in diameter, and weighs 5700 kilograms. A novel method of construction has been employed which utilizes bonded Mycalex insulating rings. This paper will describe the design, construction, and testing completed during construction. Mechanical aspects of the design will be emphasized

UNT (University of North Texas) Digital Library

DESIGN AND CONSTRUCTION OF THE 3.2 MEV HIGH VOLTAGECOLUMN FOR DARHT II*C. Peters, B. Elliott, S. Yu, S. Eylon, E. Henestroza,Lawrence Berkeley National Laboratory, Berkeley, CA 947201  INTRODUCTION   A 3.2 MeV injector has been designed and built for thesecond axis of the Dual Axis Radiographic HydrodynamicTest (Darht) facility at Los Alamos National Laboratory.Darht-II accelerator is a linac which produces a 2000ampere electron pulse with a flattop width of at least 2-microseconds and emittance of less than 0.15 π cm-radnormalized. The installation of the injector system ispresently underway and commissioning is expected tobegin in December 2000. The design and construction ofthe injector was completed by LBNL. The MarxGenerator was designed and built under subcontract toLBNL. The installation of the insulating column will beoccurring during August 2000.   The Darht-II injector High Voltage Column andCathode Assembly are housed within a 4 meter diameterby 9 meter tall vacuum enclosure. The cathode assemblyis supported on top of a vertically oriented insulatingcolumn. This arrangement is shown in figure 1. Thevolume outside of the column is vacuum and the insidevolume is insulating oil. The current stalk passes upthrough the center of the column in coaxial fashion.   A novel method of construction has been used for theinsulating column, which utilizes bonded mycalexinsulating rings. This paper will describe the design,construction, and testing of the column. Mechanicalaspects of the design will be emphasized.2  REQUIREMENTS   The primary functions of the High voltage insulatingcolumn are to physically support the cathode assemblyand to insulate the cathode from ground potential duringthe high voltage pulse. The cathode must be supported ina stable position to enable accurate alignment and stabilitywith respect to the accelerator axis. The column mustoperate reliably electrically without significant breakdownalong its length or radially to the vacuum vessel on theoutside or to the current stalk on the inside. Electricalreliability of the column is crucial and any problems inthis area will dramatically impact the operation of theInjector. The column provides a hermetic barrier betweenthe insulating medium of the marx generator and the UHVvolume of the cathode. Additionally, the columnoutgassing load must be consistent with the 5x10-8 torrpressure requirement for the Injector.3  DESIGN   The overall column design follows the conventionalapproach of using rings of insulating material separatedby metal grading spacer rings. The triple points areshielded and resistors are used to grade the voltage on thespacer rings. Figure 2 shows the typical arrangement usedthroughout the column. Insulating rings are assembledinto stacks. Nine stacks were made. Overall columnlength is 4.4 meters and diameter is 1.3 meters. Weight is5800 kg.*Work supported by the US Department of Energy undercontract DE-AC03-76SF00098Figure 1 - Injector Assembly Column and CathodeAssembly installed in Vacuum VesselFigure 2 - Cross Section of Mycalex Rings    Column orientation was chosen to be vertical. Thisavoids high bending stress that would occur if the columnwere cantilevered horizontally.   Mycalex was the material chosen for the columninsulating rings. Mycalex is a moldable and machinableceramic material first developed about 1920. It is a matrixof mica flakes suspended in a glass matrix. It isdimensionally stable, has high thermal shock resistance,low outgassing rate, high resistance to tracking due to arc-overs. It has a notch test toughness of about 1.5 ft-lbs/in.as compared to ceramic at 0.5 ft-lbs/in. It’s superiorresistance over Al2O3 to cracking under arc-overconditions has been observed during the use of bothmaterials in the high repetition rate induction machine atSRL, MA.(1)   During the design of the column, it was learned thatthere were mycalex rings of appropriate size (41.5” I.D. x47.5” O.D. x 1.3” thick) existing in a decommissionedVan de Graaff accelerator at LANL.  These rings provedto be almost ideal in all dimensions for use in the Darht-IIInjector Column. Seventy rings were found that werespares and had never been installed in the Van de Graaffmachine. Better yet, they were free. Unfortunately, therewere only about 2/3 the quantity of mycalex rings neededfor the column. After consideration of all options (newrings couldn’t be made without costly tooling), it wasdecided to make up the ring short fall with Al2O3 rings.   Total hermiticity is required of all joints in the columnin order to insure insulating oil does not migrate into thevacuum region and poison the source and contaminate thecolumn surface. O-rings were not good enough. Allbrazing vendors we spoke with either no-bid or requiredan extensive development contract in order to develop thebraze process required for this size work. Epoxy bondedjoints were used entirely to make all ceramic to metaljoints.  Extensive testing of sample joints showed a tensilestrength of 1400 psi normal and 5000 psi with themycalex grain. Ceramic to metal joints exceeded 6000 psitensile strength. All production joints were proof tested to50,000 lbs. load. Hermiticity of all stack assemblies wasmeasured with no helium leakage detected.   Tie rods along the inside diameter of the column wereadded to provide longitudinal compression of the columnto counteract the 8700 kg force differential across the topend (vacuum “pressure”) and to provide precompressionfor handling the column during handling and installation.Eight tie rods are used and produce a total of 24,000 kgcompression.   Toroidal Field Shapers were attached to the outsidediameter of the metal spacer rings to reduce the electricfields in the triple point regions to 4 kV/cm. The non-symmetrical shape is necessary to reduce the stress on theshaper lower surface to below 60 kV/cm. In order toreduce manufacturing cost and eliminate field stress risersdue to joints and fasteners, the shapers were made in onepiece. The shapers were assembled over the spacer ringsby expanding them 4 mm by heating them to 200°C.When the shapers cool their inside groove engages theedge of the spacer rings.  This process is shown in figure3.Oval shaped Shock Diffuser rings were attached to theinside edge of the spacer rings for the purpose of diffusingshock waves created by arcs in the oil. Each diffuser ringis supported with 12 radial fingers, which are springy inthe radial direction so as not to transmit forces to thespacer/ceramic joint in the event of an arc to the ring.This arrangement is shown in figure 4. Also shown in thisfigure are the MOV and resistor board assembliesmounted between each spacer ring on the 6 lower stackassemblies.4  TESTING OF MATERIALS AND SCALEMODELS   Thirty 1” diameter x 1.3” long mycalex samples werecored out of spare mycalex rings. These samples wereFigure 4 - Arrangement of MoV and ResistorBoards Installed Inside Stack AssemblyFigure 3 - Installation of Field Shapersbonded to aluminum end pieces and were pull tested tofailure. No necking of the samples was done. In all casesfailure occurred near a bonded end. The epoxy neverfailed, as there was never any epoxy remaining on themycalex part. The test determines the effective strength ofthe joint expected in the final column joints. The averagestrength was 1400 psi with a standard deviation of 100psi.   Each mycalex ring used in the column was proof testedunder internal hydrostatic pressure to test for rings withdamage or internal flaws. These rings were old and theirhistory and precise material was not well known.  Allrings were internally pressurized to 550 psi, whichproduces 3800 psi tensile stress (2/3 ultimate strength) inthe hoop direction of the ring. Several failed and werediscarded.   A ¼ scale stack was built and tested for shock survival.It was filled with oil and arcs were induced betweenelectrodes in the oil to create shock waves. The results ofthese tests were analyzed and prediction made as to thestress and conservatism of the full-scale column. BillAnderson, LANL, completed this analysis. The resultsindicate a safety factor of 4 to 10 depending onassumptions of the model. Each production stackassembly was vacuum leak checked below 10-9 std. cc/sec. Each production stack assembly was “Pull” tested to50,000 lbs. tensile load to insure no cracks existed in themycalex bond joints after assembly. One joint did fail at43,000 lbs. and was repaired.   Electrical tests included “Hockey Puck” testing, whichwas to determine breakdown field stress levels andsurface tracking behavior for the field shaper and bondedmycalex ring configuration. Three inch diameter mycalexdisks (cut out of discarded 48” diameter rings) werebonded to stainless steel rings to form the electricallyequivalent cross section of a production cross section.These tests indicate that we had a safety margin of at leasta factor of 2 in voltage holdoff in our final design.   Stack Acceptance tests were completed on each of the 9production stack assemblies. The fully configured stacks(including MOV and resistor strings) were operated undervacuum at high voltage to QA the electrical assemblywork and determine final conditioning levels andoperating margin for the final column assembly.   Thehigh voltage breakdown limit of the stack was tested to afactor of 2 above the operating voltage, and the energyand current through the MOV's and resistors were testedto greater than a factor of 1.5 above operating conditions.5  SHOCK ANALYSIS   A detailed shock analysis of the column using theexplicit non-linear finite element analysis programDYNA2D was completed by Joseph Stoner from LLNL.The general energy load assumption is based oninformation provided by Bill Anderson from LANL that1.0 kJ of energy is deposited in a cylinder of oil extendingfrom the “stalk” to the diffuser rings on the I.D. of thecolumn. This initiates a hydrodynamic pressure pulse thattravels through the oil impacting the Mycalex structure. Avertical compressive pressure was applied to the top of theMycalex to represent the preload of the tie rod on theassembled stack.   The results from this 2½ dimensional axisymmetricanalysis show that most of the energy in a shockwaveproduced by an arc will travel parallel to the axis of thestalk. Peak pressures of over 4000 psi will arrive at themycalex wall approximately 45 microseconds after thearcing event. The maximum axial tensile stress is in themycalex and is the stress of most concern; it isapproximately 900 psi and occurs at a time of 60microseconds.   The results of this analysis are not especially reassuring,as the factor of safety in the mycalex appears to be1400/900 or 1.56. However, the results of this analysis areexpected to be quite conservative and the stresses in thereal event should be less than the values predicted fromthis simulation. Three arguments can be made to theconservative nature of the results: first, no damping hasbeen modeled; second, no failure model has been used;third, the higher strength mechanical properties thatusually accompany high strain rate dynamic shocks arenot been modeled. An additional factor in favor ofconservatism is that the volume of mycalex materialsubjected to high stress is extremely small. It is just thecorner of the mycalex where it bonds to the aluminumspacer ring and azimuthally located only in the immediatevicinity of the line on the arc.   The best factor of safety for the mycalex is acombination of the analytical factor (1.56 x conservatismfactors) above and the empirical factor (4 to 10)determined from the ¼ scale model experiment describedabove.

Design and construction of the 3.2 MeV high voltage column for DARHT II

A 3.2 MeV injector has been designed and built for the DARHT II  Project at Los Alamos Lab. The installation of the complete injector system is  nearing completion at this time. The requirements for the injector are to  produce a 3.2 MeV, 2000-ampere electron pulse with a flattop width of at least  2-microseconds and emittance of less than 0.15 pi cm-rad normalized. A large  high voltage column has been built and installed. The column is vertically  oriented, is 4.4 meters long, 1.2 meters in diameter, and weighs 5700  kilograms. A novel method of construction has been employed which utilizes  bonded Mycalex insulating rings. This paper will describe the design,  construction, and testing completed during construction. Mechanical aspects of  the design will be emphasized

Design and Construction of the 3.2 Mev High Voltage Column for Darht II

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