IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983

PHERMEX STANDING-WAVE LINEAR ELECTRON ACCELERATOR

T. P. StarkeLos Alamos National Laboratory

Los Alamos, NM 87545

Summary

The PHERMEX standing-wave rf electron linac is ahigh-current pulsed electron beam generator that isused for flash x-radiography. This accelerator isbeing upgraded to 1000-A peak current and 50-MeV peakenergy over a 150-ns pulse or over three 40-ns pulses.This upgrade is a result of increasing the rf power inthe cavities and installing a new (Physics Inter-national) injector pulser.

Introduction

PHERMEX is a three-cavity, standing-wave rfelectron linac. PHERMEX began operation in 1963 at21 MeV and 300 A; was upgraded in 1967 to 27 MeV, andwill be upgraded to 50 MeV and 1000 A in 1983. 1,2PHERMEX is an acronym for Pulsed High-Energy Radio-graphic Machine Emittinq X-Rays. The primary applica-tion of the PHERMEX beam is to generate a short x-rayburst for flash x-radiographic measurements. A flashradiograph is an areal density distribution image of arapidly moving object. This image is generated when ashort duration point x-ray source is directed at anobject and x-ray film. PHERMEX differs from mostradiographic source machines in that the PHERMEX x-raysystem can resolve very small density variations inextremely thick high-atomic number objects. To gener-ate such a high-quality flash x-ray beam, the acceler-ated electron beam must be of short duration (4200 ns)to minimize object motion blur, high energy and highcurrent to maximize x-ray flux in the 2- 7-MeV x-rayenergy range, and focusable to a small spot ("1- 5-mmdiam). A standing-wave linac is well suited for gener-ating this type of electron beam.

Accelerator Configuration

Figure 1 shows a schematic of the accelerator. Ina typical pulse, first, rf energy flows into the accel-erator cavities for 1 ms, building up the acceleratingelectric field strength until the resistive cavitypower loss equals the input power. A 200-ns, 300-Aelectron pulse is injected into the first cavity thatfocuses, chops, and accelerates the electron beam. Theother cavities further focus the beam that is then

RF AMPLIFIERS

Figure 1. PHERMEX accelerator schematic.

magnetically focused to a minimum spot size on an x-rayconverter.

The original rf amplifiers are RCA 6949 models,which develop 1.13 MW pulsed at 50 MHz. As part of the1983 upgrade, these will be replaced with Eimac X2159tetrode amplifiers that deliver 5 MW pulsed at 50 MHz.This amplifier was developed by the Los Alamos Hydro-dynamic Group. It has an 80% plate efficiency and a16 dB gain for a 3-ms pulse operated once every 10 s.Figure 2 shows a circuit schematic for the tetrodeamplifier with the bias inputs and the bias isolationcircuitry omitted. The X2159 is a cylindrical tubewith the cathode, the control grid, and the screen gridconstructed of tensioned rods appropriately aligned sothat the control grid intercepts a minimum of cathodeemission, and the screen grid provides maximum capaci-tive shielding between the control grid and the plate.The primary difficulty operating this tube at 50 MHz isthat the active cathode length is approximately 0.1 X(A = 6 m at 50 MHz), and the tube is almost resonant at50 MHz. The control grid/cathode input is series reso-nant at 52 MHz, the shorted plate-screen circuit isself-resonant at 76 MHz with the corona shields inplace. A secondary difficulty is the propensity of thecontrol grid-screen grid-circuit for supporting a900-MHz parasitic. The operating line gives a 120-2plate resistance with a 35-kV plate bias, a 1500-Vscreen bias, a 1000-V control-grid bias, and a groundedcathode. The rf drive for the X2159 final amplifieroriginates at the shorted resonant line-plate circuitof an Eimac 4CX1500 driver amplifier. The 120-kWdriver power is coupled through six 25-Q RG 14-17(Scyllac) cables, 3/4 wavelength long. As the X2159control grid is near series resonant, the grid inputimpedance is very small (=0.1 Q). The six cable net-work transforms this 0.1 Q2 to 60 Q2, which is a conve-nient output impedance for the driver. The six cablesconnect symmetrically around the X2159 control grid; a4.16 Q impedance was chosen to match the control grid-transmission line impedance inside the tube. TheRG 14-17 has a resistive sheath grading the potentialat the conductor-dielectric interface; the cables areslightly attenuating at 50 MHz (3 dB/100 ft). At900 MHz, RG 14-17 is so strongly attenuating that thesix cable network is totally absorbing; consequently,

Figure 2. Rf amplifier schematic.

0018-9499/83/0400-1402$01.00( 1983 IEEE

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the 900-MHz parasitic is stabilized. The screen gridis rf biased by a small neutralization inductor toground. This inductor is adjusted to null capacitivefeedback from the plate to the control grid at 50 MHz.The plate is matched to the 60-Q output transmissionline by an adjustable near-quarter wave, 9-Q trans-mission line. The loop that couples the amplifier tothe cavity TMolo mode is adjusted to present a 60-0pure resistive load. Typically, two X2159 amplifiersare coupled to a single cavity. This load impedancelooking into the loop is

Z(M) = XL -j nQK2a ±a2+ a2 ± 1

where XL is the loop reactance,Q is the unloaded cavity Q,K2 is the loop coupling constant,n is the number of loops,

X - W0a is Q

wo0 is the cavity resonant frequency, andX is the improved frequency.

The coupling K2 is set by adjusting the coupling angleof the loops; X is typically 30 0, and a is tuned tothe frequency where Z(w) is 60 0 resistive.

The cavities are copper walled, 2.6-m-long, 2.3-m-radius cylindrical structures with 0.05-m radius aper-tures for beam transport. The unloaded Q is 125,000;the resonant freguency is 50 MHz. The cavity-electricfield strength on axis is related to the total inputpower

E(MV/m) = 3.48 P(MW)

Two 5-MW amplifiers will develop 11 MV/m, which is 0.7of the W. D. Kilpatrick limit.3 With three cavities,this gives a 50-MeV-electron beam peak energy.

The PHERMEX electron gun is biased by a 500-kVFemcor pulser generating a 200-ns, 300-A electron beam.This pulser will be replaced with a 1.25-MV PhysicsInternational pulser that will deliver three 40-nspulses or a single 150-ns pulse generating a 1000-A guncurrent. Figure 3 shows a diagram of the PI pulser.The behavior of the triggered gas switches is criticalto making the triple-pulse concept work. The switchesremain conducting several hundred microseconds afterthey are triggered, even though no voltage is appliedacross the switch electrodes. Three Marx generators,

#3 #2 #1 rl IT-I ITvuLruLILSELINE PULSELINE

Figure 3. Three pulser schematic.

three ethylene glycol 40-ns pulselines, and threetriggered SF6 switches are used. In three pulse opera-tions, Marx #1 is erected, charging Pulseline #1; thispulseline is switched out through a transfer line tothe electron gun. The triggered SF6 switch is synchro-nized with the phase of the rf in the cavities and hasa subnanosecond jitter. Marx #2/Pulseline #2 operatesthe same as System I; the Pulseline #2 voltage isswitched through Pulseline #1, Switch #1, and thetransfer line. Marx #3/Pulseline #3 operates in thesame manner. The shunt load isolates the pulser fromthe electron gun. One-hundred-fifty-nanosecond pulselengths are achieved by replacing Switches #2 and #3with a 15-ns section of ethylene-glycol pulse line,erecting all three Marx's together, and switching thepulseline voltage out through Switch #1. The pulseoutput is connected through a 50-Q series resistor to a45-ns long, 60-Q-oil pulseline and then to the electrongun. The gun load is 1000-Q; the 50-0 resistor matchesthe gun reflections preventing the 45-ns line fromringing.

The electron gun is a Pierce geometry diode with athermionic 10-cm-diam barium-oxide-impreganted cathode.Two solenoidal focusing lenses transport the beam tothe first accelerating cavity. The beam is 2-cm radiusand slightly diverging at the cavity entrance. Thedivergence is necessary to compensate for the rffocusing of the entrance aperture.

Electron Beam Characteristics

The first cavity chops and accelerates the elec-tron beam. Because the electron mass increases six toseven times in the first cavity, the exit aperturedefocusing forces are small. The second and third cav-ities accelerate the chopped beam to 21 MeV at 300-Apeak current. The beam emittance after acceleration is1 II cm-mrad (unnormalized) at 21 MeV. The beam pulse-train current profile is shown in Fig. 4. The pulsesare 3.3 ns (FWHM) long with 1 ns rise time. This pulselength gives a 600 beam fill of the rf period. As aresult of the beam fill, the 8.6-ns electron transittime of a single cavity and the 20-ns period of the rfaccelerating field, the beam energy varies 30% duringthe pulse (see Fig. 5). In addition, the beam energyvaries 10% during the pulse train due to the rf energythat each pulse extracts from the cavities as eachpulse is accelerated.

A solenoidal lens at the exit of the third cavityrefocuses the beam to a 0.5-cm radius 2 m beyond thecavity exit. Two further lenses focus the beam to a0.05-cm radius at the x-ray converter. Steering

I-zw

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a

200k

400j

600-

U 60 120 180

TIME (ns)

Figure 4. Beam current pulse train.

240 300

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35

30

n 25_wzwz 20

W 15w

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01.11 2.22 3.33 4.44 5.55 6.66 7.77 8.88

TIME (ns)

Figure 5. Beam energy variation.

magnets correct for small misalignments in the beampipe. The minimum spot size achievable is primarilydetermined by the beam energy variation, rather thanthe beam emittance. For the last 10 cm of beam travelduring focusing, the beam transits a beryllium collima-tor with a 0.5-cm entrance radius and a 0.15-cm exitradius. The presence of this collimator increases thebeam focusing, although the exact mechanism is not wellunderstood. The focused beam then passes through a0.04-cm-thick-beryllium vacuum window and a 0.175-cm-thick-tungsten x-ray converter (-112 range) to a 10-cm-thick-beryllium beam stop. The 40-kA/cm focused beam-current density is sufficient to cause a damagingthermomechanical shock in the tungsten, so the targetis rotated for each 200-ns pulse train. The berylliumwindow survives several thousand pulses beforerequiring replacement. An aluminum Compton diode isused to monitor the flux density of the x-rays gener-ated by the converter.

References

1. D. Venable, D. 0. Dickman, J. N. Hardwick,E. D. Bush, Jr., R. W. Taylor, T. J. Boyd,J. R. Ruhe, E. J. Schneider, B. T. Rogers, andH. G. Worstell, "PHERMEX: A Pulsed High-EnergyRadiographic Machine Emitting X-Rays," Los AlamosScientific Laboratory Report LA-3241 (May 1967).

2. D. C. Moir, R. J. Faehl, B. S. Newberger,L. E. Thode, "Suitability of High-Current StandingWave Linac Technology for UltrarelativisticElectron Beam Propagation Experiment," Los AlamosNational Laboratory Report LA-8645-MS (January1981).

3. W. D. Kilpatrick, Rev. Sci. Instr. 28, 824 (1957).