WO1998033228A2

WO1998033228A2 - High-gradient insulator cavity mode filter

Info

Publication number: WO1998033228A2
Application number: PCT/US1998/000798
Authority: WO
Inventors: Georges James Caporaso; Stephen E. Sampayan; Clifford C. Shang; Glen Alan Westenskow
Original assignee: United States Department Of Energy; Houck, Timothy, Lee
Priority date: 1997-01-14
Filing date: 1998-01-14
Publication date: 1998-07-30
Also published as: WO1998033228A3; AU6132498A

Abstract

A laminated high gradient insulator (HGI) structure provides an improved acceleration gap insulator in the induction cavities of high power beam accelerators or amplification cavities of microwave devices. The HGI structure for the gap insulator is a hollow cylinder composed of multiple thin annular layers of dielectric material with a thin annular conductive layer between each. The maximum insulator characteristic, and especially resistance to surface flashover, is achieved when the applied electric field traverses perpendicular to the laminate structure. HGI's have demonstrated much greater (factors of 1.5 to 4 times) vacuum surface flashover capability than insulators made from a uniform dielectric. When disposed in an acceleration or amplification cavity, the HGI insulator also provides a frequency selective transverse beam-cavity interaction impedance that may be used to inhibit the generation of unwanted oscillations in the amplification cavity or preferentially enhance the generation of desired oscillations or modes in the amplifying cavity. The HGI furthermore can be bipolar in performance since its breakdown voltage and impedance characteristics are identical when a beam propagates in either direction in the beam tube.

Description

HIGH-GRADIENT INSULATOR CAVITY MODE FILTER

STATEMENT OF GOVERNMENT INTEREST The United States Government has rights in this invention pursuant

to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory.

CROSS-REFERENCE TO RELATED APPLICATIONS This Application is a Continuation-ln-Part of United States Patent

Application serial number 08/889,587, filed 7/8/97, for "HIGH GRADIENT INSULATOR" and Provisional United States Patent Application serial

number 60/035,463, filed 1/14/97, for "HIGH GRADIENT INSULATOR

CAVITY MODE FILTER. Ail such applications are incorporated herein by

reference.

BACKGROUND OF THE INVENTION The present invention relates in general to high-power devices and,

in particular, to insulators and mode filters for such high power devices.

The invention relates especially to induction accelerators and more

particularly to the use of an insulator structure having alternating layers of dielectric and metal in the accelerating gaps of the induction cell stages in

inductive accelerators.

Induction accelerators are a unique source for high-current, high-

brightness, charged particle beams. Induction accelerators are used to

drive very-high-power microwave sources such as free electron lasers and relativistic klystrons, as a driver for intense X-ray sources for radiographic

applications and as intense beam sources for material processing. Induction accelerators are also expensive, and the higher energy induction

accelerators have been limited to major national laboratories. For these

accelerators to become commercially viable, the basic unit of the accelerator, the induction module, must be made more compact, more

efficient and less costly.

A typical induction accelerator is described in United States Patent

4,730,166, entitled, "Electron Beam Accelerator with Magnetic Pulse

Compression and Accelerator Switching", and which patent is is

incorporated by reference herein. A typical induction accelerator includes serially arranged induction cells that each have a conductor shell around a

core of ferrite or other ferromagnetic material. When a pulsed voltage is

applied to such conductor, an equivalent voltage is developed across a

portion of the conductor shell at what is referred to as the accelertor gap.

Staging such accelerator cells along the trajectory of a charged particle

beam can impart an acceleration energy to the beam. A supporting

conductive structure is used to contain and support the ferromagnetic material. Only a part of the conductive structure functions as the actual

conductor around the ferromagnetic material.

Because of an adverse interaction that can occur between air

molecules at atmospheric pressure and the electron beam which is to be

accelerated, induction accelerators use sealed and evacuated beam pipes

that thread through ferrite torroids. The evacuated beam pipe allows an unimpeded propagation of the charged particle beam. In the induction cell,

the regions surrounding the ferrite cores are typically operated at high

electric fields, so present designs use a high dielectric strength fluid, e.g.,

oil to surround the ferrite. The oil acts like a self-healing insulator and enhances the resistance to electrical breakdown over the accelerator gaps. The dielectric fluid and the vacuum needed within the evacuated pipe are

typically separated at the accelerating gap by an alumina insulator.

It is well known that a propagating charged particle beam, particularly

one propagating at speeds close to that of the speed of light, will radiate significant amounts of electromagnetic energy. Cavities, and cavities

coupled through a neck, tube or gap will express certain preferential

electromagnetic modes and/or frequencies of oscillation, e.g., resonance.

These preferential modes or frequencies are readily calculated and can be

estimated form the dimension and shape of the system, the constituent

materials and the way the electromagnetic energy is initially introduced. Such energy can excite various electromagnetic modes and/or frequencies

in the beam pipe, in the accelerating gap in the induction cell, and in the ferrite regions. Resulting interactions may produce undesirable

electromagnetic modes and/or frequencies. Such interactions can adversely affect the controlled propagation of the beam through the

accelerator by causing the beam to oscillate transverse to the direction of

propagation. Such oscillations, referred to as beam break-up instabilities

(BBU), generally start as small effects at the beginning of the beam, but may be amplified in succeeding portions of the accelerator to reach significantly

high magnitudes.

The amount of growth of these transverse oscillations can be understood and characterized by well known techniques. A useful parameter in this analysis is the transverse interaction impedance, Z_^ of the cavity (i.e., the impedance normal to the direction of propagation of the

beam). If the transverse impedance is sufficiently large at a given electromagnetic mode and/or frequency, oscillations can continue to grow.

And if the transverse impedance is sufficiently small at a given

electromagnetic mode and/or frequency, oscillations will damp at a large

rate and become inconsequential. Thus it is desirable to design an accelerator cell with as low a transverse impedance as is possible over the

frequency range of the unwanted oscillations to reduce or minimize BBU.

In current induction accelerator designs, the transverse impedance is

sufficiently high at multiple frequencies that problematic oscillations can

result. The large transverse impedances at some of these frequencies are

known to result from the accelerator gap in the conductive shell of the beam pipe in the induction cell. Because the transverse impedance from the accelerator gap is inversely proportional to the square of the beam pipe

radius, a large beam pipe has been required to prevent BBU. However, if an amount of decoupling can be realized between the beam pipe and the

induction gap at the appropriate frequencies, the transverse impedance at

these frequencies can be reduced. This in turn will allow a smaller diameter

beam pipe to be used. This will facilitate a smaller and less costly overall accelerator.

Glass, ceramic, and other such materials are universally relied on as insulator materials in high voltage systems and in present accelerators. But

such materials allow the insulators to be damaged by avalanche and

flashovers that occur when the insulator has been subjected to a voltage

over-stressing. Fine cracks can develop that lower the insulator's

breakdown voltage to successive exposures to stress voltages. Conventional bulk material insulators also tend to be very large, and the systems that incorporate them must necessarily provide enough room to

accommodate them. If a compact insulator having a high breakdown

voltage can be utiltized, the size and cost of the accelerator can be reduced.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention is to provide a more

compact, less costly and more efficient induction accelerator.

Another object of the present invention is to provide an improved

induction module for induction accelerators. A further object of the present invention is to provide a more compact induction module.

Another object of the present invention is to enable the design of an

induction accelerator having a beam pipe of reduced diameter.

Another object of the present invention is to provide an induction

module having an accelerating gap insulator having an improved vacuum surface flashover capability, especially in the presence of a cathode and an electron beam.

A further object of the present invention is to provide an accelerating gap insulator having lower field stress on the vacuum surfaces.

Yet another object of the present invention is to provide an

accelerating gap presenting reduced (low) transverse interaction impedance at beam breakup frequencies to minimize beam breakup instabilities.

A still further object of the present invention is to provide a gap

insulating structure presenting reduced (low) transverse interaction

impedance to minimize beam breakup instabilities.

Still another object is of the present invention is to provide a gap

insulator having reduced (low) longitudinal impedance at the beam

modulation frequency and harmonics to minimize power loss. Still another object is to control beam-structure interaction

impedance's in cavities for microwave sources.

Briefly, these and other objects are provided by employing a

laminated high gradient insulator (HGI) structure as the gap insulator in the induction cells of the induction accelerator. The HGI structure for the gap

insulator is a hollow cylinder composed of multiple thin annular layers of dielectric with a thin annular conductive layer between each. The maximum insulator characteristic, and especially resistance to surface flashover, is

achieved when the applied electric field traverses perpendicular to the laminate structure. HGI's have demonstrated much greater (factors of 1.5 to

4 times) vacuum surface flashover capability than insulators made from a uniform dielectric. The HGI insulator also provides a transverse interaction

impedance at the accelerator gap that inhibits beam instabilities. The HGI

furthermore can be bipolar in performance since its breakdown voltage and impedance characteristics are identical when the beam propagates in either

direction in the beam tube.

The above objects and other objects, advantages, and features of the present invention will become apparent from the following detailed

description of the invention when considered in conjunction with the

accompanying drawings wherein like reference characters represent like or

similar parts throughout the several views and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic partial cross-sectional view (taken along the

longitudinal center line of the accelerator) illustrating an accelerator

induction cell having a high gradient insulator disposed in the acceleration

gap according to the present invention; Fig. 2 is a schematic of an experimental test cavity which represents a simplified induction module which experimentally illustrated the operation

of the HGI as the gap insulator in an induction module;

Fig. 3A and 3B are plots of the transverse impedance versus

frequency of the experimental structure of Fig. 2 including measurements

with a solid insulator and with several variations in the HGI insulator-to- metal ratio;

Fig. 4A, B and 4C are a plots of the transverse impedance versus

frequency resulting from a computer simulation of the operation of the

experimental structure of Fig. 2;

Fig. 4D is a schematic drawing of the a portion of the module design

of Fig. 1 illustrating the equipotential lines in the vicinity of the module gap;

Fig. 5 is a partially cut away exploded assembly view of a hollow

cylindrical HGI made of a fused stack of metalized flat annular dielectric rings suitable for use in the accelerator gap;

Figs. 6A and 6B illustrate an alternate non laminate structure for an

HGI embodiment; and

Figs. 7A, 7B and 7C illustrate alternative gap designs and gap

insulator designs using the HGI structure in prior art accelerator module designs.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and in particular to Fig. 1 , which

shows a preferred embodiment of an induction module or induction cell 10 according to the present invention, is illustrated. The induction cell 10 is symmetrical with respect to the longitudinal centerline 12. Only the half of

the cell structure above the centerline 12 is shown to simplify the drawing as is conventional in the art. The illustrated embodiment is designed for the

Relativistic-Klystron Two-Beam Accelerator program of the U.S. Department of Energy at Lawrence Livermore National Laboratory and the Lawrence

Berkeley National Laboratory.

The induction cell 10 has a supporting cylindrical conductive shell formed by a hollow cylinder 14 forming the outer edge of the induction cell,

annular end plates 16 and 18, and interior annular magnet housing

members 20 and 22. The inner walls of the annular magnet housing members 20 and 22 form the surface of the beam pipe. The interior annular

magnet housing members 20 and 22 have similar annular channels in which

two annular low-field permanent magnet quadruples 23 and 24, respectively, are disposed.

An annular accelerator gap 26 in the wall of the beam pipe is

provided between magnet housing members 20 and 22. In operation, a

pulsed high voltage accelerating electric field is applied across the gap 26.

A HGI 28 is disposed across the gap 26. The HGI 28 incorporates larger

conductors 30 and 32 on each end that are used for mounting the insulator

in the gap 26. These two conductors are formed with a 0.5 cm radius of

curvature and a 90 degree bend on the inner radius and welded to the beam

line at a point of low surface electric field stress to create a vacuum seal. On their outer radius, the conductors 30 and 32 have a 180 degree bend terminating in an electrical slip connection that rides on the magnet housing

members 20 and 22.

Five annular induction cores 34 of ferromagnetic material (Metglas in

the illustrated embodiment) are disposed within the supporting conductive shell. The induction cores are maintained away from the supporting

conductive shell by insulating supports 36 to allow the cores to be surrounded by high strength dielectric insulating fluid (e.g., oil) in the

channels 38 between the supporting structure including the HGI 28 and the

induction cores 34. An annular microwave absorber 40 is disposed behind

the accelerator gap 26 behind the HGI 28.

The operation of the structure of Fig. 1 is illustrated by the

experimental test cavity shown in Fig. 2. The insulators were comprised of

thin disks (0.05 mm) of polycarbonate or stainless steel. Figs. 3A and 3B

show experimental impedance measurements of the test cavity with an inner

diameter of 14.6 cm and an outside diameter of 19.7 cm and different ratios

of dielectric to conducting material. Fig. 3A illustrates the impedance

spectrum of the cavity for different dielectric and conductor ratios. Fig. 3B

illustrates the effect on the primary resonance due to varying the ratio of dielectric to conducting material. The smaller peaks flanking the primary

resonance at 1.35 GHz are related to the geometry of the transmission line

and are not specific to the test cavity. Addition of microwave absorbers at

the ends of transmission line pipe reduced these impedance's while not affecting the primary 350 MHz resonance's. The TE/TM cutoff frequencies

for traveling wave modes in the transmission pipe are 1.2/1.57 GHz,

respectively. The 350 MHz resonance is not effected by the structure of the

insulator. The distribution of the layers was varied to produce different

ratios of insulator to conductor and different number of alternating layers, or

periods, for a specific ratio. The number of layers , or periods of dielectric (polycarbonate) were 9 for the 2:1 ratio, 7 for the 1 :1 ratio and 9 for the 1 :2

ratio.

With a solid insulator disposed in the gap, the test structure shows a

large transverse impedance at approximately 1.35 GHz. Insertion of a HGI structure, where the conductive film is a thin metallic plate of equal thickness to the dielectric, affords a reduction in this impedance by a factor

of 3 to 4. A similar effect was observed in extensive computer simulation as shown in Figs. 4A, 4B and 4C wherein Fig 4A illustrates the calculated

impedance spectrum of the cavity and Fig. 4B illustrates effect on the

primary resonance. The effect of increasing the number of periods while

maintaining the same ratio (2:1 )of dielectric to conductor is shown in Fig. 4C

which illustrates the convergence of the impedance spectrum with increased

number of layers, or periods of insulating material. The impedance converged rapidly with increasing number of periods. Thus it is observed in

computer simulation and also experimentally observed that insertion of the

HGI into the test accelerator cell benefits the operation of the accelerator cell. The Poisson simulation was used to determine electric field stresses

in the gap 26. An equal potential plot for the gap design is shown in Fig.

4D. The electric field across the insulator in a uniform 100kV/cm. On the oil side the of the insulator the field is 135kV/cm and at the vacuum entrance to

the gap 26 the highest field is 120kV/cm.

In comparison with the solid dielectric gap insulator of the prior art,

the use of the improved high voltage holdoff of the HGI should allow the use of a shorter accelerating gap or, alternatively, a higher voltage gradient

while the lower frequency-selective impedance and mode suppression

should allow the use of smaller diameter beam tubes for a particular accelerator design. The frequency sensitive transverse impedance of the accelerator module with the HGI insulator suggests that the HGI structure

may be used to enhance beam propagation or inhibit unwanted beam

oscillations in accelerator or amplification devices in general through

appropriate control of the transverse impedance in the amplifying cavity.

Figs. 7A-7C illustrate three prior art accelerator gap designs that may

be enhanced by the use of the HGI. Each of the designs utilizes an

insulator that has its vacuum side angled relative to the accelerator beam line to maximize the breakdown electric field. In addition, the gap in each design is curved or angled to offset the gap insulator from a straight line

path to the beam pipe. The HGI could be substituted for each of these

insulators and could include the curved vacuum facing surfaces utilized in

Figs. 7B and 7C. This should allow each of the design to be more compact while providing equivalent high voltage holdoff and the frequency-selective transverse impedance provided by the HGI. Of course the HGI could be

angled in the design of Fig. 1 to increase the breakdown voltage but this

would also eliminate the bi-directional capability of the HGI when oriented parallel to the beam line, in Fig. 7A-7C the HGI is shown with the interfaces

of the parallel layers being oriented at an angle to the beam line; however,

the HGI may be fabricated to allow the surface of the insulator to be

oriented at an angle to the beam line but the interfaces of the parallel layer oriented normal to the beam line as shown at 28c in Fig. 7C.

Design and Fabrication of a High Gradient Insulator

The partially cut-away exploded assembly view of the HGI of Fig. 5 illustrates an HGI used to fill an accelerator gap 26. An HGI 28 preferably comprises a fused stack of insulator layers, represented in part by layers

100, 102 and 104 having a period that does not exceed non millimeter. For

example, the fused stack of insulator layers 100, 102, 104 can be fabricated

from bulk dielectric materials such as fused silica, quartz glass, alumina,

ceramic, sapphire, etc. Given enough numbers to the stack, any

longitudinal length may be constructed. A series of conductive layers,

represented in part by layers 106, 108 and 110, are disposed between the

insulator layers 100, 102, 104. The structure is equivalent to a capacitive

voltage divider, and may even retain a charge that is divided among the

many layers from the cathode to the anode. The conductive layers may be

fabricated as separate metal foils or plates, or as one or more evaporated metals diffused into the respective surfaces of the insulator layers.

The conductive layers are used during fabrication to hard seal

insulator layers one to another. Pressure and heat welding, diffusion

bonding, brazing and soldering techniques may be used. The reverse should also be possible, e.g., fabricating flat metal conductor rings with oxides or other dielectric materials on their mating surfaces, and then

chemically bonding the metal oxides or other dielectric materials together.

For HGI structures fused by metal-to-metal bonding, each layer may

comprise a eutectic two part alloy with corresponding constituents that are deposited or diffused into respective faces of the insulator layers. Each conductive layer may alternatively comprise a multi-layer nanostructure foil

initiator that is ignited to bond metals that are already deposited or diffused

into respective faces of the insulator layers.

The maximum insulator characteristic, and especially resistance to surface flashover, is achieved when the applied electric field traverses

perpendicular to the laminate structure. Since the metalizations on each stacked dielectric ring run through from the inside to the outside, electric fields, and especially direct current, applied in line with the laminate

structure will find a low impedance metal conductor through the bulk of the

HGI 28.

In the radial direction, energetic radiation, especially microwave

energy, is low-pass filtered when it passes between the inside and outside

of HGI 28. At very high frequencies, the inductive reactances of the individual radial paths on the ring conductors becomes significant. Similarly, the

capacitor formed by each pair of ring conductors and intervening dielectric

exhibits significant capacitive reactance. Together, the electrical equivalent is a pi-filter with parallel capacitor inputs and outputs and a series

inductance.

Fig. 6A and 6B illustrate an alternative embodiment of an HGI 28a which does not employ a laminated structure. A dielectric wall segment

120 has both inside parallel metal rings 122 and outside parallel metal rings

124 that are, in at least one embodiment, flush at their tops to the respective surrounding dielectric material surfaces. Therefore, unlike the metal rings of

HGI 28, the metal rings of the HGI 28a do not extend all the way through.

The vacuum integrity HGI 28a would be easier to maintain, but electrical currents through the walls would not be supported. In alternative

embodiments, the depth of penetration of rings 122 and 124 into the dielectric wall 120, and/or their extension from the surface, may be adjusted

to suit particular applications.

In both HGI 28 and HGI 28a, it may be advantageous to apply to the

inner and/or outer surfaces a field emission suppression coating, a

secondary suppression material, or a thin resistive, e.g., semi-conductive,

coating. Conventional deposition means and materials may be used.

Therefore, semiconductor fabrication methods can be used to form systems

of electrically isolated rings on the inside and outside surfaces of a hollow cylinder of dielectric. Such rings preferably have a spacing period of 100- 1000 nm, and so the width of each conductor on the surface needs to be

under 50 nm. The depth of penetration needs to be no more than 1000 nm

into the surface of the dielectric. The conductive layers are used during fabrication to hard seal

insulator layers one to the another. Pressure and heat welding, diffusion bonding, brazing, and soldering techniques may be used. The reverse

should be possible too, e.g., fabricating flat metal conductor rings with oxides or other dielectric materials on their mating surfaces, and then

chemically bonding the metal oxides or other dielectric materials together.

For HGI structures fused by metal-to-metal bonding, each conductive layer may comprise a eutectic two-part alloy with corresponding constituents that

are deposited or diffused into respective faces of the insulator layers. Each

conductive layer may alternatively comprise a multi-layer nanostructure foil

initiator that is ignited to bond metals that are already deposited or diffused into respective faces of the insulator layers.

Although particular embodiments of the present invention have been

described and illustrated, such is not intended to limit the invention.

Modifications and changes will no doubt become apparent to those skilled in

the art, and it is intended that the invention only be limited by the scope of

the appended claims.

Claims

THE INVENTION CLAIMED IS

1. In a power amplification device of the type wherein a high energy beam is amplified in an amplification cavity having an acceleration gap

across which a pulsed voltage is applied amplify the beam, the improvement

being a high gradient insulator structure disposed across the

acceleration gap, said high gradient insulator structure comprising a

laminated structure of thin disks having alternating layers of dielectric material and conducting material.

2. The improvement of Claim 1 wherein the ratio of the dielectric

material to conductive material is selected to provide a low transverse

cavity-beam impedance to selectively suppress unwanted cavity

oscillations.

3. The improvement of Claim 1 wherein the ration of the dielectric

material to conductive material is selected to provide a transverse cavity

beam impedance to selectively enhance cavity oscillations.

4. In an induction module of the type wherein an electron beam propagating in a beam pipe and a pulsed high voltage is applied across an annular accelerating gap is within the module, the improvement being

an annular high gradient insulating structure disposed in said annular

gap, said high gradient insulator structure comprising a laminated structure

of thin annular disks having alternating layers of dielectric material and conducting material.

5. The improvement of Claim 4 wherein the ratio of the dielectric material to conductive material is selected to provide a low transverse

cavity-beam impedance to selectively suppress unwanted cavity

oscillations.

6. The improvement of Claim 4 wherein the ration of the dielectric

material to conductive material is selected to provide a transverse cavity beam impedance to selectively enhance cavity oscillations.

7. The improvement of Claim 4 wherein said high gradient insulator

is disposed with its interior surface oriented parallel to beam pipe and with

surfaces of the annular layers oriented normal to the direction of the beam pipe, the ration of the dielectric material to conductive material is selected

to provide a transverse cavity beam impedance to selectively enhance

cavity oscillations.

8. The improvement of Claim 4 wherein said high gradient

insulator is disposed with its interior surface oriented at an oblique angle

with respect to the beam pipe.