US3379880A - Device for forming replica images of particle distributions in a plasma stream - Google Patents

Description

A nl 23, 1968 F. H. CCENSGEN ET AL 3,379,880

DEVICE FOR FORMING REPLICA IMAGES OF PARTICLE DISTRIBUTIONS IN A PLASMA STREAM Filed Feb. 24, 1965 PULSE GENERATOR VOLTAGE SOURCE INVENTORS FREDER/C H. COENSGEN BY WILLIAM F CUM/WINS W/L L MM 5. NEXSEN JR.

ATTORNEY United States Patent DEVICE FOR FORMING REPLIQA IMAGES OF PARTICLE DISTRIBUTIONS IN A PLASMA STREAM Frederic H. Coensgen, Pleasanton, and William F. Cummins and William E. Nexsen, .lra, Liver-more, Calif., assignors to the United States of America as represented by the United States Atomic Energy (Iornmission Filed Feb. 24, 1965, Ser. No. 435,161 5 Claims. (Cl. 258-715) ABSTRACT OF THE DISCLQSURE Device for forming visible replica images indicative of instantaneous particle distributions in a plasma stream, including a semi-transmissive element for attenuating the plasma stream, and having an electrically polarized grid arranged to repel undesired charged particles from the attenuated stream and converting selected particles impinging thereon into a replica stream of electrons emerging therefrom. A scintillator screen is arranged to intercept portions of the replica electron stream, which are accelerated through light excluding means by means of an electrostatic field applied between said grid and scintillator to provide said replica image.

The present invention relates generally to devices for observing nuclear particles and more particularly to a device for forming visible replica images indicative of the spatial distribution of streaming nuclear particles termed a camera hereinafter, for simplicity.

As more is learned about the behavior of nuclear particles, the potential benefits derivable from atomic energy is greatly increased. One of the characteristics of nuclear particles not completely understood is particle interaction in the presence of electromagnetic fields, for example, encountered in the generation and utilization of nuclear particle beams and magnetically confined high temperature plasmas. In confining plasmas, various types of instability problems have been encountered which to a large extent have prevented the full realization of the potential of high temperature plasmas. As is well known, most of the instabilities result in the extremely rapid movement of the plasma across the magnetic confinin field to eventually encounter the container walls and become destroyed. By observing the spatial distribution of the plasma particles and the rapid movement of the plasma, the characteristics of the various types of instabilities can be investigated and studied. In the present art, there is usually insufficient radiation from the plasma for conventional time-resolved photographic techniques such as those using a Kerr cell as a shutter. Furthermore, even in cases where the radiation is sufiicient, interpretation of the data often is complicated by intense radiation from sources other than the plasma under observation. Another difiiculty is encountered in pulse operated plasma systems and unstable plasmas where investigations are complicated in that the plasma distribution is not reproducible. In such a case, a composite picture built up of data from several operations gives only a statistical average.

The present invention operates independently of the light or other radiations from the plasma and provides a time resolved image whose intensity is a function of the instantaneous plasma flux being viewed. As a brief description, the invention approaches the solution to the problems set forth supra in the following manner. A grid placed in the path of a stream of nuclear particles is selectively biased either positively to repel positively ice charged particles and pass negatively charged particles or neutrals, or biased negatively to repel negatively charged particles and pass positively charged particles and neutrals. Where particles other than electrons are to be observed, it is preferable to convert them to secondary electrons. These electrons, either secondary or primary, are distributed in facsimile to the particle stream cross section. By electrostatically accelerating these electrons for example, through a light-impervious, highenergy-electron pervious sheet to impinge on an energeticparticle-to-light converter, e.g., plastic scintillator, an image is produced which is a replica of the stream crosssection. An ordinary light-sensitive camera can then be used to record the image. The light-impervious, highenergy-electron pervious sheet serves to eliminate the effects of any accompanying undesirable radiation which would otherwise make it diificult if not impossible to obtain and record the image of the distribution of the particles of the nuclear particle stream.

The camera of the present invention further includes the feature of an extremely fast electronic shutter capable of shutter speeds in the nanosecond time range. The electronic shuttering action is obtained by controlling the electrostatic acceleration of the electrons which are directed to impinge on the energetic-particle-to-light converter. As is known, such conversion requires that the impinging particles have an energy above a minimum threshold level. Rapid shuttering is obtained by maintaining the electrostatic accelerating field at a level below that required to impart sufficient energy to the electrons to initiate the conversion, and at a predetermined time increase the accelerating field for a selected period to a level whereby the electrons are accelerated above the energy threshold level necessary for initiating the conversion. As a consequence of the rapid shutter characteristic of the present camera, an image representative of a substantially instantaneous distribution of selected particles of a nuclear particle stream can be obtained.

Accordingly, it is an object of the present invention to provide a device for observing the cross-sectional dis tribution of particles in a stream of nuclear particles.

A further object of the present invention is to provide a replica of the distribution of particles in a plasma stream, which replica is capable of being photographed.

Further, it is an object of the present invention to permit the application of time resolved photographic techniques to the study of plasmas.

Still another object of the present invention is to obtain plasma stream cross-sectional particle distribution data which is not complicated by radiation from sources other than the plasma stream under observation.

Another object is to provide a time resolved image whose intensity is a function of the instantaneous particle flux of the plasma being viewed.

It is yet another object of the present invention to provide a time resolved replica of particle distribution in a plasma stream.

More particularly, it is an object of this invention to capture a replica of an instantaneous cross-section of a plasma stream whose cross-section varies with time and is not reproducible.

The means of achieving these and other objects will become more apparent from the following detailed description of a preferred embodiment of the present invention, taken in conjunction with the accompanying drawing which is a cross-sectional elevation view of one preferred embodiment of the present invention.

The plasma camera, i.e., image-forming device, of the present invention is employed to selectively observe the spatial distribution of any of the various nuclear particles, e.g., positive and negative ions, electrons, or neutral particles, that may be found in a stream of nuclear particles. The camera obtains an image of the spatial distribution of the nuclear particles of a particle stream by linearly converting the energy of selected particles to light without altering the relative positions of the selected particles in the beam. The light thus generated is an indication of the spatial distribution of the selected particles of the stream, and by observing or recording this light, the particle distribution of the stream can be analyzed.

To accomplish this observation, the plasma camera of the present invention takes advantage of the coaction of the three principal sections comprising the camera: A particle selector, an image generator, and an image recorder. The particle selector is positioned to receive a stream of nuclear particles and functions to selectively extract therefrom the particles to be analyzed. In its simpliest form, the particle selector accelerates and trans mits selected particles from the particle stream to the image generator without altering the particles positional relationship while simultaneously impeding the transmission of any accompanying undesired particles. However, in the analysis of high density particle streams, it is most beneficial to attenuate the density of the particle stream prior to generating an image reproduction of the selectively transmitted particles. As will be set forth in more detail hereinafter, this can be accomplished by positioning a semi-pervious particle shield in front of the surface of the image selector facing the impinging particle stream.

The selected particles are accelerated and directed to impinge on an energelic-particle-to-light converter, i.e., image generator, wherein light is emitted therefrom in proportionate relation to the number of impinging particles to form an image of the cross-sectional distribution of the particles in the nuclear particles stream. In those cases where the plasma camera is employed to observe selected particles in the presence of extensive background light, e.g., particles originating from a plasma, the image generator may be shielded from the undesired light by means of a suitable particle-pervious-light-impervious shield. The light emitted by the image converter is an accurate representation of the spatial distribution of the selected particles in the particle stream and may permanently be recorded by the simplest of light-sensitive cameras.

As set forth hereinbefore, the present camera includes the feature of an extremely fast electronic shutter. Hence, in those cases Where it is desired to observe all types of particles of a particle stream including, for example, both positively and negatively charged particles, the particle selector could be synchronously operated with the shutter to obtain sequential observations of the various types of particles. More specifically, the particle selector would be operated to pass first one type of particles than another. Simultaneously, the accelerating electrostatic field would be adjusted to impart sufficient energy to the selected particles whereby upon impingement upon the image generator light is produced.

Although the plasma camera may talre the form of various embodiments in accordance with the present invention, the specific embodiment contemplated as best carrying out the invention is set forth in the immediately following description and accompanying drawing. Such specific embodiment may be employed, for example, by being coupled at one end of an accelerator, fusion device such as a magnetic mirror machine, etc., which is capable of delivering a charged particle or plasma beam.

In the figure, camera 11 is adapted to operate in an evacuated atmosphere and is comprised of an electrically conducting particle semi-transmissive plate 12 capping a first end 13 of an electrically conductive housing 14. End 13 of housing 14 is adapted to allow camera 11 to be mounted hermetically to a particle stream source (not shown). A plastic scintillator sheet 16 or other suitable euergetic-particle-to-light conversion means is mounted on transparent backing plate 17 which in turn is secured to a mounting ring 18 secured within housing 14 spaced apart substantially parallel to plate 12. A light-impervious high-energy-electron-pervious element 19 is disposed between scintillator 16 and plate 12. This may be a two thousand Angstrom aluminum coating insulatingly disposed on the surface 21 of scintillator 16 facing plate 12. A grid 22 having a surface which is electron emissivcly responsive to impinging particles, is mounted by stand-off insulators 23 between, parallel to, and insulatingly apart from scintillator 16 and plate 12. The second end 24 of housing 14 is provided with mounting means 26 to receive appropriate light recording means, e.g., a conventional light sensitive camera (not shown). Plate 12 and housing 14 are electrically grounded and grid 22 is connected via wire 27 to a variable voltage source 28 having a range, e.g. of $300 volts. Source 28 is referenced to housing 14 via wire 29. Metal film 19 is electrically connected via conductor 31 to a high voltage pulse generator 32 which is referenced to housing 14 via wire 33.

in practice, inch Lucite has been used as the transparent backing plate 17, Lucite being an acrylic resin, e.g., polymethyl methylacrylate, manufactured by E. I. du Pont de Nemours and Company. The scintillator can be polystyrene containing terphenyl or any of the other numerous scintillators used in high energy physics research.

The operation of camera 11 will be described with reference to its use to observe particles from a high temperature magnetically confined plasma which includes both positively and negatively charged particles. A stream of plasma particles 34 originating from a high temperature plasma moving in an evacuated atmosphere is directed to impinge on semi-transmissive plate 12. This plate 12, comprised of a stack of three TV color masks, passes approximately one-thousandth of the plasma into the chamber between grid 22 and plate 12. Although quantitative figures are unavailable, it has been observed in practice that voltage breakdown will occur between grid 22 and sheet 19 if the plasma stream density is not reduced utilizing a means such as plate 12. With a negative voltage on grid 22, the electrons in the plasma are repelled and the ions are accelerated to impinge on the electron emission grid 22. Those ions striking the grid 22 release secondary electrons. With a positive voltage on grid 22 the ions in the plasma are repelled and the secondary electrons therein are accelerated through the grid 22. In either case, electrons appearing at grid 22 are distributed in replica to the cross-sectional distribution of the plasma stream particles of the polarity which are attracted to the grid.

A positive high voltage pulse, e.g., 20 kv. applied for 0.2-0.4 microsecond, is imposed on film 19 to accelerate those electrons which appear at grid 22 towards film 19. Note that this film 19 serves as a light shield and functions as a camera shutter which opens when subjected to the high voltage positive pulse. This high voltage pulse imparts sufficient energy to some of these electrons emerging from grid 22 so that they are able to penetrate metal film 19 and produce point source fluorescence in scintillator 16. In preferred practice, the image produced on the scintillator 16 may then be recorded by ordinary photographic techniques or may be sensed by other appropriate light sensing means, e.g., an array of photocells. The most important single feature of this invention is the fact that this image has a cross section which is substantially identical to the spatial distribution of those electrons or ions flowing in that portion of the plasma stream 34 which is viewed by camera 11 a moment before the high voltage pulse is applied to plate 19. It is to be noted that particle stream cross section size that can be viewed by camera 11 is limited only by the effective size of camera 11. The effective size of camera 11 is determined by diameter of unimpeded path from plate 12 to the camera attached at the second end 24 of housing 14.

Although to date use of the camera 11 of the present invention has not been extended beyond observation of streams of charged particles, it conceivably may be used to observe streams of uncharged particles, provided a significant number of the particles possess energy sufiicient to cause secondary emission of electrons from grid 22. In such a case, grid 22 may be well placed at ground potential, there being no requirement to select particles of a particular polarity. However, if streams of charged particles are to be analyzed which include positively and negatively charged and neutral particles, at second grid having a mesh selected to allow transmission of all particles would be interposed between grid 22 and plate 12. This second grid would serve as a first stage of selectivity with grid 22 serving as a second stage of selection and electron generation. For example, to observe neutral particles from a stream including charged particles, the first grid would be charged negatively to repel the negatively charged particles, and grid 22 would be charged positively to repel the positively charged particles and allow only neutrals to impinge grid 22 and produce second-ary electrons.

In the figure, there are shown six circular metallic field aligning rings 36 insulated apart from each other and from grid 22. In practice, these rings 36 serve merely to provide a uniform electric field across the face of the scintillator 16 to prevent image distortion. It is felt that use of these rings 36 is well within the skill of the art and their use may be desirable where beam spreading problems are likely to be encountered.

As an aid to those who wish to practice the present invention, there is provided below a table of relevant design values:

Outside diameter of housing 14 12 inches.

Effective camera diameter 9.5 inches.

Front plate 12 Stack of 3 TV color masks.

Front plate 12 transmission 0.72

Grid22 58% electron trans mission. 52-mesh stainless steel.

Number of field aligning rings 36 Field aligning ring 35 separation 0.25 inch.

Front plate 12 to grid 22 separation 1 inch.

Grid 22 to scintillator 16 separation Do.

Depth of metallic coating material 19 2000 angstroms.

Grid 22 bias 300-+300 volts.

High voltage potential at accelerator electrode 19 20 kv. (approximately).

Optical camera S p e e d-Graphic Pacemaker with i n c h f5.6

Wollensak lens a n d Polaroid film back.

As this plasma camera has been used to date, the incident plasma density has not exceeded 5 X 10 ions/cm. With these ion densities, three seventeen-inch TV masks with 0.01-inch holes, obtained from Buckbee Mears, Inc., St. Paul, Minn., have been found to work Well as the attenuating front plate 12.

It may be noted that the term plasma stream as used in this application includes any stream of charged particles and is not necessarily limited to those situations where there is a net zero charge, i.e., the positive charges equal in number to the negative charges.

Although the invention has been described above in the spirit and scope of the invention. For example, by providing an appropriate particle-transmissive electrode in the position now occupied by the semitransmissive plate 12, it may be possible to eliminate that plate and also the above-mentioned camera housing 14. Also, should there be no troublesome light effects accompanying the particle beam, it may be possible to omit the light impervious metallic plate 19 from the surface of the scintillator 16. As a further modification, it may be considered that means other than a light sensitive camera may be used to sense and/or record the light image appearing on the above-mentioned scintillator 16. Furthermore, should the plasma stream contain particles of a single polarity, it may be unnecessary to provide the above-described grid means 22 to electrostatically select and reject selected particles.

Hence, the scope of the present invention is intended to be limited only by the terms of the appended claims.

What is claimed is:

1. Device for forming visible image replicas indicative of the spatial distribution of energetic particles in a plasma stream in an evacuated region, comprising, in combination:

(a) an elongated double-ended electrically conductive tubular housing in communication with said evacuated region, and arranged for entry of said plasma stream at one end for passage longitudinally therethrough;

(b) perforate semi-transmissive, electrically conductive plate means disposed across said first end of said housing to uniformly reduce and attenuate the particle density of said plasma stream passing therethrough;

(c) a grid disposed in insulated, spaced relation in said housing transversely across said attenuated plasma particle stream;

(d) a variable bipolar voltage source electrically connected between said plate and said grid for applying an electrostatic potential therebetween to repel particles of undesired polarity and to select and accelerate particles of the desired polarity from said attenuated plasma particle stream to impinge upon one side and cause emission of a corresponding replica stream of secondary electrons from the second side of said grid;

(e) sheet phosphor means disposed in spaced relation to the second side of said grid in said housing;

(f) second electrode means disposed across said tubular housing between said grid means and phosphor means, said second electrode means being pervious to high energy electrons and impervious to light; and

(g) voltage supply means connected between said grid and said second electrode means to apply an electrical pulse thereto for accelerating said secondary electrons to a sutficiently high energy level to penetrate said second electrode means and impinge upon said phosphor means.

2. Apparatus according to claim 1 for sampling high density plasma particle beams, further defined in that said first pervious electron means has a porosity sufiicient to attenuate said beam to less than about one one-thousandth of the original density thereof.

3. Apparatus according to claim 1, further defined in that voltage supply means is a voltage pulse generator for impressing upon said grid and secondary electrode means a voltage pulse of fractional microsecond duration to accelerate said secondary electrons during a preselected time interval exclusively.

4. Apparatus according to claim 3, wherein said phosphor means comprises a sheet scintillator plastic material, said second electrode means is a thin metallic film deposited on the first side of said sheet scintillator material, and a transparent backing plate is disposed at the second side of said sheet scintillator material in capping relation to said housing.

7 8 5. Apparatus according to claim 4, wherein said per- 3,002,101 9/1961 Anderson et a1. 250-213 forate semitransmissive plate means comprises three 3,012,149 12/1961 Heimann et a1. 250-213 superposed color television masks with 0.01-inch holes. 3,107,303 10/1963 Berkowitz 250-213 3,121,796 2/1964 Reed 250-213 References Cited 5 UNITED STATES PATENTS RALPH G. NILSON, Primary Examiner. 2 421,132 5 1947 Bay e 2502.213 ARCHIE R. BORCHELT, Examiner. 2,572,494 10/1951 Krieger et a1. 250-213 AU Assistant 2,903,596 9/1959 Reed 250-213

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