US3371490A - Magnetodynamic plasma accelerator device - Google Patents

Description

March 5, 1968 R. HASLUND MAGNETODYNAMIC PLASMA ACCELERATOR DEVICE 2 Sheets-Sheet 1 Filed Nov. 17, 1966 6705 DIS/f 0 RAD/Z p/m'r/o/y MIL/01? 6 #410 RAD/AL g 1940/05 ammo/v rgg/ sgggfiE INVENTOR. RALPH L. HASLZ/A D ,4 7 TOR/YE V R. L. HASLUND MAGNETODYNAMIC PLASMA ACCELERATOR DEVICE Filed Nov. 1'7, 1966 March 5, 1968 2 Sheets-Sheet 2 INVENTOR. 6741. 1. HAS'Ll/IYD United States Patent 7 3,371,490 MAGNETODYNAMIC PLASMA ACCELERATOR DEVICE Ralph L. Haslund, Mercer Island, Wash., assignor to The Boeing Company, Seattle, Wash., a-corporation of Delaware Continuation-impart of application Ser. No. 283,867, May 28, 1963. This application Nov. 17, 1966, Ser. No. 595,239

3 Claims. (Cl. 60-202) ABSTRACT OF THE DISCLOSURE A pulsed induction plasma accelerator is described wherein separate means are used to form the plasma and to impart an accelerating force to it. The structural means defining a gas ionization chamber having an axis of symmetry has a plurality of circular bands through which a time varying primary current is passed. The primary current induces an electric field which causes the ionization of a neutral gas contained within the ionization chamber, changing that gas to acurrent conducting plasma. Due to the interaction of the plasma current with the primary magnetic field, with its self magnetic field, and with its self-induced electric field, the plasma contracts about the axis of symmetry and is finally accelerated out of the chamber under the influence of a separately energized accelerationcoil. The acceleration coil is positioned such that when energized, it generates a planar radial magnetic field in the vicinity of the contracted plasma and eX- tending radially outward from the axis of symmetry of the ionization chamber.

This invention relates to accelerators, and more particularly to pulsed induction plasma accelerators.

This application is a continuation-in-part of U.S. patent application Serial No. 283,867, now abandoned, filed by Ralph L. Haslund on May 28, 1963, for Magnetodynamic Plasma Accelerator Device.

Much importance has been attributed in recent years to the urgent need for a propulsion engine capable of high-thrust density, high-specific-impulse operation for extended periods of time as required for deep journeys into space. The propulsion engine needed has been one of comparatively light weight, lending itself equally well to the maneuver and attitude control of satellites and to prime propulsion of interplanetary space-ship probes, both manned and unmanned, as required by certain missions. A reliable propulsion engine capable of efiicient operation using comparatively small quantities of propellant in a high vacuum environment has been a prerequisite to the final conquest of space.

Previous efforts to solvethe foregoing problems have resulted in very limited success, essentially due to the inherent limitations of the engine types themselves. Electrothermal engines, e.g., arc jet engines, are specific impulse limited due to operating temperatures and propellant atomic weight, and are reliability and lifetime limited due to heat exchanger, electrode and nozzle erosion. Electrostatic engines, e.g., ion engines, are thrust density (thrust per unit area) limited by reliably obtainable propellant charge to mass ratios for the electric field strengths giving the specific impulse required for space missions and are reliability limited due to electrode wear, ionsource lifetime and neutral efiiux. Electromagnetic engines, e.g., magnetohydrodynamic (MHD) or plasma engines, while, in principle, capable of both high-thrust density and high specific impulse have been efficiency limited. Direct current plasma engines are limited .due to electrode erosion and thermal losses,. initial plasma 3,371,490 Patented Mar. 5, 1968 ice generation efficiency and accelerating magnetic field trapping and distortion leading to poor acceleration efficiency. Alternating current plasma engines using electrodes are limited by electrode erosion and thermal losses, by low initial plasma formation efliciency and by weak acceleration by the discharge self magnetic field. Previous electrodeless alternating current plasma engines using induced plasma currents have had limited efiiciency due to poor accelerating field geometry, poor inductive coupling with the exciting primary field and low efliciency initial inductive plasma formation.

The teachings of this invention disclose a propulsion engine which effectively overcomes the foregoing limitations of propulsion engines known in the prior art. One advantage of this engine is that it operates with high reliability over extended periods of time. Another advantage of the teaching of this invention is operation over a Wide range of both thrust and specific impulse, providing relatively high thrust density and high specific impulse simul-- taneously. Another advantage to the teachings of this invention is the capability of the engine to operate with large power sources. Still another feature is the provision for means for separately controlling thrust and specific impulse by individually varying the induced plasma current and accelerating magnetic field parameters.

It is an object of this invention to provide a method and apparatus for the creation and control of an induced plasma current.

Another objetc of the teachings of this invention is to provide a method and apparatus for the axial acceleration of an induced plasma current by a magnetic field.

A further object is to provide a relatively simple and' a plurality of circular bands of metal which, though physically separated from one another, are electrically interconnected. The plurality of circular bands simply form a specially shaped inductive turn in a continuous circuit; the circuit is not broken. Physically, the plurality of circular bands in combination with structural means, define a gas ionization chamber having a specified geometric shape and an axis of symmetry.

Electric potential means are connected to energize the electromagnetic means. The applied potentials'vary with time and cause the conduction of parallel primary currents azimuthally in the circular bands of the electromagnetic means. No net radial or axial currents are conducted. These primary currents induce an electric field,

within the space enclosed and defined by the structuralv means and electromagnetic means and produce an azimuthal secondary plasma current in the sense of the secondary of a transformer within -a gas disposed in the space so enclosed. The induced electric field is initially ions and electrons travel in oppositedirections' and form a positive current (net charge transfer) in one direction. The ionized gas is sufficiently dense so that the particles respond as members of a continuum and the plasma responds according to continuum dynamics rather than particle dynamics characteristic of the electrostatic and low density plasma engines. The ionized gas is always a true plasma in that it exhibits volume neutrality. This feature is not true of the electrostatic engines.

-Continuing, the ionized gas immediately forms an azimuthal current upon ionization. Ion pairs are produced from the neutral gas particles by the avalanche process common to high-voltage breakdown. A single time-varying potential is applied to the plurality of circular bands. A simple approach makes use of the inductance of the bands in forming a resonating circuit. The magnetic field of the time-dependent primary current in the bands produced by this single potential induces the azimuthal secondary plasma current within the space so enclosed. This azimuthal secondary plasma current, which undergoes phase reversal with radial contraction of the plasma due to release from the magnetic containment feature of the electromagnetic system, is finally accelerated by a magnetic force (Lorentz force) produced by a coaxial accelerating magnetic field means separate from but coaxial with the plurality of circular bands. This accelerating action is achieved essentially according to the mutual magnetic dipole repulsion of the two adjacent antiparallel azimuthal currents, viz, the azimuthal secondary current of the plasma and the azimuthal current of the coaxial accelerating means. The time rate of change of the accelerating force field of the accelerating means is rapid compared to the rate of axial plasma acceleration in order that work may be done by the accelerating field on the plasma, in order that the plasma be accelerated out of the chamber before it reexpands inside the chamber and in order that penetration of the accelerating magnetic field into the plasma is very small. The accelerating coil magnetic field thus acts much like an axial magnetic piston. One novel feature of this invention includes the dynamic and magnetic containment of the current carrying plasma, within the ionization chamber prior to acceleration of the plasma therefrom, by a combination of the gas ionization chamber, the electromagnetic means and the current induced in the plasma itself. This latter combination also controls physical size of the plasma as Well.

Other novel features believed to be characteristic of my invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

FIG. 1 is a schematic cross-section of an induction chamber for plasma formation and control.

FIG. 2A is a cross-section along the line 22 of FIG. 1.

FIG. 2B is a cross-section along the line 3-3 of FIG. 1.

FIG. 3 is a schematic representation of a circular crosssection of a plasma current toroid.

FIG. 4v is a graphic representation of the transverse (axial) component of magnetic field across the median plane of the induction (and vacuum) chamber as a function of radius.

FIG. 5 is a schematic cross-section of an induction chamber and a plasma current toroid, showing the stages of plasma current toroid major radial contraction.

Conceptually, my device is of the alternating current electrodeless plasma accelerator type. The operation mechanism is of the pulse type, operating at a maximum rate of once per half cycle. One pulse of operation will be described.

With reference to the drawings, wherein like or corresponding parts are designated by the same reference characters throughout the several views, FIGS. 1, 2A and 2B showstructural means defining an ionization or vacuum chamber 1 wherein plasma is generated and controlled according to the teachings of this invention. The electromagnetic means defining in part the chamber 1 comprise a circular band 2 and side disks 3'formed of electrically conducting material. A unionized gas is placed within the vacuum chamber 1 by any convenient means such as by injection through conduits connected to a manifold (not shown) in the region nearest to outside circular band 2. The initial gas breakdown potential is a function of the gas pressure. The optimum pressure, for a given induced potential, is that which produces the greatest number of ionizing collisions and is the one which the energy gained by a gas electron over one mean free path is equal to the ionization energy of the gas atom. During continuous operation, the gas injection rate would be constant and the pulse frequency would be high enough (normally greater than 1000 cycles per second) so that there would be only slight thermal expansion of the gas from the injection region between pulses, eliminating the need for a pulsed injection valve. The apparatus or induction chamber 6, having the vacuum or ionization chamber 1, is formed additionally by structural means such as wall 5, made of electrically non-conductive material. The induction chamber 6 is connected through a short lead system 4 (normally coaxial to minimize distributed inductance) to an electrical potential source 7. -In a simple case the potential source may be in the form of a charged high voltage capacitor connected in series with the induction chamber. A high-frequency primary current of large magnitude is passed through the outside circular band 2 and side disks 3 by means of the lead system 4, as seen in FIGS. 2A and 2B. The circular band 2 and the side disks 3 are electrically in parallel; primary current passes to the side disks 3, from circular band 2, across area 32 which is electrically conducting material. A circular large electric field is induced, according to Faradays Law, within the gas now concentrated in the vacuum chamber 1. A circular arc breakdown occurs within the gas during the initial moment of the first half cycle of primary current supplied by lead system 4 as the initially induced electric field strength exceeds the breakdown field strength of the gas. Gas ionization results. The secondary current subsequently generated in the ionized gas, now a plasma, essentially acts as the secondary of an air core transformer whose primary is the circular induction chamber 6.

Referring to FIG. 3, there is shown a circular crosssection of a plasma current toroid 11 indicated for purposes of discussion; the actual cross-section being more ellipsoidal than shown in the drawing. The plasma current toroid 11, upon formation, has acting upon it a number of forces. The plasma current toroid 11 will tend to be compressed along its minor radius r, due to F the symmetric transverse force imposed upon toroid 11 by the magnetic fields of the primary antiparallel current existing in the side disks 3 of FIGS. 1, 2A, 2B and 5. The plasma current toroid 11 will additionally tend to be compressed along its minor radius 1', due to F the radial and transverse forces imposed upon plasma current filaments and created by neighboring parallel plasma current filaments. Both of the foregoing compressor forces tend to eliminate collison of the plasma current toroid 11 with the vacuum chamber 1 and wall 5. By eliminating such a collison there is reduction and possible elimination of convective and conductive heat transfer from the hot plasma medium. During this initial phase, energy is coupled into the plasma; radiation due to recombination and relaxation of the plasma is not a problem. The cyclotron and Bremmstrahlung radiation level, due to centripetal acceleration and head-on collisions of high energy particles, is minimized by limiting the operating driving potential from potential source 7 of FIGS. 2A and 2B. The overall velocity distribution for each plasma particle specie Within the current toroid 11 is essentially Maxwellian with a superimposed drift velocity. The plasma particle density within each toroid 11 is maintained large enough so that the predominant interaction mechanism is between the over-all plasma current and the existing magnetic fields, rather than between individual particles and the magnetic fields, as would be the case if the individual particle energies were allowed to become greater.

Continuing with reference to FIG. 3, the plasma current toroid 11 will tend to contract along its major radius R, under the force F produced by the local transverse component of the primary magnetic field, and will tend to expand along its major radius R due to the force P effected by the radial gradient of the inductive energy of the plasma current magnetic field. In order to prevent immediate contraction of the toroid 11 as in an ordinary pinch effect, the two given forces P and P must be equalized at the desired equalibrium radial position. The equilibrium plasma current orbit radial position is determined by the satisfaction of the magnetic field magnitude requirements at that position, much as in the case of a Betatron. The difference is that in the Betatron the density of charged particles is quite low and there is correspondingly greater individual particle energy, such that the balancing expansion force is the centrifugal force on the individual particles. The magnitude requirement for the magnetic field at equilibrium is more severe in the current interaction case (the case of which our invention teaches) than in the particle interaction case (Betatron). At equalibrium, a second set of magnetic field conditions must be imposed according to the teachings of this invention in the form of a radial magnetic field gradient so that stable spatial equilibrium will exist in both the radial and transverse directions relative to toroid 11. Stable equilibrium is important in damping out any perturbations which may tend to arise and lead to large internal energy losses. For equilibrium, the required conditions of magnetic field magnitude and radial magnetic field gradient are determined by empirically choosing the dimensions 8, 12, 13 and 14, as indicated in FIG. 1 and imposed across the initial gas breakdown or ionization region 19 as seen in FIGS. 1, 4 and 5. In FIG. 4 is shown the transverse (axial) component of the magnetic field across the median plane 18 of the vacuum chamber 1 plotted against radial direction 17. The region within which stable orbit equilibrium conditions are satisfied is indicated by reference number in FIGS. 4 and 5.

Referring to FIG. 5, the plasma toroid 11, as for any real plasma even with complete ionization, has a finite and not infinite resistivity; magnetic flux lines from the primary field generated by circular band 2 and side disks 3 penetrate it. The result is that the above given conditions for spatial stable orbital equilibrium, as imposed across region 19, become instable in time. The breakdown of spatial stability occurs before the primary current reaches its first quarter cycle of oscillation. However, if the spatial equilibrium conditions did not break down, the formed plasma current toroid 11 could not be used as an accelerator (i.e., rocket engine) propellent, because of the presence of the necessary induction chamber 6 side disks 3 which would tend to contain the plasma toroid 11 within the vacuum chamber 1.

Continuing, the flux from the primary magnetic field means, i.e., the primary current carrying conductors, circular band 2 and side disks 3, enters the plasma in the region shown by reference number 20 (FIGS. 4 and 5). This magnetic flux becomes essentially frozen inside the plasma current toroid 11 region 20 over a very short time interval. As the plasma current toroid 11 leaves the stable orbit region 20, it experiences the onset of temporal instability, and collapses and under the influence of the primary field along its major radius R, as seen in FIG. 3, toward the center of the vacuum chamber 1 of induction chamber 6. Conservation of flux, with a reduced coefficient of mutual inductance (i.e., the fraction of flux linking the external primary magnetic field and plasma current toroid 11) due to the toroids shrinking major radius R requires that the plasma current magnetic flux, and hence current magnitude, approach zero at a radial position just inside the position of initial collapse, designated as region 21 in FIG. 5. At this point, the inductive energy of the initial plasma current is converted to high radial kinetic energy contained by plasma particles in the region 21. The plasma current has now approached zero in magnitude.

Continuing with reference to FIG. 5, and as stated earlier, as the plasma toroid 11 is compressed within region 20, the trapped (frozen) magnetic flux Within the plasma toroid 11 induces a new circular electric field in a direction opposite (antiparallel) to the electric field initially induced by the primary current within the electromagnetic means, i.e., conductors circular band 2 and side disks 3. A plasma current is now created in a direction opposite to that existing upon initial breakdown of the plasma toroid 11 in region 19. This reversed direction plasma current increases in magnitude as the current toroid 11 contracts along its major radius R and, through circuit energy conservation conditions using an essentially completely ionized plasma, the current maximum magnitude exceeds that attained during the initial breakdown current existing in region 19 before release from the stable orbit region 20 because of the change in self inductance. The reversed plasma current is now in the same direction as the primary current existing in circular band 2 and side disks 3 which is at nearly the quarter cycle phase. During the plasma current collapse in region 21, the primary current magnitude in circular band 2 and side disks 3 remains essentially constant. The reversed plasma current maximum is attained at theminimum position of contraction along the major radius R of the current toroid 11 commensurate with its decreased self inductance. The major radius R at the point of maximum contraction is not zero, due to the residual enclosed flux, but does reach an over-all size which is small relative to the dimension 8 shown in FIGS. 1 and 5.

In FIG. 5 is shown a magnetic field producing means 23, e.g., coil 23, which serves as an accelerating means for the toroid 11 now within region 22 of vacuum chamber 1 within induction chamber 6. The coil 23 is mounted flush with the surface of wall 5, where wall 5 as noted earlier forms the non-conducting portion of vacuum chamber 1. The coil 23 is centered about the transverse axis of symmetry 16 of the induction chamber 6 and optimally has the same major radius as the contracted plasma. As a plasma current toroid 11 reaches the point of both greatest radial contraction and current magnitude, within region 22, a steep current pulse is passed through coil 23 (by circuit means not shown). The current pulse has a direction antiparallel to that current in the plasma toroid 11. The steep current pulse passing through coil 23 establishes an intense planar and radially directed magnetic field in the region immediately axially adjacent to the plasma which interacts with the plasma toroid current 11 under close inductive coupling, such that the resulting Lorentz force accelerates the plasma toroid 11, from region 22 out of vacuum chamber 1 through the opening 24 defined by non-conducting Wall 5. The acceleration of toroid 11 is at a high specific thrust and high specific impulse.

Because the planar (in the region of the contracted plasma) radial accelerating magnetic field developed by coil 23 is pulsed, an electric field is induced on the rising phase of the pulse which tends to cancel the large negative plasma current within the plasma toroid 11 in region 22 and convert the inductive field energy of the toroid 11 into plasma particle kinetic energy directed transversely (i.e., along axis 16). The loss of the inductive energy of the plasma current which would otherwise appear in the form of plasma thermal energy is thus prevented. Consequently, the over-all dissipative energy losses in the form of unrecoverable plasma thermal energy are held to a minimum. Because the plasma toroid 11 never comes in contact with the wall 5 of the induction chamber 6, there is no convective or conductive heat transfer from the plasma toroid 11 to the walls as indicated earlier. The reduction of both kinds of plasma thermal energy losses within the engine, therefore, increases the over-all accelerator reliability and operating efficiency.

In summary, the apparatus as described herein differs significantly from the electrical propulsion engines known in the prior art. The plasma accelerator or induction chamber 6 with accelerating coil 23 is, specifically, an electromagnetic engine, functioning by electrodeless discharge of plasma toroids. Previous electromagnetic electrodeless toroidal discharge engines were of the type which used the same magnetic field to both form the plasma and accelerate it with over-all very low etficiency while my invention is of the type which uses separate magnetic fields for each process and which are separately optimized for high over-all efiiciency. The common field type differs from the separated field type in several significant characteristics. One of the most important differences is that in the common field type the primary field inducing the plasma current also functions to accelerate the plasma current, while in the separated field induction type the functions of induction and acceleration are advantageously separated into two distinct phases: (1) the creation and control of the plasma current, and (2) the acceleration of the final form of the plasma current by a radial planar magnetic field. The net result of separating these two functions is a decided improvement in energy coupling, with a consequent marked rise in efii ciency of engine operation. Normally the final acceleration process can be accomplished with greater elfieiency than the initial plasma current formation process. To obtain the highest over-all engine efficiency the power input to the engine would be divided so that the highest efliciency stage of the process (axial acceleration) received the greatest portion of the input power.

The overall size, relative dimensions, and nature of construction according to the teachings of my invention may be varied and still satisfactorily contain, control and axially accelerate a plasma. Generally, the induction chamber 6 should be of the geometrical shape necessary to simultaneously satisfy the required local magnetic field magnitude and gradient conditions such that the induced plasma current is held under the local spatial condition of stable equilibrium. The creation of the plasma current is provided through the action of an outside circular band 2 and side disks 3 which are electrically conductive and effectively serve as the primary of a transformer. It is also noted that a means of accelerating the final form of the plasma current in the form of a toroid 11 is provided by an accelerating coil 23 which produces a pulsed planar radial magnetic field in the neighborhood of the plasma toroid to be accelerated. The accelerating coil 23, placed within the induction chamber 6, is centered about the induction chamber 6 axis of symmetry 16 and positioned with its median plane parallel to the median plane of the induction chamber 6 and in front of the induction chamber 6 side disk 3 opposite the side or wall 5 through which the plasma is axially accelerated along axis 16. The accelerating coil 23 is placed as close to the final position of the contracted plasma current toroid 11 as possible, without actually touching it and have a mean major radius equal to that of the contracted plasma, to provide optimum interaction and coupling between the plasma current and the radial magnetic field generated by coil 23 in the region axially adjacent to the plasma. The use of coils which surround the plasma as in the prior art may be used outside the induction chamber 6, but at a considerable loss in acceleration efficiency due to the reduced radial magnetic field component magnitude and inductive coupling and wasted magnetic field energy in further plasma compression by u the relatively large horizontal magnetic field component generated thereby.

Therefore, while my invention has been disclosed with respect to certain exemplary embodiments, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

I claim:

1. in a plasma accelerator for producing a pulsed gaseous plasma discharge at relatively high specific thrust and high specific impulse, the combination comprising:

(a) structural means, including electromagnetic means,

defining a gas ionization chamber of selective dimensions and having an axis of symmetry;

(b) an electric potential source connected to said electromagnetic means whereby an electric field is induced by said electromagnetic means, within said gas ionization chamber, sufficient to ionize a gas disposed therein and to form a current carrying plasma from the ionized gas, said gas ionization chamber, said electromagnetic means and the current of said current carrying plasma dynamically and magnetically contain the current carrying plasma within said ionization chamber and control the physical size of said current carrying plasma; and

(c) magnetic field producing means disposed within said structural means defining said gas ionization chamber so as to generate a planar magnetic field directed radially across said axis of symmetry in the vicinity of said plasma; whereby the magnetic field producing means, when energized, exerts a force upon said current carrying plasma so as to accelerate the plasma in the direction of the axis of symmetry of the gas ionization chamber.

2. A plasma accelerator for producing a pulsed gaseous plasma discharge at relatively high specific thrust and high specific impulse, comprising:

(a) structural means, including electromagnetic means, defining a gas evacuated ionization chamber having selective dimensions and an axis of symmetry;

(b) an ionizable gas disposed within said gas evaciuated ionization chamber;

(0) means for ionizing said ionizable gas and for the inductive formation of a current carrying plasma therefrom, comprising a source of electrical potential coupled to a plurality of primary current carrying conductors connected in parallel, said conductors comprising in part said electromagnetic means, said gas evacuated ionization chamber, said conductors and said current carrying plasma providing: (1) dynamic and magnetic containment of said current carrying plasma, and (2) control over the contraction of said current carrying plasma; and

(d) a magnetic field coil, disposed at said structural means defining said gas ionization chamber, for accelerating said current carrying plasma along said axis of symmetry.

3. A plasma accelerator for producing a pulsed gaseous plasma discharge at relatively high specific thrust and high specific impulse, comprising:

(a) structural means, including electromagnetic means, defining a gas evacuated ionization chamber having selective dimensions and an axis of symmetry;

(b) an ionizable gas disposed within said gas evacuataed ionization chamber;

(c) means for ionizing said ionizable gas and for the inductive formation of a current carrying plasma therefrom, comprising a source of electric potential coupled to a plurality of primary current carrying conductors connected in parallel, said conductors comprising in part said electromagnetic means including:

(1) a pair of electrically conductive disks, oppositely disposed, parallel to one another and centered about the axis of symmetry of said gas evacuated ionization chamber so as to enclose said chamber; and

(2) a circular band of electrically conductive material, enclosing said gas evacuated ionization chamber such that the axis of symmetry of said chamber and of said circular band are parallel and such that the radial distance from the axis of symmetry of said chamber to said circular band is greater than the radial distance from the axis of symmetry of said chamber to the outer circumference of said pair of disks; said gas evacuated ionization chamber, said conductors and said current carrying plasma providing: (i) dynamic and magnetic containment of said current carrying plasma, and (ii) References Cited UNITED STATES PATENTS 2,940,011 6/ 1960 Kolb 60202 3,138,919 6/1964 Deutsch 60-202 3,273,336 9/ 1966 Kidwell 60-202 OTHER REFERENCES Space/Aeronautics, March 1960, pages 50-54 relied on.

CARLTON R. CROYLE, Primary Examiner.

Images