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United States Patent 1 Jobes, Jr. et al.
[ Sept. 3, 1974 SYSTEMS FOR MEASURING THE PROPERTIES OF PLASMA WITH AN [ON PROBE  Inventors: Forrest C. Jobes, Jr.; Robert L.
Hickok, Jr., both of Trenton, NJ.
 Assignee: Mobil Oil Corporation, New York,
22 Filed: Oct. 15, 1971 21 Appl. No.2 189,568
 US. Cl. 324/71 EB  Int. Cl. G0ln 27/00  Field of Search 324/71, 71 EB, 24
 References Cited UNITED STATES PATENTS 3,519,927 7/1970 Holt 324/71 X 3,599,089 8/1971 Bugnolo 324/71 X OTHER PUBLICATIONS Bul. Amer. Phys. Soc., Series ll, 15, 1970; Abstract-- R DETECTOR Beam Probe Measurements of n. q. p. .l 2, E, and B,
F. C. Jobes, R. L. Hickok, & J. F. Marshall.
Primary ExaminerAlfred E. Smith Attorney, Agent, or FirmA. L. Gaboriault [5 7 ABSTRACT An ion source generates a heavy-ion beam of uniform ion velocity which is utilized to probe plasmas. The beam is scanned through a plane by deflection plates and the angle of scansion for the beam is magnified by a linear field distribution lens. The ionizing collisions between the scanned beam and the plasma results in a step charge change for the ions. Detectors are provided which are responsive to the ions undergoing a step change in charge to measure electron density of the plasma, space potential of the plasma, and the m0- mentum added by the plasma current.
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400 CHANNEL ANALYZER ION GUN N, 8 pz CIRCUITS SWEEP 20 DET MPS
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SYSTEMS FOR MEASURING THE PROPERTIES OF PLASMA WITH AN ION PROBE FIELD OF THE INVENTION sign and control magnetohydrodynamic power systems.
The properties of electron density and space potential for a plasma have been determined by studying the changes incurred by doubly-ionized ions which are created in the plasma by collisional ionization of a beam of 1+ ions. The relative number of 2+, or secondary, ions resulting from the ionization collision provides information about the electron density of the plasma and the change in energy provides information on the space potential The secondary ions, such as Cs or Tl ions, are able to provide localized measurements of plasma because the ions undergo a step change in charge at the point of creation: the electric and magnetic fields exert twice the force on the ion after the ionizing collision as before. Thus the radius of curvature of a 2+ ions is half that of the main beam, and so the 2+ ions are separated from the main beam and also from ions created at other points along the beam path. A suitably placed detector can thus monitor the plasma at any arbitrary poipt q along the beam path. This point q can be moved across the plasma by sweeping the beam and it can be moved along the beam path by moving the detector, by switching to another detector or by changing the beam energy. Maps of the plasma can therefore be readily and rapidly produced by, for example, electronically sweeping the beam and then stepping the beam energy, or by sweeping the beam and using a parallel array of detectors.
The electron density at the point of collision q can and has been determined from the relative numbers of ions produced where the electron temperature is known. The space potential of the plasma at the point q can also be and has been directly determined by the energy of the 2+ ions created there as follows. Beam energy at q is e( V (q'q)), where V is the accelerator voltage and (q) the space potential. The 2+ ions gain 2e(q) upon leaving the plasma, and so end up with a net energy of e( V+ (q)). This energy has been measured electrostatically where the energy measurement is independent of the number of ions created at q except for signal to noise considerations.
Another very important physical property of plasma is plasma current density. Although the prior art has suggested that plasma current density can be determined utilizing a heavy-ion beam probe, there has been no suggestion as to how this might be accomplished with the probe.
Furthermore, the means for measuring the electron 60 density and the means for making the other measurements which have been disclosed in the prior art do not necessarily result in a high degree of accuracy in measurement. The ion sources of the prior art are particularly noteworthy in this respect. The gas discharge ion 65 sources of the prior art do not achieve the uniformity in ion velocity which is necessary to achieve a high degree of accuracy in the measurement of plasma properties. There are also difficulties associated with the apparatus utilized for scanning the ion beam through the plasma. In the systems disclosed in the prior art, it has been necessary to place the scanning of deflection plates close to the plasma to achieve an appropriate angle of scansion and this in itself has created difficulties due to the interaction between the plasma and the deflection plates.
SUMMARY OF THE INVENTION It is another object of this invention to provlde a system for simultaneously measuring electron density, space potential and current density of plasma with an ion beam probe.
In accordance with these and other objects, a preferred embodiment of the invention comprises an ion source for generating a heavy-ion beam characterized by substantially uniform ion velocity and a sweep means for scanning the heavy-ion beam in a plane intersecting plasma. Sweep magnification means increases the angle of scansion in the plane to permit the separation of the sweep means from the plasma. Analyzing means is responsive to the ions having a step change in charge resulting from ion-collisions in the plasma so as to measure electron density in the plasma, space potential in the plasma, and the trajectory of the ions with respect to the plane intersecting the plasma.
In accordance with one important aspect of the invention, the.analyzing means comprisesdeflectio wfi'y parallel detecting surfaces angularly disposed with respect to the plane intersecting the plasma and a detecting surface for detecting the ions passing between the parallel detecting surfaces.
In accordance with a further aspect of the invention, the ion source comprises a heater, an ion emitting surface comprising zeolite doped with alkali metal ions or thallium ions where the surface is heated by the heater. An ion extractor comprising an anode plate is located at the ion emitting surface so as to provide an electric field at the plate which is substantially the same field seen at the ion emitting surface. A lens then focuses the ions emitted from the surface into a beam.
In accordance with a still further aspect of the invention, sweep magnification means are provided comprising a linear field distribution lens including a plurality of pairs of plates perpendicularly disposed with respect to the plane and having quadratically increasing potenion source.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an ion beam probe system embodying the invention;
FIG. 2 is a schematic diagram of the system detectors for measuring plasma properties;
FIG. 3 is a schematic diagram depicting the momentum added to the ions by the plasma current and the measurement of that momentum;
FIG. 4 is a block-schematic diagram of the overall system;
FIG. 5 is a sectional view of an ion beam source utilized in the system;
FIG. 5a is an enlarged view of the ion emitter for the source of FIG. 5;
FIG. 6 is an alternative construction for the ion beam source;
FIG. 7 is a schematic diagram of a circuit for the ion beam source;
FIG. 8 is a schematic diagram depicting some typical ion orbits and the effect of a linear field distribution lens on those orbits;
FIG. 9a is a schematic view of one-half of the lens shown in FIG. 8;
FIG. 9b is a plot showing the field distribution along the lens;
FIG. 10 is a tonal map obtained by measuring electron density in a plasma with the system of this invention;
FIG. 11 is a tonal map of the space potential measured utilizing the system of this invention;
FIG. 12 represents the 1 component of momentum p for two different plasmas as measured by the system of this invention;
FIG. 13 is a current density map derived from the map of FIG. 12; and
FIGS. 14a and 14b are isometric-schematic views of two different secondary ion detectors which may be used in the system.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT An ion beam probe system constructed in accordance with this invention is shown in FIG. 1. The system includes a source of a heavy-ion beam 22 (e.g. Cs+ or Tl+) which is directed through scanning or sweep means comprising deflection plates 24. The deflection plates 24 sweep the beam through an angle in the xy plane which is then magnified by a linear field distribution lens 26. The heavy-ion beam then enters a plasma 28 where ionizing collisions occur in various points q on a grid 30.
As a result of the ionizing collisions, the Cs+ or Tl+ ions emanating from the source 20 undergo a step change in charge to create Cs2+, Cs3+, or Cs4+ ions or Tl2+, Tl3+, or Tl4+ ions. Because electric and magnetic field exerts different forces on ions having different charges, the radius of curvature for the various pendicular to the xy plane and momentum analyzer deflection plates 42 parallel to the xy plane and located in the path of the 2+ secondary ions provide measurements of plasma current density as well as the electron and space potential of the plasma 30.
It will be understood that these properties of the plasma are measured at various points q within the plasma 28 which are located at the intersections of the grid lines in the grid 30. The location of these intersections or points of measurements q in the plasma 28 is determined (for a fixed magnetic field) by the beam energy, the beam path and the detector location. With the detector fixed, the location is determined by the accelerator voltage of the source 20 and the sweep voltage of the deflection plates 24 respectively where both quantities may be controlled electronically as will be explained subsequently with reference to FIG. 4. As the beam is swept across the plasma by the plates 24, the detector 34 as well as the other detectors see one point for each beam path with the locus of these points representing a detector line 44. As shown, this line is slightly curved and makes about a 45 angle with respect to the beam path. The detector line can then be moved across the plasma by changing the beam energy at the source 20. The result then is the detection grid 30 where the more vertical lines are detector lines (including line 44) with the high energy lines to the left. The more horizontal lines are lines of constant sweep voltage.
In accordance with one very important aspect of the invention, the current density of the plasma 30 is determined by measuring the momentum added to the 2+ ions by the plasma current. As shown in FIG. 3, this momentum is actually measured by determining the trajectory of the 2+ ions with respect to the xy plane or along the z axis. It may be seen that dz/dr, the rate of change of ion position with respect to the z axis as a function of distance r in the xy plane, is zero for primary 1+ ion beam at both the entrance to and the exit from the plasma. However the secondary 2+ ions have a finite dz/dr at the point of exit from the plasma which varies from point q to point q within the plasma and results from the current in the plasma.
In order to measure this finite dz/dr which represents the trajectory of the 2+ ions along the z axis or with respect to the xy plane, the detector 34 comprising sensors 45 and 46 is provided. The plates 45 and 46 are split into halves 45a and 46a and 45b and 46b with halves 45a and 46a connected via preamplifier 245a and 246a and resistors 48a and 48b to one input of a differential amplifier 48, and halves 45b and 46b connected via preamplifier 245b and 246b and resistor 48c and 48d to the other input of a differential amplifier 48. The output from the differential amplifier is applied to the lower momentum plate 42. The other analyzer plate 42 is connected to ground. By utilizing this feedback arrangement, the deflection plates 42 will maintain the 2+ ions centered on the gap between the plates 45a and 45b where the outputsignal of the amplifier 48 represents the z axis momentum p which is added to 2+ ions by the plasma current.
The signal to the plates 45 and 46 themselves as shown in FIGS. 1 and 2 provide measurements of the electron density as well as the space potential of the plasma. As indicated in FIG. 1 and more clearly disclosed in FIG. 2, 2+ secondary ions pass between the momentum analyzer plates 42 and into the space between the energy analyzer plates 40. The upper plate 40 is connected to an accelerator voltage source V ACC while the lower plate includes an aperture 50 to permit the 2+ ions to enter the space between the plates 40 and exit therefrom through an aperture 52 located a distance D from the aperture 50. Guard rings 54 are provided between the energy analyzer plates 40. The plates 45 and 46 of the split detector 34 are centered on the aperture 52 to intercept the 2+ ions passing through the analyzer. The difference between the signals to the two plates 45 and 46 are applied to the inputs of a differential amplifier 53 with the output of the amplifier 53 applied as a correction voltage to the lower plate 40. This correction voltage adjusts the distance traveled by the ions so that they always stay centered on the end 45e of the plate 45 no matter what energy the ions have. The correction voltage to do this must therefore be the space potential :1) itself since the ions which have an energy of e( V+ da) lost 2e upon entering the analyzer and therefore start their trajectory inside the analyzer with an energy E of e(V- (b). The electric field is (V )/H where the distance between plates 40 and the distance traveled is DE/(V q5) b) The electron density at a point q within the grid 30 shown in FIG. 1 may be determined from the relative number of ions produced there, if the electron temperature is known. Since the number of 2+ ions is approximately N f(T (in the thin target approximation) where N is the electron density and T is the electron temperature, N and T can be determined separately by measuring the relative numbers of ions created by two different reactions having different f(T Thus, N and T may be determined separately from the relative number of ions detected by the 2+ ion detector 34 and the 3+ ion detector 36.
Reference will now be made to FIG. 14a wherein the four-quadrant nature of the split detector 34 is shown in detail. As shown there, the detector 34 comprises the narrow plate 45 mounted on top of the wider plate 46 with the detecting surfaces being parallel to the xz plane. The plates 45 and 46 are then split by a plane parallel to the xy plane to form a narrow gap 56. .The split between the plates 45 and 46 detects only those changes in trajectory with respect to the yz plane or along the x axis while the momentum detector only detects those changes in the trajectory with respect to the xy plane and along the z axis. The four-quadrant detector 34 should be mounted inside a shielding box (not shown) to collect any secondary electrons emitted from the detecting surfaces. The signals from each plate 45a, 45b, 46a and 46b are amplified by preamplifier 245a, 245b, 246a and 246b respectively and then passed through the resistors 48a, 48b, 48c and 48d to amplifier 48 and through resistors 53a, 53b, 53c and 53d to amplifier 53.
A modified four-quadrant detector is shown in FIG. 14b. Again parallel sensing plates 45' and 46' are utilized as the energy detector. But rather than split the plates again and separate them by a narrow gap as shown in FIG. 14a, a plurality of parallel sensing plates 58 pependicular (a lesser angle could be used) to the plates 45 and 46' and parallel to the xy plane are utilized. Where the 2+ ions are in a trajectory parallel to the xy plane, very few of the 2+ ions will be detected at the detecting surfaces of plates 58. However, where the trajectory forms an angle with respect to the xy plane, the detecting surfaces of the plates 58 will detect the 2+ ions thus providing a measurement of the momentum added by the plasma current. As in the case of the four-quadrant detector shown in FIG. 14a, the signals generated by the detection of 2+ ions at the plates 58 and the parallel plates 45 and 46' are applied to the inputs of the amplifier 48 and 53' through preamplifier and summing resistors not shown.
A block diagram of the overall system for providing measurements of the plasma current as well as the electron density and space potential of the plasma will now be described with reference to FIG. 4. As shown there, a pulser 60 steps a 400 channel analyzer 62 repeatedly through all 400 channels. A V staircase which is proportional to the channel numbers reset every 20th channel is generated. A V, staircase signal that steps every 20th channel is also generated. A control circuit 64 coverts the staircase signal to voltages which are applied to a high voltage supply 66 and a sweep amplifier 68 to provide the detection grid 30 as shown in FIG. 1. The 2+ ions which pass through the electrostatic energy and momentum analyzers are detected and the resulting signals are amplified. Appropriate feedback signals are then generated in the case of d) and p with the measurements N, d) and p, being stored in the appropriate channels of the analyzer 62.
To make a measurement with this system, the analyzer 62 is stepped through all 400 channels with a measurement point q following in an appropriate fashion. At each point q, the parameter to be measured (n, qb or p is sampled and digitized and stored in the appropriate channel in the analyzer 62. This process can be repeated with the digitized signals being added together each time a specific channel is passed thus effectively providing signal averaging. The measured signal can then be displayed as a tonal map on the analyzer display cathode ray tube 74 in the form of the maps shown in FIGS. 10, 12 and 13.
In both detectors shown in FIGS. 14a and 14b, the trajectory of the 2+ ions with respect to the xy plane is determined. In the case of the detector of FIG. 2a, the displacement of the 2+ ions with respect to the xy plane is detected. In the case of the detector of FIG. 2b, the angle of the 2+ ions with respect to the xy plane is detected.
In accordance with another important aspect of the invention, the heavy-ion source 20 as shown in FIG. 1 generates an ion beam comprising ions having substantially uniform velocity. The details of an ion source capable of providing ions of uniform velocity will now be discussed with reference to FIG. 5.
The ion source or ion gun of FIG. 5 comprises an io emitter and an extractor 102. A glassy cinder comprising a Cs or T1 zeolite is contained within a heater 104 such that an ion emitting surface 106 is located substantially at an anode plate 108 of the extractor 102. By locating the ion emitting surface 106 at the anode plate 108 the electric field generated by the extractor 102 as seen at the plate 108 is substantially the same electric field that is seen at the ion emitting surface 106. This results in the extraction of ions having substantially uniform velocity. V
The resulting ion beam passes through a first lens 110 on to a target 112 having a small aperture 114. The ions pass through a small aperture 1 14 and into a drift space 116 before entering a second lens 118. A final accelerator gap 120 is provided for the ions passing between deflection plates 122 which are schematically represented as deflection plates 24 in FIG. 1.
As mentioned previously, the thermionic ion emitter 100 comprises a zeolite doped with an appropriate ion. Zeolite is then heated to approximately I,400 K. and placed in an electric field. The ions can be created chemically long before being incorporated in the ion gun, and the zeolite merely serves as a storage medium. Although many different ions may be incorporated into zeolites, alkalai metal ions or ions with similar chemistry have been found to be particularly well adapted for use in the ion source. One very heavy atom which has a chemistry similar to the alkalai metals and is therefore considered as a suitable alkalai metal is thallium (204.4 amu) with a valence of one so that a zeolite ion source can provide ion beams in convenient steps throughout the whole atomic mass range from 7 to 204 amu.
The zeolite may be prepared by taking a standard sodium zeolite and chemically exchanging sodium for the desired ion. This can be accomplished by washing the zeolite in an appropriate salt solution until an equilibrium mixture of sodium and the particular ions utilized is reached in both the zeolite and the solution. Then the zeolite is permitted to settle out, the salt solution decanted off and replaced with a fresh solution, and a new equilibrium reached with a higher ion concentration. In this fashion almost all the sodium could be replaced with the desired ion, except that in some zeolites larger ions (especially Cs+) cannot enter all of the sites where .the Na+ ions exist, so that in these zeolites some sodium (perhaps 20 percent) remains. After the final washing, the zeolite is dried, ground to a powder and stored.
The ion emitter 100 is constructed as shown in FIG. a. A five turn heater coil 124 of .025 cm. tungsten wire is made by wrapping the wire around a small machine screw with the leads bent to come in axially along one end of the coil 124. This coil 124 is then inserted into an alumina holder 126 which serves to support the coil 124 and the zeolite, and also to insulate the tungsten leads. The aluminum holder 126 is a commercially available cylindrical insulator, approximately 5mm. In diameter and about 25mm. in length. Heater leads 128, which are inserted through two small holes 130 in the holder, are each wrapped with a second tungsten wire which serves to cool the leads and to lower the joule heating. Without this precaution, the heaters are subject to a thermal instability in which when the power is first turned on, the leads heat first, because they have less thermal mass than the coil and zeolite. As a result, the leads have a greater resistance and so absorb a greater fraction of the power. The end result is that the leads 128 become white hot while the coil and zeolite would only become dull red.
The zeolite is put into the heater coil 124 in the form of a paste made by adding water to a small quantity of zeolite. Either'a spatula or an eye dropper may be used to pack the zeolite into the heater coil 124 and an electric current (approximately 4 amps) runs through the heater to dry the paste; during the drying the zeolite is shaped and smoothed with the spatula to form the flat surface 106. The zeolite is then heated to a bright red heat to convert irreversibly the zeolite to an amorphous glassy form which has a greater mechanical strength and which is the actual ion-emitting substance. In order to protect the tungsten wire from oxidation, this final heating is done in a vacuum.
In the particular gun shown in FIG. 5, the ions were extracted across a 1.27 cm. gap by a 3-10kv/cm. field, then focused by a first lens on the aperture 114 having a hole .0343 cm. in diameter. This aperture was then the object for the projection lens 118 and the beam size at one meter was just the image of this aperture 114 magnified by the projection lens 118.
The lenses 110 and 113 are electrostatic unipotential lenses-three element lenses with the outer elements at the same potential. These lenses can be designed with a wide variety of ways, but all such designs have two features in common: any focal length from infinity down to the lens size itself can be achieved merely by changing the voltage on the center element, and any given lens design can be scaled to any size.
One of the difficulties in the ion gun of FIG. 5 is that it is difficult to improve on the ion beam. In order to decrease the beam size, for instance, the target hole 114 can be made smaller, or the object distance of the projection lens 118 can be increased, but either change decreases beam current, unless the aperture of the projection lens 118 is also increased. The current passing through the target whole 114 is therefore essentially space-charge limited and cannot be readily increased. The ion gun of FIG. 6 which utilizes the space-charged design principles developed by J. R. Pierce, Theory and Design of Electron Beams (D. Van Nostrand Co., Inc., New York, 1954), 2nd ed., Chap. 9, p. 145, Chap. 10, 173, for electron guns solves this problem.
In the gun of FIG. 6 (elements and sections which are analogous or equivalent to the elements and sections of FIG. 5 are labeled with the same reference characters followed by the letter a), the extraction field is shaped so that the ion flow is parallel to the extraction field. This is accomplished by suitably shaping the negative extraction electrode 124 to act as a diverging lens, giving a virtual image of the ion emitter at approximately 1.7d behind the emitter surface 106a, where d is the separation between the emitter and the extraction electrode 124. Because this image is formed from parallel flow, it is notan image of the emitter surface 106a, but rather the velocity distribution of the beam. The center of this image consists of ions traveling the optic axis and any circle centered on this central point consists of ions with constant ]VJ I The gun of FIG. 6 is extremely efficient; approximately 30 percent of the extracted beam can be delivered as the usable ion beam, as compared to approximately 0.3 percent with the ion gun of FIG. 5. It can deliver a lOkv, 1p. A beam of Tl+ or Cs+ with a diameter of about 4mm. at one meter. A greatly increased efficiency permits the zeolite temperature to be much lower than in the case of the gun of FIG. 5. This in turn results in a drastic increase zeolite life-time (from minutes to at least hours in the case of a Tl+ source).
The ion beam energy for the ion sources of FIGS. 5 and 6 is controlled electrostatically by connecting a well regulated, programmable 30KV power supply to a center tap of a transformer winding 132 supplying the current to the zeolite heater filament. To within the voltage difference between the transformer center tap 130 and the emitter surface 106 or 1060 (less than 1 volt), the ion energy (in a drift space at ground potential) is eV where V is the output voltage of the power supply. Within the ion optic system, however, the ion energy is determined by the extractor power supply 134, which is connected between the negative extractor plate and the 30KV power supply. Lens power supplies 136 and 138 are also connected between the 30KV power supply and the negative plates of the first and second lens 110 and 118 respectively.
In accordance with another important aspect of the invention, the linear field distribution lens 26 is utilized to magnify the sweep of the ion beam while maintaining a substantial separation between the deflection plates 24 and the plasma 28 as shown in FIG. 8. Because of the vacuum magnetic fields in the region of the plasma 28, there is a very strong focusing effect on the ion beam generated by the ion source 20 and swept by the deflection plates 24. In the absence of the lens 26, the primary ion orbits would be focused to a point shortly after passing through the plasma thereby resulting in poor spatial resolution.
However, by utilizing the lens 26, the focusing properties of the vacuum magnetic field are overcome so as to achieve the substantially parallel ion orbits represented by lines 142 where lines 140 represent secondary paths. With the lens 26, the secondary detectors may be located a substantial distance from the plasma 28 in a vacuum duct 144 while still maintaining good spatial resolution. Furthermore, with the arrangement shown in FIG. 8, the sweep plates 24 may be maintained a substantial distance from the plasma 28 and the sweep voltage may be minimized while still achieving the parallel orbits represented by the lines 142. As a result, there is no high current loading of the sweep amplifier for the plates 24 and there is little risk of breakdown between the sweep plates 24. The use of the lens 26 also permits the aperture to the plasma 28 to be minimized thereby facilitating differential pumping between the area of the plasma 28 and the area in which the sweep plates 24 are located. As aresult, an increase of pressure at the plasma 28 prior to firing will not effect an increase in pressure at the sweep plates and this too avoids high current loading of the sweep amplifier and the risk of severe breakdown problems between the sweep plates 24.
The lens 26 comprises first and second sets of parallel I plates 146 which extend substantially parallel with the beam plate from the ion source 20 to the plasma 28 and perpendicular to the xy plane. The plates 146 are maintained at quadratically increasing potentials with respect to a centerline 148 to develop a linear field distribution which will now be described with reference to FIGS. 9a and 9b.
A first set of plates 146a and a second set of plates 146b on only one side of the centerline 148 are depicted in FIG. 9a. It will be understood that the beam passes between the sets of v plates 146a and l46b through the space 150 extending along the xy plane. The field at various locations within the lens is represented by the ordinant of FIG. 9b for various distances from the centerline 148 as represented by the abscissa in FIG. 9b. In order to achieve this linear field distribution within the gap 150, the plates 146 are maintained at quadratically increasing voltages as the distance from the centerline 148 increases.
Utilizing the previously described ion beam probe system, it is possible to map the momentum P,, the electron density n, and the space potential (it as shown in FIGS. 10, 12 and 13 respectively. It will be noted that the electron density n map of FIG. 12 and the space potential 1) map of FIG. 13 are similar except the peak in (1) lies above and to the left of the peak in n.
Once having determined and mapped the momentum p, as shown in FIG. 10, it is then possible to determine and map the current density of the plasma as shown in FIG. 11. This is possible since the current flow in the plasma produced an additional magnetic field which will alter the orbits of both the primary and secondary beams. The calculations are carried out in a cylindrical coordinate system where the z axis is perpendicular to the plane of symmetry and intersects the symmetry plane at the center of the plasma (center of the limiter).
It is apparent from FIG. 3 that the orbits of secondary ions emitted at various points along the primary beam path, diverge in the z direction after leaving the plasma and that the z displacement at any radial position is closely related to the radius at the emission point. If the current density has only a z component and the distribution has cylindrical symmetry, and if the vacuum magnetic field has only a 2 component, then it can be shown that dz /dr is proportional to B 9 e and, to a good approximation dz /dr is also proportional to B e where z', and z, are the velocity and the displacement of the secondary beam at r the radial position of the detector, r is the emission radius and B 9 e is the magnetic field due to the plasma current at the emission radius. Consider first the equation from the primary beam from the starting radius r, to the emission radius where a e/mc and B 9 is the magnetic field due to the plasma current (a function of r only). Initial conditions are t=z,,=z',,=0 at r=r,,
Si /or a3 9 (r) an. e... mm. 21 B0(r)dr 3) For the secondary orbit i and z, at some radial position r is Eq. 6 shows that the rate of change of the 2 component of momentum evaluated at some radius r is a direct measure of the field produced by the plasma current at the emission radius r,,. Unfortunately, the 2 component of momentum is small compared with the x-y components and is rather difficult to measure.
Substituting vds for dt in Eq. 5, taking the derivative with respect to r yields For beam orbits that escape the magnetic field (which are the only ones considered) S(r) is nearly a linear functon of r, so the differential in the last integrand nearly cancel. Secondary orbits for r and r Ar will be nearly parallel and 6S /8r will be less the unity. The integral over B 9 (r) is of the order of r B 9 (r,,) where r is the plasma radius. The length of the secondary path (S S is in general at least five times larger than r,,. Consequently, 8z /8r should be at least an order to magnitude more sensitive to B 9 (r the field at the emission point, than it is to be integrated field over the path. The reason for this is that all the secondary ions sample the same field from the edge of the plasma to the detector, but due to the curvature of the paths in the vacuum B field dr/dt is not the same for all of them. In effect, some orbits take a longer time to reach the detector, and consequently have a larger z displacement due to the initial z velocity. This is a small effect giving rise to the small correction represented by the second integral in Eq. 7. The first integral is due to the variation of the initial 2 velocity at the emission point .and, again, since the integrated field sampled by all the primary orbits will be nearly the same, the initial 2 ve locity will not be a strongly varying function of the emission point. In general, then, the B 9 (r field at the emission point will dominate 6z /6r so that an experimental determination of zd/8r provides a direct measure of the current produced field, and consequently the current distribution.
It is to be understood that the invention as shown herein and described is the preferred embodiment only and that various changes may be made without departing from the spirit of the invention as set forth in the appended claims.
What is claimed is:
1. A system for simultaneously measuring electron density, space potential, and current density of a plasma with an ion beam probe comprising:
an ion source for generating a heavy-ion beam characterized by substantially uniform ion velocity;
a sweep means for scanning said heavy-ion beam in a plane intersecting said plasma;
a sweep magnification means for increasing the angle of scansion in said plane to permit the separation of said sweep means from said plasma; and
an analyzing means responsive to the ions having a step change in charge resulting from ionizing collisions in said plasma so as to measure electron density in said plasma, space potential in said plasma, and the change in trajectory of said ions with respect to said plane which results from the current in said plasma, said analyzing means comprising: deflection means for deflecting said ions with respect to said plane; sensors for sensing ions having different trajectories with respect to said plane; and means for generating a signal representing the difference between the number of ions sensed by one of said sensors and a number of ions sensed by another of said sensors, said signal being applied to said deflection means and representing the current density of said plasma. 2. The system of claim 1 wherein said sensors comprise adjacent and parallel planar detecting surfaces.
3. The system of claim 1 wherein said sensors comprise a plurality of spaced parallel detecting surfaces angularly disposed with respect to said plane and a detecting surface for detecting the ions passing through the spaces between said parallel detecting surfaces.
4. A system for simultaneously measuring electron density, space potential, and current density of a plasma with an ion beam probe comprising:
an ion source generating a heavy-ion beam characterized by substantially uniform velocity comprising: a heater; an ion emitting surface comprising a zeolite doped with an alkali ions or thallium ions, said surface being heated by said heater; an ion extractor comprising an anode plate located substantially at said surface so as to provide an electric field at said plate which is substantially the same field seen at said surface; and a lens for focusing the ions emitted from said surface into a beam; a sweep means for scanning said heavy-ion beam in a plane intersecting said plasma;
a sweep magnification means for increasing the angle of scansion in said plane to permit the separation of said sweep means from said plasma; and
an analyzing means responsive to the ions having a step change in charge resulting from ionizing collisions in said plasma so as to measure electron density in said plasma, space potential in said plasma and the change in trajectory of said ions with respect to said plane which results from the current in said plasma.
5. The system of claim 4 wherein said sweep magnification means comprises a linear field distribution lens including a plurality of pairs of plates perpendicularly disposed with respect to said plane and having quadratically increasing potentials on either side of a centerline extending toward said ion source.