US2809314A - Field emission ion source - Google Patents

Description

Oct. 8, 1957 R G, HERB 2,809,314

FIEL EMISSION ION SOURCE Filed Jan 27, 1956 2 Sheets-Sheet l Oct. 8; 1957 R. G. HERB FIELD' EMISSION 10N SOURCE 2 Sheets-Sheet 2 Filed Jan. 27, 1956 [IIL Patented Get. 8, 1957 man ntvirssioN roN Sonaca Raymond LvG. Herb, Madison, `Wis., assigner to High Voltage Engineering Corporation, Camhridge, lvass., a corporation of Massachusetts Application January 27, 1956, Serial No. 561,818

19 Claims. (Cl. 313-63) This invention relates to the generation oi ions by emission from metallic points under the influence of very high electric fields, and in particular to an ion source comprising at least one hollow member having a point, such as a hollow needle, the interior of which is supplied with the gas to be ionized, and high-voltage means for producing a very high electric ield at the point of the needle, so that gas which permeates through the needle is ionized by the electric iield after it arrives at the point of the needle.

While the invention is not limited to any particular application, an ion source embodying the invention is especially yadapted for use in particle accelerators, since it provides an ion beam which can be very precisely focussed and collimated, and permits reduction in the size of accelerating tube apertures and production of a better tube vacuum. These improvements, in turn, lead to a reduction of the long tube eilect, yand better shielding of the beam from static inucnces in the accelerating tube.

An ion source constructed in accordance with the invention provides a substantially monoencrgetic ion beam. In the conventional ion source, the ions are created by means of a discharge through a gas, and the voltages which are necessary to produce the discharge cause ions to be accelerated by varying amounts up to the maximum voltage drop in the ion source. F or example, most capillary-arc sources and radio-frequency sources have an energy spread of more than l() electron volts. in the ion source of the invention, the only energy spread is that due to the thermal agitation of the gas at the point of the needle, which may be as low as one-iortieth of an electron volt. Thus the invention provides a beam of ions in which the energy spread is very small and comparable to that encountered in an electron source.

An ion source constructed in accordance with the invention is capable of providing a substantial well-collimated ion beam, because the degree of random distribution of the position from which and the direction in which the ions are emitted is extremely small. That is to say, each ion in the beam originates from some point within a very smal area, and each starts out in a direction within a very small solid angle. Substantial ion currents are possible, despite the smallness of the emitting area, owing to the high luminosity of the ion source of the invention, where "l` minosity is a measure of the number of ions emitted "ci unit area of the emitting surface. For example, a capi 3e source may provide an ion from an area of about s on the order or" l sity is l square mi limeter, and cui milliampere; so that the lumi per square centimeter.

in the ion source of the invention may be as low as 6.3 X-10 square cen current as low as l microampere would of 1600 amperes per square centimeter.

The angular limites of the emitted ions is proportional to the ratio of the magnitude of the random velocities possessed by the ions in the source to the gradient of the accelerating voltage at the source. As previously noted, the random velocities in the ion source of the invention are very small. Moreover, the accelerating voltage gradient at the source is on the order of 108 Volts per centimeter, which far exceeds the gradients in conventional sources. The high initial gradient further assists in providing a well-collimated beam by virtue of the fact that it reduces the eect of space charge.

It is possible to pulse an ion source constructed in accordance with the invention, and by the application of short electric pulses relatively high instantaneous currents may be realized. A simple calculation shows that there are enough atoms at the point of the needle to provide a short, high current pulse. Assuming the gas layer on the point to be 1 atom thick, and assuming that the point is a hemisphere of radius 5x10-5 centimeter, if the pulse interval is 10-9 second, the current can be of the order of 10 milliamperes. Moreover, the rapid replenishment of the gas at the point permits a high pulse repetition rate.

As will appear from the following detailed description thereof, an ion source constructed in accordance with the invention is mechanically simple. Moreover, the gas flow required to provide a given proton llow is less than that required in conventional ion sources.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings, in which:

Fig. l is a view in central section of one embodiment of the invention;

Fig. 2 is an enlarged view of the ion emiter shown in Fig. l;

Fig. 3 is a diagram showing the electrical circuit associated with the apparatus of Fig. l;

Fig. 4 is a view similar to that oi Fig. 2 and showing .one modification of the apparatus or Fig. 2;

Fig. 5 is a View similar to that of Fig. 2 and showing another modiiication of the apparatus of Fig. 2; and

Fig. 6 is a view similar to that or Fig. 2 and showing apparatus for producing a pulse ion beam in accordance with the invention.

The phenomenon of field emission, by which appreciable amounts of current can be drawn from metals even at low temperaturesby the application of very high electric elds at the surface, has been known since 1897 in connection with the emission of electrons. Classical theory could give no reasonable mechanism which explained field emission. In the quantum mechanical picture, some of the multitude of conduction electrons penetrate the potential barrier of the metal surface when this barrier is narrowed by a strong applied iield.

Recently the phenomenon of eld emission has been observed in connection with positive ions. Field emission of ions dii-ters from eld emission of electrons in that the metal at whose surface the high electric tield is created does not ordinarily possess ion reservoir comparable to the vast multitude of 'hilo electrons which are available for electron emission. Consequently, the principal problem in causing the held emission of ions from a metallic point is the provision at the point of atoms or molecules which will be ionized by the high electric iield. The problem is augmented in the case of an ion source suitable for use in particle accelerators and similar devices by the necessity for continuous replenishment of the ionizable atoms or molecules at the point. While limited currents have been achieved with metal points of special material and with metals in which hydrogen gas has been absorbed, the construction of a practical ion source has been thwarted by inadequate replenish ment ofthe ionizable material at the emitter point.

The invention provides the, necessary replenishment of ionizable material at the emitter point by using a hollow needle filled with gas under high pressure, and, in some cases, by also heating the hollow needle. The invention makes use of two phenomena: first, the ability of certain gases to permeate certain metals under certain conditions; second, the fact that under certain circumstances the gas which has permeated the metal to the surface thereof will remain there, forming a layer of molecules which can ow along the surface of the metal. Accordingly, the gas which is within the hollow needle at high pressure permeates through the walls of the needle to the outer surface thereof. One might suppose that the flow of gas to the point of the needle would be very limited, owing to the fact that the thickness of the needle in the immediate vicinity of the point must be relatively thick in order to provide the necessary mechanical support for the point. However, owing to the second phenomenon mentioned above, the gas need not ilow through this thick part of the needle Wall in order to reach the point. needle wall, forms a layer of molecules on the outer surface of the needle, and then ows along the outer surface of the needle to the point. In this way, the gas which is pulled off the emitter point by an intense electric field is replenished by the surface flow, the magnitude of which is sufficient to replenish gas at the point at the high emission rates required for practical use.

f course, the gas selected and the material of which the needle or other hollow pointed member is composed must be such that the gas will permeate through the needle. Fortunately, the gas which is most frequently used in ion sources for particle accelerators is also a Vgas which is capable of permeating most metals. This gas is hydrogen, including its isotopes deuterium and tritium; and in the claims the term isotopes of hydrogen includes hydrogen, deuterium and tritium. While most metals are permeable to isotopes of hydrogen, palladium is particularly so and for this reason is a preferred material for the hollow needle. However, palladium has low tensile strength, and so in some cases it may be desirable to use a stronger metal, such as iron or nickel, in view of the large force exerted on the needle by the intense electric eld. Copper has a relatively low permeability to isotopes of hydrogen, and so those portions of the gas enclosure through which the gas is conveyed 'to the hollow needle may conveniently be made of copper.

Referring to the drawings, and first to Fig. l thereof, the ion source therein Vshown is constructed in accordance with the invention and comprises an evacuated chamber 1 which contains a needle assembly 2 and an accelerating f lens system 3, 4, 5. The chamber 1 is evacuated by means of a suitable pump (not shown). Y

The chamber 1 includes an insulating column, consisting of insulating rings 6 separated by and sealed to apertured thin electrode disks 7,V and a lead-in flange 8. Metallic rings 9 it over the sharp edges of the exposed Instead, it flows through the thin parts of the electrode disks 7 Vto prevent sparking and corona discharge from theseV sharp edges. Resistors (not shown) may be provided between adjacent electrode disks-7 to provide the desired distribution of potential along the column and to prevent formation of any high, localized potential gradients which might cause breakdown.

T he needle assembly 2 is shown in detail in Fig. 2. VA copper tube 10, one-quarter inch in diameter, is silver soldered into the lead-in ange 8 (Fig. l). The end of the copper tube 1f; is drilled out slightly, and a copper snout 11 is inserted into the resultant annular recess 12 in the copper tube 10 and is silverv soldered in place. Y A hollow needle 13 is silver soldered in the snout 11, as shown in Fig. 2. lt should be emphasized that the invention is not limited to the particular needle assembly shown, but includes any suitable hollow member having a point. For example, the needle assembly 2 shown in Fig. 2 may be modilied by machining the tube 10 and the 4 Y snout 11 out of a single copper rod, thereby reducing the number of silver-soldered joints.

In accordance with the invention, gas is fed at high pressure into the needle assembly 2 from a suitable gas source 14. The needle 13 consists of a material which is permeable to the gas from the gas source 14; and so, as appears from Fig. 2, the wall through which the gas will flow is that portion of the needle 13 which extends from the extremity of the snout 11 nearly to the point 15 of the needle 13. For example, if the needle 13 is a rod one-sixteenth inch in diameter and two centimeters long, and if a hole one-thirty-second of an inch in diameter is drilled nearly the entire length of the rod, as shown in Fig. 2, the needle 13 will have an inner area of about one-half square centimeter available as a permeation surface, while the wall thickness is approximately .045.

centimeter. Y

The high pressure alone may be sullcient to cause the gas to permeate through the wall of the needle 13 so as to reach the outer surface of the needle 13 and, owing along the outer surface, arrive at the emitter point 15. If not, the temperature of the needle 13 may be increased to assist in such permeation by means of a heater coil 16, which is energized by a suitable power supply 17. If the needle 13 is to be raised to hightemperature, the heater coil 16 may be an electron emitter which bombards the needle 13 with electrons so as to raise its temperature by a large amount.

lf the needle 13 of the foregoing example is made of palladium, and if hydrogen is introduced into the needle 13 at a temperature of 300 K. and a pressure of 1500 pounds per square inch in excess of the pressure prevailing outside the needle 13, the flow rate through a wall may be calculated to be V1 l05 cubic centimeters per second. This flow rate corresponds to a current of 100 microamperes. lf the needle 13 is made of iron, and if hydrogen is introduced into the needle 13 Vat a temperature of 500 K. and a pressure of 1500 pounds per square inch in excess of the pressure prevailing outside the needle 13, the ow rate may be calculated to be 2X109 cubic centimeters per second. This flow rate corresponds to a current of .02 microampere. Thus a palladium point easily supplies the necessary flow rates, While an iron point must be heated to raise its permeability.

Gas arriving at the emitter point 15 is ionized by a very high electric field which is produced at the point 15 by impressing a high-voltage potential difference between the point 15 and the lens system 3, 4, 5. The electric field at the point 15 increases as the potential difference between the point 15 and the lens system 3, 4, 5 increases, and also increases as the radius ofcurvature of the point 15 decreases. The maximum size of the point 15 thus depends upon the electric field necessary and the potentialV difference available.

Field strengths of (2 to 6) l08 volts per centimeter are necessary for proper operation of the ion source. An accurate calculation of the voltage produced by a given emitter point can be made following a method which assumes that the shape of the point can, for calculation purposes, be considered as a sphere on a cone.V However, a calculation assuming the point to be a paraboloid will suliice Where only an approximate value of the field produced is desired. The latter assumption leads to the formula: Y

rin-

where F is the electric eld in volts per centimeter, U is the applied potential in volts, r is the radius of the emitter, and l?. that of the extractor, both in centimeters. Calculations from this formula and from certain data indicate that with a maximum voltage of 35 kilovolts a point Vradius of (2 to 3))(10-5 centimeters is required.

The general order of the magnitude of r and U may be calculated as follows:

Since F must be of the order l08 volts per centimeter, r (in centimeters) must equal U (in kilovolts) times lO-S.

Fine points such as this have been produced in several ways. Among these are mechanical grinding, chemical etching, oxidation in a flame, and electrolytic etching. Of these methods electrolytic etching seems to produce the smoothest surfaces with the least irregularities. Such a surface is desirable in the present invention primarily because of its greater mechanical strength. Iron points were successfully etched in a molar solution of potassium chlorate, dissolved in a 30 percent solution of hydrochloric acid. A nickel helix about 2 inches in diameter was made one electrode, and the emitter point the other. Sixty-cycle alternating current of about volts potential was used and is satisfactory. The point was etched for a short period of time, then inspected under an optical microscope, then etched again. This etching was continued until the radius just passed the limit of resolution of the microscope. This places the radius of the point at approximately 3XlO5 centimeters. For an accurate knowledge of the point, an electron micrograph must be made; although the general shape of the point can be determined through observation with a high power optical microscope. The points which were etched were approximately paraboloids. The same methods can be used to etch palladium points.

Any suitable means may be used to create the necessary high electric eld at the point l5. In general, however, it will be desirable to focus the emitted ions into a beam, and in that event the electrode to which the ions are attracted from the emitter point may comprise part or" a suitable lens system 3, 4, 5. The invention is not limited to any particular lens system; nor, indeed, is it necessary to the operation of the invention that a lens system be used at all. Merely by way of exemplifying one possible embodiment of the invention, the lens system 3, 4, 5, shown in Fig. l, is a tube type einzel lens, and comprises three lens electrodes 3, 4, and 5, which are constructed of one-siXteenth-inch wall seamless steel tubing, and each of which is polished and the ends carefully rounded. The electrodes 3, 4, 5 are fastened into stainless steel disks 1S thirty mils thick, which are supported on the electrode disks 7 of the chamber 1.

The electric circuit for the lens system 3, 4, 5 is shown in Fig. 3. Referring thereto, electrodes 3 and 5 are grounded, and a high-voltage supply 19 maintains the needle assembly 2 at a suitable potential, such as 50 kilovolts, which may be positive or negative depending on Whether the ion source is to provide positive or negative ions, respectively. The middle electrode 4 is maintained at a potential between ground and the potential of the needle assembly 2 by means of a potential divider 2li. lf a pulsed ion beam is desired, the high-voltage supply 19 may comprise a suitable circuit for producing highvoltage pulses at the needle assembly 2; or, alternatively, any other suitable pulsing means may be employed. As hereinbefore stated, an ion source embodying the invention is well adapted to the production of high-current ion pulses of short duration.

he lens system 3, 4, 5 serves to focus the ions into a beam after they leave the emitter point 15. The primary reasons for choosing a tube type einzel lens for this purpose were: (l) simplicity of construction-the lens itself is simple, and the voltage supply 19 which is used for the emitter point 15 can also serve for the lensY system 3, 4, 5; (2) if the lens voltage is a definite fraction of the emitter voltage, the focal length of the lens does not depend on the voltage applied to the emitter point. To provide the lens with a variable focal length, the voltage supply for the lens is taken from the high-voltage supply i9 through the potential divider 20. ln this system the focal length of the lens is determined by the setting ot the potentiometer 20. Once this setting has been made the emitter voltage may be varied at will without destroying the focus of the beam.

The rst electrode 3 is equipped with an aperture 21, which serves both as a limiting diaphragm defining the beam, and as an extractor for the emitter point 15. The aperture 2l shown in Fig. l is designed to permit a sixtydegree cone of protons to enter the lens from an emitter point 15 one centimeter distant.

In a uni-potential lens the dimensions of the center electrode 4 are the only critical dimensions. The other two electrodes 3, 5 lare designed to serve the purpose of shielding the beam from any static charge which might build up on the insulating walls of the column of the chamber 1. Therefore the electrodes 3, 4, 5 are made reentrant by one-quarter inch, and are made long enough to cover all insulating surfaces exposed to the beam.

A major application of the invention is its use as a positive-ion source for particle accelerators, and especially as a source of protons and deuterons for such particle accelerators. However, the invention is not limited to such application, but includes lother applications where a point source of ions is desired. For example, the invention may be embodied in a point source for negative ions, in which event the undesired emission of electrons from the metallic needle is minimized by the layer of gas which covers the outer surface of the metal, as hereinbefore set forth. Any electrons which are emitted may easily be removed from the negative-ion beam by conventional means, such as causing the beam to pass through a magnetic tield strong enough to deflect the electrons yet not strong enough to attect the negative ions appreciably. When the invention is embodied in a point source for negative ions, the eld strength required at the point is 'only of the order of l0I volts per centimeter, rather than the 108 volts per centimeter required for positive-ion operation.

As hereinbefore stated, the ion source of the invention comprises at least one hollow member having a point, and in the embodiment of the invention which is shown in Figs. l and 2 said hollow member having a point comprises the needle assembly 2. Only part of the hollow member need be of a material permeable to the gas compressed within it, and in the embodiment shown in Figs. l and 2 only the hollow needle 13 is permeable to the gas. However, other constructions of the hollow member having a point are possible without departing from the spirit and scope of the invention, and two alternative constructions are shown in Figs. 4 and 5.

Referring now to Fig. 4, therein is shown a needle assembly 2' which is similar to the needle assembly 2 shown in Figs. l and 2, except that the tube 1l) and snout 1i terminate in a solid needle 13'. The solid needle i3 is easier to manufacture than the hollow needle 13 (Fig. 2), but the gas flow is reduced because the gas must permeate through more solid material. Nevertheless, if the material of which the solid needle 13 is composed readily transmits the gas to be ionized, the construction shown in Fig. 4 may be used. For example, a solid needle 13 composed of palladium would transmit hydrogen sundciently readily to permit the use of the embodiment of the invention shown in Fig. 4. In the embodiment of the Vionized permeates through the material of the pointed member to the-outer surface thereof, and then travels along such outer surface to the point of the pointed member. For this reason, it is not necessary that the point or tip of the pointed memberbe especially permeable to the gas, and in some cases it may be desirable that the tip of the pointed member be composed of a material different from that of which the rest of the pointed member is composed. For example, palladium is highly permeable to hydrogen, but it is soft. In accordance with the invention, a pointed member composed of palladium may be provided with a tip composed of harder material such as tungsten, since the material at the tip need not be permeableV to the. gas. Referring to Fig. 5, therein is shown a hollow needle 13 whose shaft 21 is composed of a material, such as palladium, which is permeable to the gas to be ionized, but whose tip 22 is composed of a different material, such as tungsten, and may be welded to the shaft 21 or otherwise aliixed thereto. in the embodiment of the invention shown in Fig. 5 the tip 22 holds its shape better and is less apt to be deformed, lasts longer, and is easier to manufacture than point 15 of the needle 13 in the embodiment of the invention shown in Fig. 2.

lt has hereinbefore been suggested that, if a pulsed ion beam is desired, the high-voltage supply 19V (Fig. 3) may comprise a suitable circuit for producing high-voltage pulses at the needle assembly 2. However, the highvoltage supply 19 must provide a voltage of the order of 104 volts, and pulsing a voltage of this magnitude is a dimcult task. ln accordance with the invention a pulsed ion beam may be produced in a much simpler manner by modifying the power supply 17 (Fig. l) so that it creates a controllable potential dierence of the order of a few hundred volts, or even less, between the heater coil 16 and the needle 13. The needle 13 is kept at a fixed potential,Y and the voltage'of the coil 16 with respect to the needle 13 is pulsed by means of the power supply 17. As shown in Fig. l, the power supply 17 is at the high potential end of the ion source, and it need deliver pulses or" only a few hundred Volts or less.

For improved pulse control, the embodiment of the invention shown in Fig. 6 may be used. The needle aspulse-voltage power supply 17.' V'lhehigh-voltage supply 19 keeps the needle assembly 2 at a high potential, such i as 25 ltilovolts, and the pulse-voltage power supply 17 delivers voltage pulses of the order of hundreds of volts or less between the plate 23 and the needle assembly 2. i' Assuming that the apparatus of Fig. 6 isdesigned to emit lpositive ions, the needle assembly 2 will be at a high positive potential, such as +25, kilovolts. As the potential ot the plate 23 becomes more positive than VLI-Z kilovolts, the tield pattern at the point 15 will become distorted in such a way that fewer eld lines will end at the point 15, thereby reducing the iield strength and decreasing the ion emission from the point 15. Conversely, as the potential ofrthe plate 23 becomes less positive than +25 kilovoltsfion emission from the point 15 is increased. For example, field emission may be cut off when the potential of the plate 23 reaches +26 kilovolts. y in that event, l-kilovolt pulses from the power supply 17 would. suffice to produce a pulsed ion beam.

' ln general, the voltage output required of the power supply 17 will be on the order of hundreds ofvolts or less.

The effect of the plate 23 upon the ion emission from the point 15 is analogous to the effect of the grid in a conventional triode on the electron current to the plate `of the triode, since small variations in the potential diierence between the needle assembly 2 and the plate 23 change the 'eld strength at the point 15 a great deal, thereby controlling the ion emission from the point 15. As the plate 23 is moved back further away from the point 15 than the position shown in Fig. 6, sensitivity is reduced. As the plate 23 is moved forward towards the tip 15, sensitivity is increased. Sensitivity is greatest when the plate 23 is in front of the tip 15, but in that event some bad effects might be encountered: for example, it might not be possible to attain the necessary 108 volts per centimeter at the point 15 for positive-ion emission, or the necessary 107 volts per centimeter for negative-ion emission. Y

Having thus described the principles of the invention,

" Vtogether with several illustrative embodiments thereof,

it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the inventionrbeing set forth in the following claims.

1 claim:

l. An ion source comprising in combination a hollow member at least a part of which is permeable to a gas and having at least one point on its external surface, means to compress said gas into said hollow member, and means to produce an electric ield at said point vof suicient strength to cause ionization of said gas in the Yvicinity of said point.

2. A positive-ion source in accordance with claim 1, wherein the strengthV of said electric eld is at least of the order of l03 volts per centimeter, and wherein said electric lield is so oriented as to attract positive ions from said point.

3. A negative-ion source in accordance with claim l, wherein the strength of said electric held is at least of the order of 107 volts per centimeter, and wherein said electric lield is so oriented as to attract negative ions from said point.

4. An ion source in accordance with claim l, wherein said hollow member comprises a hollow tube terminating in a hollow needle the lateral wall VofY which is permeable to said gas.

5. An ion source in accordance with claim 1, wherein said hollow member comprises a hollow tube terminating p in a solid needle which is permeable to said gas.

6. An ion source in accordance with claim l, wherein said hollow member comprises a hollow tube terminating in a needle having a shaftV permeable to said gas and having a hard point.

7. An ion source comprising in combination a hollow member at least a part of which is permeable to a gas and Vhaving at least one point on its external surface, means Y to compress said gas into said hollow member, an electrode spaced from said point, and means to impress a potential difference between said point and said electrode of sufficient magnitude to cause ionization ofV said gas .in the vicinity of said point. Y

8. A positive-ion source in vaccordance with claimr7, wherein the Vradius of curvature of Vsaid point in centimeters is ofthe order of .2 times said potential difference in volts divided by the electric iield in volts per centimeter at said point, wherein said'electric eld is atleast of the order-of 108 volts per centimeter, and wherein said point is at a positive potential with respect to said electrode. Y Y

9. A negative-ion source in accordance with claim 7,

- wherein the radius of curvature of said point in centimeters is of the order of .2 times said potential dierence inrvolts divided by the electric field in volts per centimeter at said point, wherein said electric eld is at least of the order of l0" volts per centimeter, andrwherein said point is at a negative potential with respect to saidelectrode.

An ion source comprising in combination: a hollow member at least a part of which is permeable to a gas and having at least one point on its external surface,

means to compress said gas into said hollow member, means to produce an electric field at said point of sufficient strength to cause ionization of said gas in the vicinity of said point, and means to heat said point.

ll. An ion source comprising in combination a hollow member at least a part of which is permeable to isotopes of hydrogen and having at least one po-int on its external surface, means to compress at least one isotope of hydrogen into said hollow member, and means to produce an electric iield at said point of suiiicient strength to cause ionization of said isotope of hydrogen in the vicinity of said point.

12. An ion source in accordance with claim l1, wherein the permeable part of said hollow member is composed of palladium.

13. An ion source in accordance with claim 11, wherein the permeable part of said hollow member is composed of iron and wherein means is provided to heat the permeable part of said hollow member.

i4. An io-n source in accordance with claim ll, wherein the permeable part of said hollow member is composed of nickel and wherein means is provided to heat the penreable part of said hollow member.

l5. An ion source comprising in combination: a hollow member at least a part of which is permeable to a gas and having at least one point on its external surface, means to compress said gas into said hollow member, electrode 'spaced from said point, and means to apply voltage pulses of short duration between said point and said electrode of suliicient magnitude to cause ionization of said gas in the vicinity of said point during said pulse.

16. An ion source comprising in combination: a hollow member at least a part of which is permeable to isotopes of hydrogen and having at least one point on its external surface, means to compress at least one isotope of hydrogen into said hollow member, means to produce an electric eld at said point of sufficient strength to cause ionization of said iso-tope of hydrogen in the vicinity of said point, and means for focusing into a well-collimated beam the ions which are pulled from said point by said electric field.

17. An ion source comprising in combination: a hollow member at least a part of which is permeable to a gas and having at least one point on its external surface, means to compress said gas into said hollow member, a first electrode spaced from said point, means to impress a potential diierence between said point and said iirst electrode of suicient magnitude to cause ionization of said gas in the vicinity of said point, a second electrode in the vicinity of said point, means to impress a Voltage diierence between said point and said second electrode of a magnitude small relative to the potential difference between said point and said tirst electrode, and means to vary said voltage dilerence so as to control the emission of ions from said point.

18. An ion source comprising in combination: a hollow member at least a part o which is permeable to a gas and having at least one point on its external surface, means to compress said gas into said hollow member, a first electrode spaced from said point, means to impress a potential diierence between saidvpoint and said lirst electrode of sufficient magnitude to cause ionization of said gas in the vicinity of said point, a second electrode in the vicinity of said point, and means to impress voltage pulses between said point and said second electrode of a magnitude small relative to the potential difference between said point and said irst electrode.

19. An ion source comprising in combination: a hollow tube terminating in a needle at least a part of which is permeable to a gas, means to compress said gas into said tube, an electrode spaced from the tip of said needle, means to impress a potential diierence between said needle and said electrode of suicient magnitude to cause ionization of said gas in the vicinity of the tip of said needle, an apertured plate surrounding said needle in the vicinity of the tip thereof, means to impress a voltage ditte-rence between said needle and said apertured plate of a magnitude small relative to the potential difference between said needle and said electrode, and means to vary said voltage difference so as to control the emission of ions from the tip of said needle.

References Cited in the iile of this patent UNITED STATES PATENTS 2,287,620 Kallmann lune 23, 1942 FOREIGN PATENTS 697,105 Great Britain Sept. 16, 1953

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