US3032490A - Destruction of neutral particles in a device for producing a high density plasma - Google Patents

Description

May 1, 1962 A. SIMON 3,032,490

DESTRUCTION OF NEUTRAL PARTICLES IN A DEVICE FOR PRODUCING A HIGH DENSITY PLASMA Filed April 28, 1958 n (p= 0 p) ION 24 SOURCE INVENTOR.

Alber1 Simon ATTORNEY BY Eu M% United States Patent assignor to the United United States This invention relates to a novel method and apparatus for burning ou neutral particles in a region where a plasma is to be formed and contained such, for example, as disclosed in co-pending application Serial No. 728,754, filed April 15, 1958, entitled, Method and Apparatus for Trapping Ions in a Magnetic Field, by John S. Luce, and the present invention is an improvement over the system of this co-pending application.

In the device described in that application, high-energy molecular ions are injected into a confining magnetic field perpendicular to the lines of magnetic force. At some point in the orbit of these ions in the magnetic field, a portion of them are caused to dissociate and/or ionize to form atomic ions. These resultant atomic ions have one-half the momentum of the original molecular ions and hence have one-half the radius of curvature in the field. If the center ofthe orbits of these atomic ions coincides with the axis of the magnetic field, the ions-will circulate in a ring. If the center of the orbits and the axis of the machine do not coincide, the atomic ion orbit will precess about the point of origin of the atomic ion. The ions will circulate until-a charge exchange occurs with one of the neutral gas atoms in the system.

In any such device there are present, particularly at startup, a large number of residual neutral particles (-3x10 3 l0 particles per cm?) in the reaction vessel before any reaction is begun. These neutral particles are gas molecules which remain in the device even after evacuation, since available vacuum pumps cannot, of course, create a perfect vacuum. These neutral particles are at relatively low energies-(cold) compared to ions in the plasma, and are deleterious in that they will remove hot ions from the system through the charge exchange process. When one of the circulating ions undergoes charge exchange, the hot ion strikes a neutral and picks up an electron, thus becoming a fast neutral particle, while the original low-energy neutral thus becomes a cold ion. The fast neutral, not contained by a magnetic field, is lost to the system and the cold ion left behind may be readily lost through the ends of the reaction vessel (through the mirrors in a mirror type machine). In this manner input energy is drained out of-the system. If, therefore, any appreciable number of neutrals remain in the system they will remove hot ions by charge exchange and few, if any, of the ions will survive to form a plasma. No further progress towards building up a hot plasma can be made until the neutral background is greatly reduced. For ions of 300 kev. energy, for example, neutral pressure must be reduced to less than one part in 10 of the ion background, since the cross section for charge exchange is substantially as great as that for coulomb deflection. By direct pumping, an initial pressure of 10- mm. Hg or less would be required. The trapped beam itself helps to destroy the neutral particles by ionization, however, allowing a plasma to.form.' The temperature. and density of the plasma cannot be raised, however, until substantially all the neutrals are removed.

Faced with this apparent limitation on plasma growth and heating, applicant has found a method by which the neutral particles may be effectively removed, or burned out of the system. He has discovered that for each ice plasma volume and pressure in the region of the plasma there exists a critical current of atomic ions which, when trapped by a magnetic field as a result of dissociation and/or ionization of an injected molecular beam by a high intensity are discharge to form a trapped circulating atomic ion beam, will destroy neutral particles as fast as the particles are flooding into the system. He has further found that by changing one of the controlled conditions, such as pressure, or increasing the injected molecular ion current, more atomic ions than the critical current may be provided, thus allowing a plasma to build up substantially unhindered by cool neutrals. Such build up might allow the arc to be extinguished and the gas feed continued, using the molecular breakup occuring in the plasma itself.

Accordingly, it is an object of this invention to provide a method and apparatus for burning out neutral particles in an evacuated region to allow a plasma to build up substantially unhindered by cool neutrals.

It is another object of this invention to burn out neutral particles in an evacuated region bycontrolling the relationship between the pressure within the region, and the injection voltage and current of the molecular ions to provide a critical input current of the atomic ions for thus producing a plasma which'is essentially free of neutral particles.

These and other objects will become apparent upon a consideration of the following detailed specification when considered with the drawing in which:

FIG. 1 shows an example of a plasma generator in which the principle of this invention may be effected, and

FIG. 2 shows a graph of critical current curves for various pressure conditions in the device of FIG. 1.

Since there are no available complete physical descriptions of devices for forming and containing energetic ionized plasmas, this invention will be described particularly with reference to the device of FIG. 1 for purposes of clarity.

Referring to FIG. 1, a device is shown in which high energy molecular ions are injected into a chamber normal to a containing magnetic field and high intensity arc, for igniting a plasma by trapping atomic ions in a manner fully described above. The device of FIG. 1 comprises an outer cylindrical shell 10 with joining end walls 13 and 14. End wall 13 has a circular opening to which a tubular member 17 is affixed. Member 17 has an end closure member 19 in which the cathode 1 is fixedly mounted. End wall 14 has a circular opening to which is afiixed a tubular member 18. Member 18 has an end closure 20 in which the anode ,2 is fixedly mounted. Outer shell 10 is provided with a circular opening to which is attached a tubular member 21 which in turn has affixed thereto an end closure member 22. Fixedly mounted in said member 22 is tubular member 23, provided with a reduced portion which connects with an aperture in liner '7. A conventional ion accelerator tube 29 communicates with member 23, and serves to accelerate molecular ions from an external ion source 24 to relatively high energies. The accelerator tube may be energized by a conventional high voltage generator. A suitable high current source of ions may be provided by apparatus'such as Set forth on page 18 of Nucleonics,

vol. 9(3),(l95l); Rev. Sci. Instr., vol. 24, p. 394 (1953),

and. 28 have circular openings in axial alignment with the anode and cathode. Surrounding the cathode 1 and anode 2 are suitable tubular baffles 3 and 4, respectively, which extend through the openings in said walls 27 and 28, respectively. Liner 7 has a pair of circular openings in alignment with a pair of circular openings in outer shell 10. The aligned openings are. joined by insulating bushings 25 and 26, respectively. The inner chamber, formed by liner 7 and walls 27 and 28, is connected to a vacuum through openings 15 and 16 of bushings 25 and 26, respectively. Outer liner 10 also has a pair of additional openings 11v and 12 connected to a vacuum, said openings being connected to an outer chamber located between the shell 10 and inner liner 7. A circular mag,-

netic mirror coil is mounted .on apertured wall 8 and is disposed around the outside of inner liner 7 between the ion source tube 23 and bushing 25. Another circular magnetic mirror coil 6 is mounted on apertured wall 9 and is disposed around the outside of inner liner 7 between ion source tube 23 and bushing 26. These mirror coils provide acontaining magnetic field whose direction.

is shown, by the arrow H.

In operation of the device set forth in FIG. 1, a high intensity are discharge is initiated between the cathode and anode electrodes in a conventional manner. The inside and outside chambersare evacuated and the pressure of the inside chamber is maintained at. approximately mm. Hg, while the outside chamber pressure is maintained at approximately 10- mm. Hg, for example. The mirror coils 5 and 6 have an inside diameter of. 17 inches and a spacing between the inner faces of the coils of 18% inches. With these dimensions, a cylinder can be inscribed whose rims just touch the inner edge of the coils and the volume of such a cylinder is then equal to 69x10 cm. The plasma which is formed by dissociation of the. high energy molecular ions as they pass through the high intensity are is confined and trapped within the said inscribed volume by the magneticv field H. The gas used for the ion source input is deuterium, and the injection voltage of the molecular ions is approximately 500 kev., for example, which results in atomic ions of substantially 250 kev. energy.

As stated above, the device of FIG. 1 at startup will have large numbers of neutral particles in it, and these neutral particles will remove hot ions from the system because of the charge exchange process. The cross section for charge exchange is a steeply decreasing function of the atomic ion velocity above about 30 kev.

However, even at 300 kev. it has a value of about 3 X 10" cm. which is very much larger than the coulomb cross sections involved in energy loss and particle deflection. Every charge exchange destroys a neutral, since, as discussed above, the hot ion strikes a neutral and picks up an electron, thus becoming a fast neutral particle which is not contained by the magnetic field and is readily lost to the system. The ionization caused by the fast ion will remove many neutrals by this means, since the ionization cross section is more than 20 times as large as that for charge exchange at 300 kev. As stated, applicant has discovered that there is a critical value for the input atomic current at which the ions burnout the neutrals as fast as they are flooding into the plasma. Once this value is exceeded, the ions get ahead of theneutrals and the plasma builds up within the abovementioned inscribed volume, hence burning out additional neutrals and the system cleans out the neutrals in short order. This process is known as burnout and, in this instance is provided by the energetic ions. Thus the achievement of burnout would result in rapid buildup in ion density. The hot ions, now persisting much longer in the system since not lost by charge exchange, again are degraded somewhat in energy as a result of energy transfer to for example,

colder electrons and ions. Concurrently, their motion would randomize, and a high-temperature, high-density plasma would be formed.

To determine the actual magnitudes of critical current, pressure, and other controlled conditions, the effect of burnout by energetic ions has been investigated numerically by applicant. The buildup of ion density in the plasma in a mirror-type. device such as that of FIG. 1 can be expressed by the time-dependent equations.

where n =atomic ion density n =neutra1 density I =atomic ion current S=surface of area of plasma N=density of neutrals in external, manifold, which is substantially 3X10 pressure in mm. Hg

P=the probability of scattering into the escape cone,

and is equal tov 1cos 0 V=volume occupiedby the plasma v=velocity of ions v =thermal velocity of: neutrals a =coulomh cross; section in the plasma for scattering by multiple small-angle collisions through degrees.

a =chargeexchange cross section a =ionization cross section The escape cone referred to in term P above, is that region bounded by a surface forming an angle with the axis equal to the critical angle for containment; This critical angle (0 is obtained from the expression where R=mirror ratio of the device; that is, the, ratio of the magnetic field strength in the mirror region (between the coils) to that in the uniform central region.

The charge-exchange cross section values may be taken from the measured values published in Physical Review 103, 896, (1956) assuming for deuteron that the cross section is the same at the same relative volocity. The value. is 5.5. 10- cm. at 50 kev., for example. The ionization cross section values may be computed from the formula given by Bethe and Ashkin, Experimental Nuclear Physics, vol. 1, part II, published in 1953. The value is 1.0 lO- cm. at 250 kev., for example. The coulomb cross section values may be computed from the formula where e is the charge on the electron, and E is the average energy of an ion in the plasma.

In the first equation above, the first term on the right represents the constant source input; the second term takes into account mirror losses; and the third term represents loss by charge exchange. In the second equation, the first term on the right represents the streaming of neutrals into the plasma; the second term represents the out-streaming from the plasma; and the third term shows the effects of neutral burnout by ionization and charge exchange.

As discussed above, applicant has found that burnout occurs at the critical point at which the neutrals are being ionized at a rate equal to the rate of their entry into the system. The number of neutrals destroyed by a fast ion before the ion itself is lost can be expressed as where I is the total current of neutrals streaming into the plasma as defined by the formula:

The input current I used herein is the value of atomic ion current produced as a result of dissociation and/ or ionization of the molecular ion beam. If the critical value of molecular ion current is desired, the value of 1 obtained in Equation 3 must be multiplied by a factor proportional to the break up efficiency of the arc. For normal operation at 150 volts, 300 amperes, this (factor is 25%. Since the neutral in-streaming varies linearly with pressure, the value of critical current also varies linearly with pressure.

Some of the parameters of the above equations for the device of FIG. 1 for computing the critical current curves of FIG. 2 are as follows. The volume inscribed by the mirror coils is 6.9 10 cm. as aforementioned. Since only a part of this volume will be occupied by a plasma, the parameter V was chosen to be approximately 4x10 cm. It was further assumed that the plasma would have a semi-spherical shape with a typical radius of 8 inches. Therefore, the surface-tovolume ratio would be about 3/ r or 0.15 cm.'- and S would then be about 6x10 cm}. At the energy involved v is 1.9x l0 cm./sec. a is 4.l l0'- v=l.0 l0 cm./sec., 01 is 1.0 1()- cm. and o' is 5.5 l0 cm. At a pressure of mm. Hg, N is about 3x10 The injection atomic ion energy is 250 kev. and the molecular ion energy is 500 kev. At these energies, the critical ratio I /I from Formula 3 above, can be determined by substituting the values of w and a as given above, into the formula. The numerical value of the critical ratio at the energy involved. The value of 1 can be determined by multiplying the measured value of the injected molecular ion current by the known break-up efficiency of the arc discharge. For example, a molecular ion current of 80 ma., with an arc break-up efliciency of 25%, would produce an atomic ion current of 20 ma.

For a given pressure and energy, all of the parameters in Equations 1 and 2 above are known and are fixed for a given machine, except n and n which vary as 1 is varied. Since the value of 1 can be determined, as discussed above, the values of n and n can be determined by solving the two diflerential Equations 1 and 2 for each value of 1 For the same energy and a different pressure, the value of N will change since it varies linearly with the pressure as set forth above, and is substantially 3X10 times the pressure in mm. Hg. Therefore, it can be seen that n and n can be determined for different values of 1 for different selected pressures for plotting the curves shown in FIG. 2. Where the two curves (n, and n for a given pressure cross each other gives an indication of the value of critical atomic ion current required for burnout. Equation 3., when solved, agrees with the results of Equations 1 and 2, as plotted on FIGURE 2. It can thus be seen that use of Equation 3 is a more direct way of obtaining the value of I for a given machine operating at a given energy and pressure. 5

The results of the computations for the final steadystate values are shown in FIG. 2 when plotted as a func tion of pressure and input current. These results also show the linear relationship between the critical current 6 V and the pressure. The values of input current predicted by Equation 3 are shown by the arrows. Thus, it may be seen that for a machine of the size shown in FIG. 1, input current of at least about ma. is required for burnout when the pressure is 10- mm. Hg (IO- but only about 8 ma. is required if the pressure is 10" mm. Hg (IO- p). The pressures are shown in FIG. 2 as microns. They could be shown as mm. Hg, if desired,

since one micron is equal to 10- mm. Hg.

Burnout is not a suddenly occurring phenomenon as the current is increased but rather a smooth transition over a relatively-narrow range of current. It has been shown that for currents well above the critical value, the steady-state neutral density, n can be expressed as:

This shows that the neutral density approaches zero as I becomes very large.

It will be apparent to those versed in the art that the absolute values shown in FIG. 2 will vary depending upon the parameters selected for a particular machine. However, the general relationships will not vary, so that a value of critical current can be calculated for any machine of this type and that a current in excess of this value will produce burnout of the neutrals in the machine.

The value of the critical atomic ion input current in any machine of this type can be controlled in at least four ways. First, the current of molecular ions can be varied by changing the conditions in the ion source, such as by varying the source are voltage for example and thus changing the quantity of the injected molecular ions. Second, the injection voltage of the ion source may be varied and thus varying the energy of the injected molecular ions. Third, the value of the critical atomic ion current can be controlled by changing the break up efficiency of the mechanism causing ionization and/or dissociation of the molecular ion beam. If an arc is being used, a change in voltage or current of the are changes the percentage of atomic ions formed. A fourth control over the value of the critical atomic ion current can be effected by changing the pressure within the plasma chamber.

The use of the burnout principle is not necessarily limited to mirror-type machines. For example, a plasma can be formed in a portion of a stellarator-type machine by providing one section with temporary magnetic mirrors. The plasma, once formed, would be permitted to fill a larger portion of the machine by gradually moving the mirrors outward from each other. The high energy injection, if the input current is sufficiently large will cause burnout of the neutrals in such a machine. Once burnout has been achieved, the competing charge exchange process is eliminated and hot ions will collide with cold electrons, heating the electrons.

By using high-current, high-energy injection, it is probable that the added pumping effect of the trapped plasma on the entire vacuum system of a machine is sufficient to reduce the neutral density external to the plasma region (N in Equation 2). Under some conditions of operation this may be sufficient to evacuate the entire system even before the burnout condition, as described above, has been reached.

This invention has been described by way of illustration with a particular class of devices, but it should be apparent that the invention is equally applicable in devices other than those described. It will be appreciated by those versed in the art that applicant has discovered and herein described a method for forming from a group of circulating ions moving in an atmosphere of neutral particles, wherein the charge-exchange process predominates greatly, a high-density, high energy plasma.

What is claimed as novel is: V 1. In a method of forming a plasma trapped within a magnetic field by evacuating a region within said field,

the current of neutral particles I entering said field being directly proportional to the pressure therein, injecting molecular ions of deuterium at substantially 500 kev. into said field, and dissociating said ions With an energetic arc discharge to form a current of I substantially 25-0 kev. atomic ions, said atomic ions being trapped and circulating in a ring, the improved method for heating the cold ions and electrons associated with said atomic ions comprising establishing within said field an atomic ion current to neutral particle ratio 1 /1 of at least .05 2, whereby neutral particles are removed by said atomic sms f te t n the nt id re ion Th meth of l m 1 ere ai pr s ur ithin said region is substantiallylod mm, Hg and said current I is at least 8 milliamperes.

,8 References Cited in the tile of this patent 'UNITED STATES PATENTS 7 O HE REF E E CES Atornics and Nuclear Energy, February-19.58, pp. 58,. 9

Nucleonics, February 1958, pp. 90-93, 151-155.

Atom, No. 25, November 1958, Monthly Information Bulletin of the United Kingdom Energy Authority, p. 13.

Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic Energy, vol. 31, United Nations, Geneva, 1958, pp. 298-304.

Gow et a1. Apr. 28, 1953 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,032A90 May 1, 1962 Albert Simon It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 4, line 21, for "N" read N line 46, for

"volocity" read velocity line 47, for "50 read 250 column 5 line 33 after "cm./sec.", first occurrence insert a comma; column 6, lines 15 to 17 after the equation insert Signed and sealed this 20th day of November 1962.

(SEAL) Attest:

DAVID L. LADD Commissioner of Patents ERNEST W. SWIDER Attesting Officer

Claims (1)

1. IN A METHOD OF FORMING A PLASMA TRAPPED WITHIN A MAGNETIC FIELD BY EVACUATING A REGION WITHIN SAID FIELD,

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