US3429501A - Ion pump - Google Patents

Description

Feb. 25, 1969 A. R. HAMILTON ET AL 3,429,501

ION PUMP Feb. 25, 1969 Filed Aug. 30, 1965 A. R. HAMILTON ET AL ION PUMP Sheet 3 @f5 United States Patent O 3,429,501 ION PUMP Allen R. Hamilton and Thadeus S. Graczyk, Rochester, N.Y., assignors, by mesne assignments, to The Bendix Corporation, Detroit, Mich., a corporation of Delaware Filed Aug. 30, 1965, Ser. No. 483,401 U.S. Cl. 230-1 5 Claims Int. Cl. F04b 37/14, 49/00; H013 9/38 ABSTRACT F THE DISCLOSURE A high eiciency ion pump and method of operating same. Efficiency of the operation of an ion pump is improved by providing power sensing and power regulating means in the circuit between the pump power supply and the pump. A feedback loop from the sensing means to the regulating means is provided and the combination is arranged such that power is supplied to the -pump at approximately the optimum power level over an extended range of pump pressures.

The present invention relates to high vacuum pumps and in particular to ion pumps.

A recent innovation in the eld of high vacuum pumps has been the development of pumps variously designated as electrical vacuum pumps or ion pumps. These pumps operate on a principle similar to the early Penning gauges.

Brielly, molecules of a gas to be pumped are ionized by an electron discharge in an area between an anode and a collector and are accelerated 'by an electric field toward the collector. The electron discharge may be obtained by providing an electric iield between pump electrodes such as the anode and the collector in a diode pump or between the anode and a separate cathode in a triode pump. A source of sputtering material may be located within the pump envelope and material sputtered therefrom by some of the gas ions as they move toward the collector. The source of sputtering material may be in the form of a cathode in a triode pump. A portion of the s-puttered material moves to the collector and covers the ions previously driven into the collector to prevent their return to the pump envelope. In this way molecules are pumped by the sputtered material thereby permanently removing them from the space to be evacuated.

Such pumps have been found very etective in achieving substantially higher vacua than has previously been possible with the various pumps known in the art. However, because such pum-ps are most useful and most efiicient at low pressures it is normally necessary to provide a forepump for reducing the pressure within the vessel t0 be evacuated to the upper pressure limits of the ion pump. Such a pressure is normally -2 torr. However, even at this pressure the typical ion pump is ineliicient and subject to stalling or cessation of pumping due to the relatively heavy gas load.

In addition, there are other problems encountered in the operation of the pump. Ion pumps are most ellicient when operating at optimum power dissipation levels. This is the level at which the pump has its greatest net pumping ability. Above this level the pump electrodes become so hot that gas molecules are released rather than held. Below this level the pump is operating at less than capacity and is inefficient. However, this optimum level of operation is achieved by conventional ion pumps and power supplies in a very limited range relative to the pressure ranges over which the pump is operable. At lts upper pressure limit it is characteristic of an ion pump t0 have a high current flowing between the pump electrodes and hence the voltage between the electrodes is quite 3,429,501 Patented Feb. 25, 1969 ICC low. The high current is due to the relatively large quantities of gas present which, in turn, yields a large quantity of positive ions owing toward the collector and a relatively low difference of potential between the pump electrodes. Under these conditions, the pump has a very low gas-handling capacity because it is operating considerably below optimum power level and hence its efliciency is quite low.

Below the high-pressure range is a second range 0f pressures wherein the pump still experiences a relatively high current, and the voltage between the electrodes increases at a rapid rate as the amount of gas to be pumped and hence the pressure Within the pump decreases. This combination of effects contributes to a rapid rise in the amount of the power dissipated by the pump to the point where it exceeds the optimum operating level. When this condition occurs two effects are encountered. First, the pump experiences excessive heating which may result in damage to the pump by virtue of melted electrodes and second, operation at relatively high temperatures results in a slow net-pumping speed again detracting from optimum pump elhciency.

The need for preventing excessive amounts of power from being dissipated by the pump has already been recognized and regulating means of various types have 'been provided in the power supplies used with ion pumps in order to maintain the maximum power supplied to the pump electrodes at or below safe operating levels. In the main these regulating devices have been current limiting and voltage regulating devices which prevent the current or Voltage from exceeding certain maximum limits.

While successful in preventing the pump from destroying itself by exceeding its rated capacity, the regulating devices have done nothing to enhance the eiiiciency of operation of a typical pump. In elTect, the prior art regulators have served only to cut off the peaks of the various curves representing voltage, current and power supplied to the pump but have not reshaped these curves or adjusted pump parameters to cause the pump to operate at its maximum possible elliciency over the maximum possible range of pressures.

The present invention is directed to such an end. It attacks the problems of inefiicient pumping by providing a power supply that limits the pump operation to optimum levels and holds the actual operating level substantially at the optimum for the particular pump over an extended range of pump pressures regardless of the changing impedance of the pump as the pressure changes. The limits of the range over which the power can be held constant are determined by characteristics of the supply itself, viz., the maximum current capacity at the high pressure end of the pump operating range and the maximum output voltage at the low pressure end. The supply is designed such that the maximum current and maximum voltage obtainable from the supply correspond to the upper and lower operating limits of the pump respectively.

In terms of apparatus, the invention provides a power supply for an ion pump that has means for detecting changes in the power supplied to the pump and powerregulating means which are operated in response to the power change detecting means so that the power supplied to the pump is maintained substantially at the optimum level.

In terms of method, the invention comprises the steps of connecting an energized power supply to an ion pump to provide a difference 0f potential and an electron discharge in the interelectrode space of the pump, monitoring the power supplied to the pump by the power supply and controlling the power supplied in response to the monitored variable such that the power supplied to the pump is maintained substantially at the optimum operating power level over a substantial portion of the operating range of the pump.

The method of this invention provides for maintenance of the power supplied to the pump at a substantially constant level over a substantial portion of the operating pressure range of the pump. Ideally, this constant level is the optimum power dissipation level of the pump in question. In practice, approximations of the ideal are still capable of effecting substantial improvements in pump throughput.

To obtain the preferred power characteristic curve, a power supply is provided for producing a curve which increases rapidly from zero to the optimum dissipation level of the pump for nearly all values of current flowing between the pump electrodes and then rapidly approaches zero at the low-pressure limit of the pump. The present invention relates to various embodiments of the power supply for producing or closely approximating such a curve. All embodiments are based on the principle that if the power supplied to the pump is maintained nearly constant at the optimum power level of the pump, operation will be most etiicieut. By adapting one of the various specific embodiments of this invention to an ion pump, improvement has been found to be on the order of 250% for air.

These advantages and others will be more fully understood by reference to the appended detailed description in conjunction with the following figures:

FIG. l depicts a typical power characteristic and voltage regulation curve for a conventional prior art ion pump power supply;

FIG. 2 depicts the power and voltage regulation characteristics for an ideal power supply;

FIG. 3 is a block diagram of the circuit of this invention;

FIG. 4 is a schematic diagram of one embodiment of the circuit in FIG. 3;

FIG. 5 is a set of characteristic curves related to the circuit shown in FIG. 4;

FIG. 6 is a schematic of an alternative embodiment of the circuit shown in FIG. 3, and

FIG. 7 is a set of characteristic curves related to the circuit shown in FIG. 6.

In FIG. 1 are depicted the power and voltage regulation curves of a conventional power supply used with a typical ion pump. Power and -voltage are plotted along the ordinate 2 and current along the abscissa 4. Indicated by dotted line 6 is the optimum power dissipation level of the pump. Curve 8 representing the power characteristic, rises from a value of zero at the origin to a maximum 9 which is considerably above the rated level 6 and then diminishes again to zero at a current value on the order of onehalf ampere. The area adjacent the origin 5 corresponds to the low end of the pump pressure range while the high current portion corresponds to the high end of the pressure range. Typically, the pump pressure range extends from 10-2 torr to approximately l0lo torr. Above 1()-2 torr the gas load on the pump is too heavy and the pump cannot be started. Below 10-10 Itorr, the number of molecules being released back into the space to be evacuated may equal the number ybeing pumped per unit time resulting in a net pumping speed of zero.

Curve 10 represents the voltage-current characteristic of a conventional power supply and as shown therein the voltage is relatively high at low pressures and diminishes approximately to Zero at the high end of the pressure range.

Also depicted in FIG. 1 are two sectors 12 and 14. Sector 14 is bounded by the points at which the power curve falls below the power rating 6 and the upper operating limit of the pump. Sector 12 is defined by the two points at which the power curve passes through the power dissipation level 6. In sector 12 excessive heating of the electrodes and other parts within the pump envelope is encountered because the power supplied to the pump exceeds the pump rating. Sector 14 is essentially an area of relatively low voltage and relatively constant current. In this area, the power supplied to the pump is considerably below rated pump power and hence pump capacity in this area is very low.

Ideally the operating characteristics of an ion pump power supply should approach the power characteristic represented by curve 16 and the voltage characteristic represented by curve 18 depicted in FIG. 2. At the upper threshold of operation corresponding to the high pressure end of the pump operating range, the power increases at a rapid rate to the level corresponding to the optimum power dissipation level of the pump aud maintains this value throughout the operating range. Immediately prior to the lower threshold (the low pressure end of the pump range), the power approaches zero. Similarly, the voltage regulation curve 18 diminishes from a very high value at the low pressure end to a relatively low value throughout most of the operating range of the pump and approaches zero at the upper limit of the pump.

A block diagram of a power supply in accordance with this invention is shown in FIG. 3. In that figure a pair of input terminals 22 connect a source of AC power (not shown) to a power regulating means 24 such as a current limiting device or a voltage regulator. Connected to regulator 24 is a low reactance transformer 26 for stepping up the input voltage to a level suitable for use with the ion pump. The secondary of the transformer 26 is connected to two nodes of a full wave diode rectifier bridge 28. The remaining two nodes of the bridge 28 are connected to an output power-sensing device 30 and thence to output terminals 32 which connects the supply to the dischargeproducing electrodes of an ion pump 33. The dischargeproducing electrodes may be the anode and collector in a diode pump or the anode and cathode in a triode pump. A feedback connection 34 is provided from the powersensing device 30 to the input power regulator 24. Ideally, device 30 continually monitors power supplied to the pump and as this value tends to deviate from the optimum value, a control signal is fed back to the regulator to increase or decrease power drawn from the source. In practice, one or more fixed threshold devices such as silicon controlled rectiiers, saturable reactors, relays or other switching means may be used in the power regulator.

As will be illustrated in subsequent embodiments there are a number of alternative ways in which the power supply of this invention can be constructed in order to perform the desired function of regualting the power supplied to the pump to thereby increase pump efliciency. One alternative is illustrated in FIG. 4.

In that gure input terminals 36 are connected to a power source (not shown), to one side of a primary winding 38 of a transformer 40 and to an armature 46 of relay 45. Primary winding 38 is provided with a tap 42 connected to a relay contact 44. The other side of the primary winding is connected to a second relay contact 48. The secondary winding 39 of the transformer 40 is connected to two nodes of a full wave rectifier diode bridge 48. The output from the bridge is taken from nodes 50 and 52.

Output terminals 54 and 56 connect the power supply to an ion pump 57. Connected in series between node 50 and output terminal 56 is a relay coil 58 for the armature 46 having a by-pass capacitor 60 connected in parallel therewith. Node 52 of bridge 48 is connected to the other output terminal 54. Coil 58 provides the means whereby relay 45 is operated to select the desired number of turns on a primary winding.

In operation, this circuit performs as follows. The ion pump is usually used in conjunction with a forepump for pumping the space to be evacuated down to approximately 10"2 torr before the ion pump operation is initiated. Input terminals 36 are connected to a source of power and the output terminals 54 and 56 are connected to the ion pump 57. When a suiiiciently low pressure has been reached by the forepump, the source of power energizes the power supply and the ion pump.

At high pressure, power is drawn from the supply at a high current and relatively low voltage. This is due to the presence of relatively large numbers of gas molecules in the space discharge between the pump electrodes. The high current supplied to the pump energizes the coil 5S and draws relay armature 46 against contact 48 thereby putting the full number of turns in the primary winding 38 into the circuit. In this condition, (with a relatively high number of turns in the primary) the voltage induced in the secondary winding is relatively low, and the conditions for preferred operation of the pump are satisfied, that is, a high current and a low voltage and power consumption approximating the optimum level. As pump operation continues and the pump pressure begins to decrease, the current flow between the anode and collector decreases until it reaches the dropout current rating of the relay coil 58. As current diminishes toward this drop-out value, the potential differenceV between the anode and cathode increases such that the amount of power supplied to the pump remains relatively the same.

When the drop-out current rating of the relay has been reached, relay armature 46 releases from contact 48 and engages relay contact 44, decreasing the number of turns in the primary winding of the transformer. 'Iliis decrease in the number of turns in the primary winding has the effect of increasing the voltage induced in the secondary winding 39 so that even though current supplied to the pump has now been reduced substantially compared to the initial operating values, the amount of power supplied to the pump is maintained relatively constant by virtue of the increase in the voltage supplied to the pump as the result of the release of the armature 46. As the pump pressure reduces still further, the decrease in current liowing between the electrodes of the pump can no longer be compensated by the increased voltage supplied in the secondary and the total power supplied to the pump begins to diminsh from approximately the optimum value for the pump towards the cut-off point, namely, the low pressure operating limit of the pump.

In FIG. 5 are .shown the voltage-current curve 62 and power characteristic curve 64 for the ion pump and power supply of FIG. 4. As depicted therein, the power is maintained at a value more nearly approximately the optimum power value 61 for the pump over a substantially larger pressure range than is possible with power supplies of the prior art. The power and voltage-current characteristic curves for a prior art power supply are shown `by the dotted characteristic curves 65 and 66. As shown in FIG. 5, the power characteristic curve 64 rises from zero at the high-pressure end of the pump range to a peak at 68 and then begins to diminish. However, by virtue of the adjustment of the number of turns on the primary, characteristic curve 64 tends to remain relatively constant over an extended range below point 68 and consequently the gas absorbed by the pump is substantially increased in that range. Similarly the voltage curve 62 undergoes an abrupt change at the points of relay operation.

The drop-out and pull-in points of the relay coil 58 are indicated at 70 and 72 respectively. Any current value exceeding the pull-in value 72 causes the relay to operate and to be held in the closed position until the drop-out value 70 is reached. The dotted extensions 67 and 74 of the voltage-current characteristic curve indicate the voltages that would be supplied to the pump were it not for the operation of the relay. By virtue of the operation of relay 45 at the drop-out point 70, the number of turns in the primary winding 38 is decreased and the voltage supplied to the pump increases substantially to maintain the power supplied at the value approximating the optimum level for the pump. Comparing curves y64 and 66 it can be seen that the power supply of the present invention extends the range of relatively constant power from the supply for a considerable range above and below prior art supplies.

For purposes of analysis FIG. 5 has been divided into four zones or quadrants. In Zone I corresponding to pump start-up and the high pressure zone of pump operation higher current is available with a consequent gain in pump capacity relative to pumps operated with conventional supplies. In Zone II, the midrange of pump operation (approximately 1x10-3 to 5 l0F5 torr) the power characteristic curve 64 is below curve 66 and therefore the electrodes do not become so hot that pumped molecules are released, again providing a net gain in pumping capacity relative to conventional power supplies. The transition from Zone II to Zone III corresponds to the area of relay operation. As pressure diminishes, the relay opens causing the power supply to switch from the voltage-current and power characteristics 74 and `69 respectively to characteristics 67 and 63 respectively. The availability of more current and higher voltage means that the supply can provide more power and hence greater pumping capacity in the lower operating pressure range (5 105 to l l0'7 torr) of the pump than has heretofore been available. In Zone IV (below l l0-r1 torr) it is immaterial which set of characteristic curves the pump is operating on since all of the curves approximate each other in this region. In order to obtain pumping the power supply must be capable of relatively high output voltages.

FIGS. 6 and 7 depict a power-control circuit and characteristic curves in accordance with another embodiment of the invention. As shown in FIG. 6, the circuit comprises input terminals 76 which are connected through voltage relay 78 and current relay '80 to the primary winding of a transformer `82. Relay 7-8 comprises a single pair of contacts 75 and 77 and is shown in the unener-gized condition. IRelay '80 comprises contacts '79, 81 and 91 and a iganged armature 83 and is also shown in the unenergized condition. The secondary winding v87 of transformer '82 is connected to a diode bridge 84. Output node 92 of the bridge is connected through relay winding 86 to output terminal `89. Winding 88 and voltage dropping resistor 93 are connected across output nodes 92 and 94. Windings l86 and 88 are associated with relays 80 and 78, respectively. Output terminal 90 is connected to the bridge output node 94.

Assuming operation at the high pressure end of the pump, a high current iiows between electrodes as the pump is turned on due to the presence of a relatively large amount of gas in the space to be evacuated. Due to the high current, relay coil `86 has a sucient current flowing therethrough to operate relay f80. The pull-in and dropout current value for relay 80 is indicated in FIG. 7 at 103 and 105, respectively. The pull-in and dropout points for relay 78 are indicated at 95 and 97, respectively. With relay l80 operated, contact 75 on relay 718 and contacts 81 and 91 on relay '80 are closed and the secondary winding 87 of the transformer 82 sees a relatively low number of turns on the primary winding and therefore a relatively high voltage is supplied to the pump in the area under the power characteristic curve between the dropout voltage rating of relay 78 and the start-up point of the pump (Zone I). The voltage-current characteristic curve 102 depicts the almost instantaneous rise of the voltage (approximating the high-voltage current characteristic 101) at pump start-up. By virtue of inducing an increased voltage in the secondary, the operating power follows characteristic curve 99 and the power supplied to the pump is more rapidly brought to the optimum pump level 96 shown in FIG. 7.

As the space to be evacuated is pumped, relay 80 remains operated and relay 78 operates when the voltage across the output of bridge 84 reaches the pull-in volta-ge 95. When relay 78 operates, contact 77 closes. In this condition (Zone II), a relatively large number of turns on the primary are presented to the secondary and the voltage induced therein is diminished. This is in keeping with the desired operation of the pump because, by virtue of the reduction of pressure in the pump, voltage supplied to it tends to increase while the current is still at relatively high values. Were the voltage in the secondary winding not reduced, power supplied to the pump would be in excess of the optimum level as is shown by the power characteristic curve -100 of a conventional power supply. By operation of relay 78, the supply switches to the low power and voltage- current curves 103 and 104 and consequently the total power supplied to the pump continues to approximate the optimum value 96.

Operation of the pump continues under these conditions and pressure within the pump continues to drop vuntil current between the pump electrodes diminishes to the dropout value of relay vS and this relay releases (Zone IH). By virtue of de-energization of relay 180, the number of turns on the primary is again reduced to a relatively low number, and the voltage induced in the secondary is increased and the supply switches back to the high power and voltage- current curves 99 and 101 respectively. Relay '80 performs essentially the same function as is performed by relay -45 in FIG. 4 and tends to maintain the amount of power supplied to the pump at the rated level. As shown by the voltage-current characteristic 102, the voltage supplied to the pump by the circuit of FIG. 6 is substantially higher in comparison to dotted characteristic curve 104, the characteristic curve that would be obtained were the number of turns on the primary not decreased by virtue of operation of relay `80. As shown by the power characteristic curve -98 in FIG. 7, the power supplied to the ion pump by the supply of PIG. 6 approximates the optimum value 96 of the pump over a substantial portion of the pump-operating range above and below the prior art power supply characteristic curve 100. Curve 107 represents a prior art voltage-current characteristic curve. The result is a substantial increase in the capacity of the pump in both the high and low pressure ran-ges relative to conventional pumps. The vario-us quadrants indicated in FIG. 7 correspond to those described in conjunction with FIG. 5.

As indicated previously, the present invention provides a method and apparatus for improving the operation of an ion vacuum pump by controlling the power supplied to the pump. In addition to the use of relays to adjust the number of turns in the primary winding of a transformer for obtaining this control, other means such as silicon controlled rectiers and saturable reactors can be used to control the power delivered by the supply to the pump without departure from the scope of the invention. Similarly, switching means other than relays can be employed. Moreover, adjustment of the number of turns on the secondary of a transformer to change the output voltage of the supply accomplishes the same objective as varying the number of primary turns and the preceding detailed description of circuits in which the number of turns on the primary are varied should not be regarde as limiting.

What is claimed is: 1. An ion pump for evacuating gases in a space comprising:

a pump envelope connectable to the space to be evacuated, electrode means mounted within the envelope for producing an electron discharge to ionize a portion of the gases within the envelope. means for producing a magnetic eld extending between the electrode means, a source of sputtering material within the pump envelope, a pair of terminals for connecting the electrode means to a source of electric power, transformer means having a primary and a secondary winding, the primary winding being connected to the pair of terminals,

means for adjusting the number of turns in the primary Winding,

rectifying means connected to the secondary winding of the transformer means, and

power-sensing means connected between the rectifying means and the electrode means for operating the means for adjusting turns on the primary winding for causing the amount of electric power delivered to the electrode means to be maintained substantially at the optimum operating power level of the pump over an extended range of pump pressures.

2. An ion pump for evacuating gases in a' space comprising:

a pump envelope connectable to the space to be evacu ated,

electrode means mounted within the envelope for producing an electron discharge to ionize a portion of the gases within the envelope,

means for producing a magnetic iield extending between the electrode means,

a source of sputtering material within the pump envelope for covering ions driven into the electrode means,

a pair of terminals for connecting the electrode means to a source of electric power,

transformer means having a primary and a secondary winding, the primary winding being connected to the pair of terminals and having a tap for adjusting the number of turns in the primary winding,

rectifying means connected to the secondary winding of the transformer means, and

coil means connected between the rectifying means and the electrode means and operatively engaging the tap on the primary winding whereby power supplied to the electrode means is maintained substantially at the optimum operating power level for the pump over an extended range of pump pressures.

3. An ion pump for evacuating gases in a space comprising:

a pump envelope connectable to the space to be evacuated,

electrode means mounted within the envelope for producing an electron discharge to ionize a portion of the gases within the envelope,

means for producing a magnetic field extending between the electrode means,

a source of sputtering material within the pump envelope for covering ions driven into the electrode means,

a pair of terminals for connecting the electrode means to a source of electric power,

transformer vmeans having a primary and a secondary winding, the primary winding being connected to the pair of terminals and having a tap for adjusting the number of turns in the primary winding,

full-wave rectifying means connected to the secondary winding of the transformer means, and first and second coil means connected between the rectifying means and the electrode means and operatively engaging the tap on the primary whereby the power supplied to the electrode means is maintained substantially at the optimum operating power level for the pump over an extended range of pump pressures.

4. An ion pump for evacuating gases in a space comprising:

a pump envelope connectable to the space to be evacuated,

electrode means mounted within the envelope for producing an electron discharge to ionize a portion of the gases within the envelope,

means for producing a magnetic iield extending between the electrode means,

a source of sputtering material within the pump envelope for covering ions driven into the electrode means,

a pair of terminals for connecting the electrode means to a source of electric power,

transformer means having a primary and a secondary winding, the primary winding having a tap for adjusting the number of turns in the primary winding,

means for connecting one of the pair of terminals to the one end of the primary winding,

switch means connected to the other of the pair of terminals and selectively connectable to the tap and the other end of the primary winding,

full-wave rectifying means connected to the secondary winding of the transformer means, the rectifying means having a pair of outputs,

coil means connected between one of the rectifying means output and the electrode means and operatively engaging switch means, and

means for directly connecting the other rectifying means output to the electrode means whereby the power supplied to the electrode means is maintained substantially at the optimum operating power level for the pump over an extended range of pump pressures.

5. An ion pump for evacuating gases in a space cornprising:

a pump envelope connectable to the space to be evacuated,

electrode means mounted within the envelop for producing an electron discharge to ionize a portion of the gases within the envelope,

means for producing a magnetic eld extending between the electrode means,

a source of sputtering material within the pump envelope for covering ions driven into the electrode means,

a pair of terminals for connecting the electrode means to a source of electric power,

transformer means having a primary and a secondary winding, the primary winding having a tap for adjusting the number of turns in the primary winding,

means for connecting one of the pair of terminals to one end of the primary winding,

rst and second switch means connected to the other of the pair of terminals and selectively connectable to the tap and the other end of the primary winding,

full-Wave rectifying means connected to the secondary Winding of the transformer means, the rectifying means having a pair of outputs,

iirst and second coil means connected between one of the rectifying means output and the electrode means and operatively engaging the first and second switch means respectively, and

means for directly connecting the other of the rectifying means output and the electrode means whereby the power supplied to the electrode is maintained substantially at the optimum operating power level for the pump over an extended range of pump pressures.

References Cited UNITED STATES PATENTS 2,721,969 10/1955 Van Ryan et al. 323-435 2,992,379 11/1961 Rosin S23-43.5 3,319,153 5/1967 Livingston 323-435 3,078,388 2/1963 Hanks et al. 315-107 3,275,883 9/1966 Watters 315-107 2,264,495 12/1941 Wilner 230-69 X 2,282,401 5/ 1942 Hansell 230-69 2,954,156 9/1960 Meyer 230-69 2,993,638 7/1961 Hall et al. 230-69 3,032,703 5/ 1962 Lawrence 323-7 3,186,632 1/1965 Conner 230-69 3,159,332 12/1964 Rutherford 230-69 3,249,291 5/ 1966 Ackley 230-69 W. L. FREEH, Primary Examiner.

U.S. Cl. X.R.

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