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A Method for Greatly Enhancing the Pumping Action of a Penning Discharge
A Method for Greatly Enhancing the Pumping Action of a Penning Discharge By W. M. BRUBAKER Research Division, Consolidated Electrodynamics Corporation, Pasadena,
California
Although the Penning discharge has long been used as a vacuum gauge, it is well-known that it exhibits a pumping action; recently this phenomenon has been used to make a vacuum pump. However, the various ideas expressed in the literature on the nature of this pumping action are not in agreement. In the present work mass spectrometric methods showed that argon and other noble gases are pumped at the cathode of the discharge. We have found that the addition of a third electrode to the usual diode Penning structure eliminates the most serious shortcomings of the simple Penning discharge as a vacuum pump for the noble gases. The mechanism whereby the third electrode greatly enhances the pumping action of the discharge is discussed. Experimental data are presented which compare the pumping behavior of the discharge in the two-electrode and the three-electrode structures. Apparatus
Introduction The Penning discharge has long been used as a vacuum gauge. The fact that the discharge current is even approximately proportional to the pressure indicates that the ionizing electron stream intensity is independent of the pressure. Its magnitude must, therefore, be controlled by the space charge of the electron stream itself. That an electrical discharge causes the pressure in a vacuum vessel to decrease has been known for more than a hundred years.1 Dushman, in his book Vacuum Technique devotes an entire chapter to "Chemical and Electrical Clean-up of Gases at Low Pressures". Alpert2 used the pumping action of a so-called ion gauge to achieve exceptionally low pressures. Gurewitsch and Westendorp3 described the use of a simple Penning discharge as an ion pump. Hall4 has described an adaptation of the Penning discharge which is particularly suited for pumping, and which has been made available commercially. Although much has been written about the pumping action of the discharge for the noble gases, there is not a general agreement as to the mechanism. In a recent article Hall4 says: "Pumping of noble gases is not well understood, but it appears that it is probably accomplished by burial of ions in cathode surfaces at high velocity, or by plastering of atoms on to anode walls by sputtering titanium' \ It is the purpose of this paper to describe experiments which demonstrate that the noble gases are pumped at the cathode. Further, it is shown how the efficiency of pumping for the noble gases can be greatly enhanced by the addition of a third electrode.
The experiments which we will report today were made in the apparatus shown in Fig. 1. The test chamber is evacuated by oil diffusion pumps through a trap. The latter is either a charcoal or a copper foil trap. Gas is admitted to the system through either a needle valve from a balloon or from a glass flask at low pressure through a molecular leak. The experimental discharge units are placed in the tent chamber, and when they operate as a pump they are is parallel with the conventional pumping system. The total pressures are monitored by the ion gauge, and the partial pressures by the mass spectrometer. The latter has a negligible pumping speed relative to the Bayard-Alpert ion gauge, and so makes a minimum disturbance to the pressures being measured. /MASS\ SPECTROMETER
FIG. 1. Vacuum system schematic.
1 S. Dushman; Scientific Foundations of Vacuum Technique p. 687. John Wiley New York (1949). 2 D . Alpert; J. Appl. Phys. 24 860 (1953). 3 A. M. Gurewitsch and W. F. Westendorp ; Rev. Sei. Instrum. 25 389(1954). 4 L. D . Hall; Rev. Sei. Instrum. 29 367 (1958).
Experiments It is easy to verify that the evaporation of titanium from a filament in the vicinity of a discharge greatly enhances the pumping action of the discharge for the noble gases. Using 302
Enhanced Pumping of a Penning Discharge
the mass spectrometer to monitor the partial pressures of the various gases, we have performed experiments which demonstrate the pumping action of the Penning discharge for the noble gases. In Fig. 2 is shown the geometry of the first experiment. Here we have a short cylindrical anode. On the axis of the apparatus is a tungsten-titanium filament. The molybdenum discs are so placed that the active portion of the cathodes cannot see the hot filament. The anode, however, is fully illuminated. The evaporation of titanium from this filament made a negligible change in the pumping speed of the discharge for argon.
303
TITANIUM CATHODE
CYLINDRICAL ANODE
y/////////////////////// I _ _J
TUNGSTEN-TITANIUM FILAMENT 1.500" LONG
^—TITANIUM CATHODE
-TITANIUM CATHODE MOLYBDENUM DISCS
///////////////////////A FIG. 3. Two-electrode unit designed to test the influence of a freshly deposited layer of titanium on the cathode surfaces.
TUNGSTEN-TITANIUM FILAMENT
-4L
z
^-TITANIUM CATHODE
τ_ /
FIG. 2. Two-electrode unit designed to test the influence of a freshly deposited layer of titanium on the anode alone.
In Fig. 3 is shown a similar anode and cathode assembly, but with the tungsten-titanium filament placed differently. Now the active portion of the cathodes is illuminated, but the inside of the anode is not. With this arrangement, the evaporation of titanium from the filament increases the pumping speed for argon thirty-two times! Further, the current passed by the discharge is essentially that which would be required to do the pumping if it is assumed that each ion formed is permanently removed ! Several conclusions can be drawn from the results of the experiments just described. First, the pumping action for the noble gases is by ion burial in the cathode. Second, the current at the cathode is due almost entirely to the charges brought in by the incident ions. Many ions strike the cathode for each electron that leaves. Hobson and Redhead5 have demonstrated that the Penning discharge is stable at pressures as low as 10~12 mm Hg. This is further evidence that the ionizing electron stream remains space-chargelimited, and that the individual electrons advance toward the anode only as the result of interactionwith the molecules in the vessel. 6
J. P. Hobson and P. A. Redhead ; Canad.J. Phys. 36 271 (1958).
The experiments also illustrate why the simple diode geometry is so very inefficient for the pumping of the noble gases. The ions are driven into the cathode, but not very far. The erosion of the cathode which results from the continued ionic bombardment releases the molecules which had previously been buried as ions. On the assumption that the discharge current at the cathode is carried essentially by positive ions, the data indicate that the molecules of argon are ionized, driven into the cathode, liberated by surface erosion, and that the process occurs many times before they reach a final resting place. Finally, they land on a portion of the cathode where the density of the incident ions is low, and where the rate of deposition of the cathode material from other, more active portions, exceeds the erosion rate due to local sputtering. Here they are buried deeper by continued operation of the discharge. Three-electrode structure For most metals the sputtering rate increases as the energy of the incident ions increases. Further, ions incident at a grazing angle do more sputtering than those which strike the surface at normal incidence. Through the use of three electrodes it is possible to take advantage of these two effects to increase the sputtering. The third electrode, which we have called the auxiliary cathode, is placed between the anode and the two plate cathodes. Through a suitable choice of geometry, it is possible to cause a small portion of the ions to strike the auxiliary cathode at nearly grazing incidence. Further, by making this electrode the most negative part of the apparatus, it is feasible to have the ions strike the auxiliary cathode with several times*as much energy as they have when they strike the main cathode. As sputtering has been shown to be a momentum transfer phenomenon, the material sputtered from the auxiliary cathode can be expected to shower directly on to the main cathode.
304
W.
M.
By using a few of the ions to cause severe erosion on the auxiliary cathode, the efficiency of trapping of the ions on the main cathode can be greatly increased. Since the ions which strike the main cathode do so at nearly normal incidence, and with reduced energy, the sputtering (erosion) rate from the main cathode is held to a minimum. Thus it is possible for the sputtering action of the few ions which strike the auxiliary cathode to offset the sputtering action at the main cathode very effectively. Pumping Action of the Three-electrode as Compared to that of the Two-electrode Unit—Two-electrode units of many different configurations have been tested. They have one common characteristic: they are poor pumps for the noble gases. Further, they tend strongly to exhibit instabilities with time. That is, their effective pumping speed may vary greatly from minute to minute. This is illustrated in Fig. 4. With argon being admitted to the apparatus, and the conventional pumps valved off, we have the record of the argon pressure. This is mass spectrometer data, with time as the independent variable. We note that the pressure of argon periodically becomes quite high. The peaks are thirty-six times as high as the valleys. During most of the time, the discharge demonstrates good pumping action. The duration of the high pressure periods is longer at higher argon admission speeds. None of the other gases present are found to partake of the violent pressure excursions experienced by the argon. The trace of Fig. 4 illustrates one of the kinds of instability observed. Occasionally the wave form of the pressure vs. time records is nearly sinusoidal. Usually the harmonic content is quite high. Any change in the operating conditions is likely to start a pressure oscillation. Small line voltage variations are reflected as unexpectedly large pressure variations. «x«C
IX10**^
_J
Jl 10 15 TIME IN MINUTES
F I G . 4. Mass spectrometer record of the pressure of argon in a system pumped solely by a two-electrode unit. This is an example of the instabilities in the pumping of the two-electrode unit.
In Fig. 5 is shown a geometry which illustrates the threeelectrode principle. Conical pieces of titanium are inserted on the axis of the apparatus, just in front of the cathodes. They are electrically insulated from the cathodes. We observed the performance of the device as a pump with the potential of the auxiliary cathodes (cones) as an independent variable.
BRUBAKER
v
t CATHODE
CYLINDER
Z)
CATHOD E
FIG. 5. Three-electrode unit. T h e cones of titanium, placed on the axis, constitute the third electrode. When they are energized negatively with respect to the main cathode, the pumping speed for argon is greatly enhanced.
In Fig. 6 we have a record of the argon pressure, as seen by the mass spectrometer. The pressure of the argon, in the absence of any ionic pumping, is about 2 x l 0 ~ 5 m m . Initially, the diode (zero voltage on the points) reduced the pressure by a factor of 6.3. However, instabilities soon developed and the pressure began to oscillate; at first the amplitude was so large that the discharge was alternately acting as a pump and as a gas (argon) source. Then the amplitude tended to stabilize at a lower value. After 30 min of this, the cones (auxiliary cathodes) were energized at —2 kV. Immediately the pressure fell to a value lower than it had been before, and the oscillations ceased. After 40 min the pressure stabilized at 6.5 X 10~6. The increased pumping speed which results from the application of a negative potential to the cones is due to the increased ion collection efficiency on the main cathodes. The titanium which is showered from the cones on to the main cathodes discourages the release of the pumped gas by subsequent ion bombardment. The molecules of argon which are driven into the cones as ions are released by subsequent ion bombardment of the cones, but only a small portion of the total number of ions formed strike the cones. The increased pumping speed is not thought to result from an increase in the ion production rate. It is difficult to see how the negative potential on the cones would increase the intensity of the ionizing electron stream, which is located adjacent to the anode. T o test the relative merits of a three-electrode and two electrode structure, the apparatus of Fig. 7 was assembled. Here we have a different type of three-electrode unit placed in the same envelope with a two-electrode unit. This thirdelectrode (auxiliary cathode) is of cellular structure, energized at —3 kV. The two units were operated in parallel in an argon pressure of 10~6 mm Hg. Periodically they were operated singly and their pumping speeds measured. The pumping speed of the three-electrode, in liters per second, is shown in Fig. 8. No trend toward any change of pumping speed is apparent in the data. Fig. 9 shows the same data for the two-electrode
Enhanced Pumping of a Penning Discharge
305
3XK>"
0
FIG. 6. Mass spectrometer record of the pressure of argon in the unit shown in Fig. 5. Note the lower, steady pressure which results from the application of potential to the cones (auxiliary cathodes).
VACUUM W A L L ^
AUXILIARY^ CATHODEM
Ξ 0.08 0.04
3 ELECTRODE AND 2 ELECTRODE UNITS IN COMMON ENVIRONMENT
FIG. 7. A two-electrode and a three-electrode unit in the same environment. These were operated simultaneously in argon at a pressure of 10~6 mm. unit. Here we note a decrease in pumping speed with time. Finally, in Fig. 10, we show the ratio of the observed pumping speeds for the two units. In the two-electrode unit, the sputtering action of the positive ions erodes the cathodes, forming a depression near the axis. Ions which strike the sides of the depression tend to sputter titanium from one side to the other. This is not conducive to effective pumping. In another test, a three-electrode unit has pumped air at a
40
80 Time,
120
160
200
240
hrs at 10—β mm Argon
FIG. 8. Speed of three-electrode unit, liters per second as a function of time. pressure of 5 x l 0 ~ 6 mm for 1,000 hours with no apparent loss of efficiency. The pumping speed for air is about four times that for argon. Discussion We have many records of the type illustrated by Fig. 4, and by Fig. 6. We also have records which show a reasonable stability for a period of time. However, the two-electrode units are prone to exhibit rather severe pumping instabilities
W. M.
306
when pumping the noble gases. The three-electrode units, on the other hand, are comparatively free from this defect. Further, they are only slightly influenced by small voltage fluctuations as compared to the two-electrode units. In summary, the addition of the third electrode to a unit with a given anode gives the following improvements :
BRUBAKER
4. Life of pump is very greatly increased, and the efficiency of operation remains high throughout the life of the pump. Speed of 3 electrode
.
.
, .
x x -— , t . o s a function of time M of. 02 electrode Speed
1. T h e pump operates stably with time. (No intermittancies in pumping.) 2. The pumping speed for argon is increased ten times. 3. The pumping speed for air is increased four times.
h
Speed of diode as a function of time 0.024
0.020
_ -
•
b -c » φ| at C CL
P 3
•
0.016
0.012
_
0.008
-
a.
\ .
50 ^ v
·
• ^S.
!
·
* •
*
0.004 i
1
L_J 40
L__! 80
Time,
100
150
200
250
Hours of operation at 10"*6 Hg Argon
! 120
1 ! t f \ L_ ! 160 200 240
hrs at \J~
mm H j Argon
F I G . 9. Speed of two-electrode unit, liters per second as a function of time.
1
F I G . 10. Ratio of the pumping speeds of the three-electrode to the two-electrode unit as a function of time.
Acknowledgment It is a pleasure to acknowledge the benefits of many helpful discussions of this work with Dr. C. E. Berry. His continued support and encouragement is much appreciated. Further, the aid given by Mr. F. Pickett through his craftsmanship in the design, construction, and assembly of the apparatus is gratefully remembered.
California
Although the Penning discharge has long been used as a vacuum gauge, it is well-known that it exhibits a pumping action; recently this phenomenon has been used to make a vacuum pump. However, the various ideas expressed in the literature on the nature of this pumping action are not in agreement. In the present work mass spectrometric methods showed that argon and other noble gases are pumped at the cathode of the discharge. We have found that the addition of a third electrode to the usual diode Penning structure eliminates the most serious shortcomings of the simple Penning discharge as a vacuum pump for the noble gases. The mechanism whereby the third electrode greatly enhances the pumping action of the discharge is discussed. Experimental data are presented which compare the pumping behavior of the discharge in the two-electrode and the three-electrode structures. Apparatus
Introduction The Penning discharge has long been used as a vacuum gauge. The fact that the discharge current is even approximately proportional to the pressure indicates that the ionizing electron stream intensity is independent of the pressure. Its magnitude must, therefore, be controlled by the space charge of the electron stream itself. That an electrical discharge causes the pressure in a vacuum vessel to decrease has been known for more than a hundred years.1 Dushman, in his book Vacuum Technique devotes an entire chapter to "Chemical and Electrical Clean-up of Gases at Low Pressures". Alpert2 used the pumping action of a so-called ion gauge to achieve exceptionally low pressures. Gurewitsch and Westendorp3 described the use of a simple Penning discharge as an ion pump. Hall4 has described an adaptation of the Penning discharge which is particularly suited for pumping, and which has been made available commercially. Although much has been written about the pumping action of the discharge for the noble gases, there is not a general agreement as to the mechanism. In a recent article Hall4 says: "Pumping of noble gases is not well understood, but it appears that it is probably accomplished by burial of ions in cathode surfaces at high velocity, or by plastering of atoms on to anode walls by sputtering titanium' \ It is the purpose of this paper to describe experiments which demonstrate that the noble gases are pumped at the cathode. Further, it is shown how the efficiency of pumping for the noble gases can be greatly enhanced by the addition of a third electrode.
The experiments which we will report today were made in the apparatus shown in Fig. 1. The test chamber is evacuated by oil diffusion pumps through a trap. The latter is either a charcoal or a copper foil trap. Gas is admitted to the system through either a needle valve from a balloon or from a glass flask at low pressure through a molecular leak. The experimental discharge units are placed in the tent chamber, and when they operate as a pump they are is parallel with the conventional pumping system. The total pressures are monitored by the ion gauge, and the partial pressures by the mass spectrometer. The latter has a negligible pumping speed relative to the Bayard-Alpert ion gauge, and so makes a minimum disturbance to the pressures being measured. /MASS\ SPECTROMETER
FIG. 1. Vacuum system schematic.
1 S. Dushman; Scientific Foundations of Vacuum Technique p. 687. John Wiley New York (1949). 2 D . Alpert; J. Appl. Phys. 24 860 (1953). 3 A. M. Gurewitsch and W. F. Westendorp ; Rev. Sei. Instrum. 25 389(1954). 4 L. D . Hall; Rev. Sei. Instrum. 29 367 (1958).
Experiments It is easy to verify that the evaporation of titanium from a filament in the vicinity of a discharge greatly enhances the pumping action of the discharge for the noble gases. Using 302
Enhanced Pumping of a Penning Discharge
the mass spectrometer to monitor the partial pressures of the various gases, we have performed experiments which demonstrate the pumping action of the Penning discharge for the noble gases. In Fig. 2 is shown the geometry of the first experiment. Here we have a short cylindrical anode. On the axis of the apparatus is a tungsten-titanium filament. The molybdenum discs are so placed that the active portion of the cathodes cannot see the hot filament. The anode, however, is fully illuminated. The evaporation of titanium from this filament made a negligible change in the pumping speed of the discharge for argon.
303
TITANIUM CATHODE
CYLINDRICAL ANODE
y/////////////////////// I _ _J
TUNGSTEN-TITANIUM FILAMENT 1.500" LONG
^—TITANIUM CATHODE
-TITANIUM CATHODE MOLYBDENUM DISCS
///////////////////////A FIG. 3. Two-electrode unit designed to test the influence of a freshly deposited layer of titanium on the cathode surfaces.
TUNGSTEN-TITANIUM FILAMENT
-4L
z
^-TITANIUM CATHODE
τ_ /
FIG. 2. Two-electrode unit designed to test the influence of a freshly deposited layer of titanium on the anode alone.
In Fig. 3 is shown a similar anode and cathode assembly, but with the tungsten-titanium filament placed differently. Now the active portion of the cathodes is illuminated, but the inside of the anode is not. With this arrangement, the evaporation of titanium from the filament increases the pumping speed for argon thirty-two times! Further, the current passed by the discharge is essentially that which would be required to do the pumping if it is assumed that each ion formed is permanently removed ! Several conclusions can be drawn from the results of the experiments just described. First, the pumping action for the noble gases is by ion burial in the cathode. Second, the current at the cathode is due almost entirely to the charges brought in by the incident ions. Many ions strike the cathode for each electron that leaves. Hobson and Redhead5 have demonstrated that the Penning discharge is stable at pressures as low as 10~12 mm Hg. This is further evidence that the ionizing electron stream remains space-chargelimited, and that the individual electrons advance toward the anode only as the result of interactionwith the molecules in the vessel. 6
J. P. Hobson and P. A. Redhead ; Canad.J. Phys. 36 271 (1958).
The experiments also illustrate why the simple diode geometry is so very inefficient for the pumping of the noble gases. The ions are driven into the cathode, but not very far. The erosion of the cathode which results from the continued ionic bombardment releases the molecules which had previously been buried as ions. On the assumption that the discharge current at the cathode is carried essentially by positive ions, the data indicate that the molecules of argon are ionized, driven into the cathode, liberated by surface erosion, and that the process occurs many times before they reach a final resting place. Finally, they land on a portion of the cathode where the density of the incident ions is low, and where the rate of deposition of the cathode material from other, more active portions, exceeds the erosion rate due to local sputtering. Here they are buried deeper by continued operation of the discharge. Three-electrode structure For most metals the sputtering rate increases as the energy of the incident ions increases. Further, ions incident at a grazing angle do more sputtering than those which strike the surface at normal incidence. Through the use of three electrodes it is possible to take advantage of these two effects to increase the sputtering. The third electrode, which we have called the auxiliary cathode, is placed between the anode and the two plate cathodes. Through a suitable choice of geometry, it is possible to cause a small portion of the ions to strike the auxiliary cathode at nearly grazing incidence. Further, by making this electrode the most negative part of the apparatus, it is feasible to have the ions strike the auxiliary cathode with several times*as much energy as they have when they strike the main cathode. As sputtering has been shown to be a momentum transfer phenomenon, the material sputtered from the auxiliary cathode can be expected to shower directly on to the main cathode.
304
W.
M.
By using a few of the ions to cause severe erosion on the auxiliary cathode, the efficiency of trapping of the ions on the main cathode can be greatly increased. Since the ions which strike the main cathode do so at nearly normal incidence, and with reduced energy, the sputtering (erosion) rate from the main cathode is held to a minimum. Thus it is possible for the sputtering action of the few ions which strike the auxiliary cathode to offset the sputtering action at the main cathode very effectively. Pumping Action of the Three-electrode as Compared to that of the Two-electrode Unit—Two-electrode units of many different configurations have been tested. They have one common characteristic: they are poor pumps for the noble gases. Further, they tend strongly to exhibit instabilities with time. That is, their effective pumping speed may vary greatly from minute to minute. This is illustrated in Fig. 4. With argon being admitted to the apparatus, and the conventional pumps valved off, we have the record of the argon pressure. This is mass spectrometer data, with time as the independent variable. We note that the pressure of argon periodically becomes quite high. The peaks are thirty-six times as high as the valleys. During most of the time, the discharge demonstrates good pumping action. The duration of the high pressure periods is longer at higher argon admission speeds. None of the other gases present are found to partake of the violent pressure excursions experienced by the argon. The trace of Fig. 4 illustrates one of the kinds of instability observed. Occasionally the wave form of the pressure vs. time records is nearly sinusoidal. Usually the harmonic content is quite high. Any change in the operating conditions is likely to start a pressure oscillation. Small line voltage variations are reflected as unexpectedly large pressure variations. «x«C
IX10**^
_J
Jl 10 15 TIME IN MINUTES
F I G . 4. Mass spectrometer record of the pressure of argon in a system pumped solely by a two-electrode unit. This is an example of the instabilities in the pumping of the two-electrode unit.
In Fig. 5 is shown a geometry which illustrates the threeelectrode principle. Conical pieces of titanium are inserted on the axis of the apparatus, just in front of the cathodes. They are electrically insulated from the cathodes. We observed the performance of the device as a pump with the potential of the auxiliary cathodes (cones) as an independent variable.
BRUBAKER
v
t CATHODE
CYLINDER
Z)
CATHOD E
FIG. 5. Three-electrode unit. T h e cones of titanium, placed on the axis, constitute the third electrode. When they are energized negatively with respect to the main cathode, the pumping speed for argon is greatly enhanced.
In Fig. 6 we have a record of the argon pressure, as seen by the mass spectrometer. The pressure of the argon, in the absence of any ionic pumping, is about 2 x l 0 ~ 5 m m . Initially, the diode (zero voltage on the points) reduced the pressure by a factor of 6.3. However, instabilities soon developed and the pressure began to oscillate; at first the amplitude was so large that the discharge was alternately acting as a pump and as a gas (argon) source. Then the amplitude tended to stabilize at a lower value. After 30 min of this, the cones (auxiliary cathodes) were energized at —2 kV. Immediately the pressure fell to a value lower than it had been before, and the oscillations ceased. After 40 min the pressure stabilized at 6.5 X 10~6. The increased pumping speed which results from the application of a negative potential to the cones is due to the increased ion collection efficiency on the main cathodes. The titanium which is showered from the cones on to the main cathodes discourages the release of the pumped gas by subsequent ion bombardment. The molecules of argon which are driven into the cones as ions are released by subsequent ion bombardment of the cones, but only a small portion of the total number of ions formed strike the cones. The increased pumping speed is not thought to result from an increase in the ion production rate. It is difficult to see how the negative potential on the cones would increase the intensity of the ionizing electron stream, which is located adjacent to the anode. T o test the relative merits of a three-electrode and two electrode structure, the apparatus of Fig. 7 was assembled. Here we have a different type of three-electrode unit placed in the same envelope with a two-electrode unit. This thirdelectrode (auxiliary cathode) is of cellular structure, energized at —3 kV. The two units were operated in parallel in an argon pressure of 10~6 mm Hg. Periodically they were operated singly and their pumping speeds measured. The pumping speed of the three-electrode, in liters per second, is shown in Fig. 8. No trend toward any change of pumping speed is apparent in the data. Fig. 9 shows the same data for the two-electrode
Enhanced Pumping of a Penning Discharge
305
3XK>"
0
FIG. 6. Mass spectrometer record of the pressure of argon in the unit shown in Fig. 5. Note the lower, steady pressure which results from the application of potential to the cones (auxiliary cathodes).
VACUUM W A L L ^
AUXILIARY^ CATHODEM
Ξ 0.08 0.04
3 ELECTRODE AND 2 ELECTRODE UNITS IN COMMON ENVIRONMENT
FIG. 7. A two-electrode and a three-electrode unit in the same environment. These were operated simultaneously in argon at a pressure of 10~6 mm. unit. Here we note a decrease in pumping speed with time. Finally, in Fig. 10, we show the ratio of the observed pumping speeds for the two units. In the two-electrode unit, the sputtering action of the positive ions erodes the cathodes, forming a depression near the axis. Ions which strike the sides of the depression tend to sputter titanium from one side to the other. This is not conducive to effective pumping. In another test, a three-electrode unit has pumped air at a
40
80 Time,
120
160
200
240
hrs at 10—β mm Argon
FIG. 8. Speed of three-electrode unit, liters per second as a function of time. pressure of 5 x l 0 ~ 6 mm for 1,000 hours with no apparent loss of efficiency. The pumping speed for air is about four times that for argon. Discussion We have many records of the type illustrated by Fig. 4, and by Fig. 6. We also have records which show a reasonable stability for a period of time. However, the two-electrode units are prone to exhibit rather severe pumping instabilities
W. M.
306
when pumping the noble gases. The three-electrode units, on the other hand, are comparatively free from this defect. Further, they are only slightly influenced by small voltage fluctuations as compared to the two-electrode units. In summary, the addition of the third electrode to a unit with a given anode gives the following improvements :
BRUBAKER
4. Life of pump is very greatly increased, and the efficiency of operation remains high throughout the life of the pump. Speed of 3 electrode
.
.
, .
x x -— , t . o s a function of time M of. 02 electrode Speed
1. T h e pump operates stably with time. (No intermittancies in pumping.) 2. The pumping speed for argon is increased ten times. 3. The pumping speed for air is increased four times.
h
Speed of diode as a function of time 0.024
0.020
_ -
•
b -c » φ| at C CL
P 3
•
0.016
0.012
_
0.008
-
a.
\ .
50 ^ v
·
• ^S.
!
·
* •
*
0.004 i
1
L_J 40
L__! 80
Time,
100
150
200
250
Hours of operation at 10"*6 Hg Argon
! 120
1 ! t f \ L_ ! 160 200 240
hrs at \J~
mm H j Argon
F I G . 9. Speed of two-electrode unit, liters per second as a function of time.
1
F I G . 10. Ratio of the pumping speeds of the three-electrode to the two-electrode unit as a function of time.
Acknowledgment It is a pleasure to acknowledge the benefits of many helpful discussions of this work with Dr. C. E. Berry. His continued support and encouragement is much appreciated. Further, the aid given by Mr. F. Pickett through his craftsmanship in the design, construction, and assembly of the apparatus is gratefully remembered.