Ionization Gauges:
Cold Cathode Type : The cold cathode gauge uses a confined discharge to sustain a circulating electron current for the ionization of gases. The absence of a hot cathode provides a far more rugged gauge, the discharge requires less power, outgassing is much reduced,and the sensitivity of the gauge is high, so that simpler electronics can be used. Based on these facts, the gauges would appear ideal substitutes for the hot-cathode type of gauge. The fact that this is not yet so where precise measurement of pressure is required is related to the history of cold cathode gauges. Some older designs of such gauges are known to exhibit serious instabilities (Lange et al., 1966). An excellent review of these gauges has been published (Peacock
et al., 1991), and a recent paper (Kendall and Drubetsky, 1997) provides reassurance that instabilities should not be a major concern with modern gauges.
Operating Principles: The first widely used commercial cold cathode gauge was developed by Penning (Penning,1937; Penning and Nienhuis, 1949). It uses an anode ring or cylinder at a potential of 2000 V placed between cathode plates at ground potential. A magnetic field of 0.15 tesla is directed along the axis of the cathode. The confined Penning discharge traps a circulating cloud of electrons,substantially at cathode potential, along the axis of the anode. The electron path lengths are very long, as compared to the hot cathode gauge, so that the pressure measuring sensitivity is very high, permitting a simple microammeter to be used for readout. The gauge is known variously as the Penning (or PIG), Philips, or simply cold cathode gauge and is widely used where a rugged gauge is required. The operating range is from 10¯2 to 10¯7 torr.
Note that any discharge current in the Penning and other cold cathode discharge gauges is extinguished at
pressures of a few torr. When a gauge does not give a pressure indication, this means that either the pressure is below 10¯7 torr or at many torr, a rather significant difference.
Thus, it is necessary to use an additional gauge that is responsive in the blind spot of the Penning—i.e.,
between atmospheric pressure and 10-2 torr.
The Penning gauge has a number of limitations which preclude its use for the precise measurement of pressure.It is subject to some instability at the lower end of its range, because the discharge tends to extinguish. Discontinuities in the pressure indication may also occur throughout the range. Both of these characteristics were also evident in a discharge gauge that was designed to measure pressures into the 10¯10 torr range, where discontinuities were detected throughout the entire operating range (Lange et al., 1966). These appear to result from changes between two or more modes of discharge. Note, however, that instabilities may simply indicate that the gauge is dirty.
The Penning has a higher pumping speed (up to 0.5 L/s) than a Bayard-Alpert gauge, so it is even more essential to provide a high-conductance connection between gauge and the vacuum chamber.
A number of refinements of the cold cathode gauge have been introduced by Redhead (Redhead et al., 1968), and commercial versions of these and other gauges are available for use to at least 10¯10 torr. The discontinuities in such gauges are far less than those discussed above so that they are becoming widely used.
Cold Cathode Type : The cold cathode gauge uses a confined discharge to sustain a circulating electron current for the ionization of gases. The absence of a hot cathode provides a far more rugged gauge, the discharge requires less power, outgassing is much reduced,and the sensitivity of the gauge is high, so that simpler electronics can be used. Based on these facts, the gauges would appear ideal substitutes for the hot-cathode type of gauge. The fact that this is not yet so where precise measurement of pressure is required is related to the history of cold cathode gauges. Some older designs of such gauges are known to exhibit serious instabilities (Lange et al., 1966). An excellent review of these gauges has been published (Peacock
et al., 1991), and a recent paper (Kendall and Drubetsky, 1997) provides reassurance that instabilities should not be a major concern with modern gauges.
Operating Principles: The first widely used commercial cold cathode gauge was developed by Penning (Penning,1937; Penning and Nienhuis, 1949). It uses an anode ring or cylinder at a potential of 2000 V placed between cathode plates at ground potential. A magnetic field of 0.15 tesla is directed along the axis of the cathode. The confined Penning discharge traps a circulating cloud of electrons,substantially at cathode potential, along the axis of the anode. The electron path lengths are very long, as compared to the hot cathode gauge, so that the pressure measuring sensitivity is very high, permitting a simple microammeter to be used for readout. The gauge is known variously as the Penning (or PIG), Philips, or simply cold cathode gauge and is widely used where a rugged gauge is required. The operating range is from 10¯2 to 10¯7 torr.
Note that any discharge current in the Penning and other cold cathode discharge gauges is extinguished at
pressures of a few torr. When a gauge does not give a pressure indication, this means that either the pressure is below 10¯7 torr or at many torr, a rather significant difference.
Thus, it is necessary to use an additional gauge that is responsive in the blind spot of the Penning—i.e.,
between atmospheric pressure and 10-2 torr.
The Penning gauge has a number of limitations which preclude its use for the precise measurement of pressure.It is subject to some instability at the lower end of its range, because the discharge tends to extinguish. Discontinuities in the pressure indication may also occur throughout the range. Both of these characteristics were also evident in a discharge gauge that was designed to measure pressures into the 10¯10 torr range, where discontinuities were detected throughout the entire operating range (Lange et al., 1966). These appear to result from changes between two or more modes of discharge. Note, however, that instabilities may simply indicate that the gauge is dirty.
The Penning has a higher pumping speed (up to 0.5 L/s) than a Bayard-Alpert gauge, so it is even more essential to provide a high-conductance connection between gauge and the vacuum chamber.
A number of refinements of the cold cathode gauge have been introduced by Redhead (Redhead et al., 1968), and commercial versions of these and other gauges are available for use to at least 10¯10 torr. The discontinuities in such gauges are far less than those discussed above so that they are becoming widely used.
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1 comment:
Good. But can u also give construction of penning gauge????
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