Fundamentals of Vacuum Technology (1248463), страница 35
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Argon might escapefrom the vessel (cover opens, bell jar rises). For such and similarapplications, pressure switches or vacuum gauges that are independent ofthe type of gas must be used (see section 3.2).3.3.3 Ionization vacuum gaugesIonization vacuum gauges are the most important instruments formeasuring gas pressures in the high and ultrahigh vacuum ranges. Theymeasure the pressure in terms of the number density of particlesproportional to the pressure. The gas whose pressure is to be measuredenters the gauge heads of the instruments and is partially ionized with thehelp of an electric field. Ionization takes place when electrons areaccelerated in the electric field and attain sufficient energy to form positiveions on impact with gas molecules. These ions transmit their charge to ameasuring electrode (ion collector) in the system. The ion current,generated in this manner (or, more precisely, the electron current in thefeed line of the measuring electrode that is required to neutralize theseions) is a measure of the pressure because the ion yield is proportional tothe particle number density and thus to the pressure.The formation of ions is a consequence of either a discharge at a highelectric field strength (so-called cold-cathode or Penning discharge, see3.3.3.1) or the impact of electrons that are emitted from a hot cathode (see3.3.3.2).Under otherwise constant conditions, the ion yield and thus the ion currentdepend on the type of gas since some gases are easier to ionize thanothers.
As all vacuum gauges with a pressure reading that is dependent onthe type of gas, ionization vacuum gauges are calibrated with nitrogen asthe reference gas (nitrogen equivalent pressure, see 3.3). To obtain the truepressure for gases other than nitrogen, the read-off pressure must bemultiplied by the correction factor given in Table 3.2 for the gas concerned.The factors stated in Table 3.2 are assumed to be independent of thepressure, though they depend somewhat on the geometry of the electrodesystem. Therefore, they are to be regarded as average values for varioustypes of ionization vacuum gauges (see Fig. 3.16).Given the presenceof predominantly(type of gas)HeNeArKrXeHgH2COCO2CH4higherhydrocarbonsTable 3.2 Correction factorsCorrection factorbased on N2(nitrogen = 1)6.94.350.830.590.330.3032.40.920.690.83.3.3.1Cold-cathode ionization vacuum gauges(Penning vacuum gauges)Ionization vacuum gauges which operate with cold discharge are calledcold-cathode- or Penning vacuum gauges.
The discharge process in ameasuring tube is, in principle, the same as in the electrode system of asputter ion pump (see section 2.1.8.3). A common feature of all types ofcold-cathode ionization vacuum gauges is that they contain just twounheated electrodes, a cathode and an anode, between which a so-calledcold discharge is initiated and maintained by means of a d.c. voltage (ofaround 2 kV) so that the discharge continues at very low pressures. This isachieved by using a magnetic field to make the paths of the electrons longenough so that the rate of their collision with gas molecules is sufficientlylarge to form the number of charge carriers required to maintain thedischarge.
The magnetic field (see Fig. 3.12) is arranged such that themagnetic field lines of force cross the electric field lines. In this way theelectrons are confined to a spiral path. The positive and negative chargecarriers produced by collision move to the corresponding electrodes andform the pressure-dependent discharge current, which is indicated on themeter. The reading in mbar depends on the type of gas. The upper limit ofthe measuring range is given by the fact that above a level of several10-2 mbar the Penning discharge changes to a glow discharge with intenselight output in which the current (at constant voltage) depends only to asmall extent on the pressure and is therefore not suitable for measurementpurposes.
In all Penning gauges there is considerably higher gas sorptionthan in ionization vacuum gauges that operate with a hot cathode. APenning measuring tube pumps gases similarly to a sputter ion pump(S Å 10-2 l/s). Here again the ions produced in the discharge areaccelerated towards the cathode where they are partly retained and partlycause sputtering of the cathode material.
The sputtered cathode material1 Small flange DN 25 KF;DN 40 KF2 Housing3 Ring anode with ignition pin4 Ceramic washer5678Current leadthroughConnecting bushAnode pinCathode plate0.1 Ð 0.4Fig. 3.12 Cross-section of PENNINGVAC PR 35 gauge83HomeVacuum measurementforms a gettering surface film on the walls of the gauge tube. In spite ofthese disadvantages, which result in a relatively high degree of inaccuracyin the pressure reading (up to around 50 %), the cold-cathode ionizationgauge has three very outstanding advantages.
First, it is the leastexpensive of all high vacuum measuring instruments. Second, themeasuring system is insensitive to the sudden admission of air and tovibrations; and third, the instrument is easy to operate.3.3.3.2Hot-cathode ionization vacuum gaugesGenerally speaking, such gauges refer to measuring systems consisting ofthree electrodes (cathode, anode and ion collector) where the cathode is ahot cathode. Cathodes used to be made of tungsten but are now usuallymade of oxide-coated iridium (Th2O3, Y2O3) to reduce the electron outputwork and make them more resistant to oxygen. Ionization vacuum gaugesof this type work with low voltages and without an external magnetic field.The hot cathode is a very high-yield source of electrons.
The electrons areaccelerated in the electric field (see Fig. 3.13) and receive sufficient energyfrom the field to ionize the gas in which the electrode system is located. Thepositive gas ions formed are transported to the ion collector, which isnegative with respect to the cathode, and give up their charge there. Theion current thereby generated is a measure of the gas density and thus ofthe gas pressure. If i- is the electron current emitted by the hot cathode, thepressure-proportional current i+ produced in the measuring system isdefined by:i+ = C á iÐ á p undnitrogen this variable is generally around 10 mbar-1. With a constantelectron current the sensitivity S of a gauge head is defined as the quotientof the ion current and the pressure.
For an electron current of 1 mA and C= 10 mbar-1, therefore, the sensitivity S of the gauge head is:S = i+ / p = C á i- = 10 mbar-1 á 1 mA = 10 mbar-1 á 10-3 A = 1 á 10-2 A/mbar.Hot-cathode ionization vacuum gauges also exhibit gas sorption (pumpingaction), which, however, is considerably smaller than with Penning systems,i.e. approx. 10-3 l/s. Essentially this gas sorption takes place on the glasswall of the gauge head and, to a lesser extent, at the ion collector. Here useis made of nude gauges that are easy to operate because an externalmagnet is not needed. The upper limit of the measuring range of the hotcathode ionization gauge is around 10-2 mbar (with the exception of specialdesigns). It is basically defined by the scatter processes of ions at gasmolecules due to the shorter free path at higher pressures (the ions nolonger reach the ion collector = lower ion yield).
Moreover, uncontrollableglow or arc discharges may form at higher pressures and electrostaticdischarges can occur in glass tubes. In these cases the indicated pressurepI may deviate substantially from the true pressure pT.At low pressures the measuring range is limited by two effects: by the X-rayeffect and by the ion desorption effect. These effects results in loss of thestrict proportionality between the pressure and the ion current and producea low pressure threshold that apparently cannot be crossed (see Fig.
3.14).(3.3)+p=ii− ⋅ C(3.3a)The variable C is the vacuum gauge constant of the measuring system. ForAnodeUCUA(+ 50V) (+ 200V)Indicated pressure mbarIon collectorCathodUAActual pressure mbarUCI Pressure readingwithout X-ray effectII Apparent low pressurelimit due to X-ray effectIII Sum of I and IIi+: ion currenti-: electron currentFig. 3.13 Schematic diagram and potential curve in a hot-cathode ionization vacuum gaugeFig.
3.14 Apparent low pressure limit due to X-ray effect in a normal ionization vacuum gauge84HomeVacuum measurementThe X-ray effect (see Fig. 3.15)The electrons emitted from the cathode impinge on the anode, releasingphotons (soft X-rays). These photons, in turn, trigger photoelectrons fromsurfaces they strike. The photoelectrons released from the ion collector flowto the anode, i.e. the ion collector emits an electron current, which isindicated in the same manner as a positive ion current flowing to the ioncollector. This photocurrent simulates a pressure.
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