Sec. 4-5] 



E I. EC • T UK 'A I. T /,'. 1 AN Dl'C E US 



231 



4s 



A 



the two electrodes C and A. The electric field causes a deflection of 

 the electron beam which can be observed on the fluorescent screen F. 



The method is very sensitive. Warren, 1 who has combined this 

 system with a feedback arrangement that com- 

 pensates the beam deflection by an auxiliary 

 magnetic field, has measured fields from 1,000 

 down to 0.2 volts/cm. 



The gas in the tube tends to cause a diffusion of 

 the electron beam. With a gas discharge tube of 

 a diameter of 1 cm and a beam velocity of about 

 20 kV, the application of the method is limited 

 to pressures below about 1 mm Hg. 



-B 



//■ 



For further literature references, see the paper by 

 Warren, ibid. 



4-53. Method Based on Luminous-gas Dis- 

 charge in rf Fields 



f 



Fig. (4-5)2. Electron- 

 beam system for the 

 measurement of elec- 

 tric fields: A and C, 

 electrodes between 

 which an electric gas 

 disci large passes; 67, 

 electron gun; .B, elec- 

 tron beam; F, fluores- 

 cent screen. 



A system for the measurement of high field 



strengths (of the order of several hundred volts 



per centimeter) in rf fields has been described by 



Lion. 2 The system is shown schematically in 



Fig. (4-5)3. A spherical tube B containing gas 



under reduced pressure is brought into the electric rf field. If the 



field strength is sufficiently high, an electrodeless glow discharge 



will arise in the tube. The tube is surrounded by an insulating, light- 

 tight shield S which may be constructed 

 as a light guide. The light emitted from 

 (Q- tjj\ the tube falls upon a photoelectric cell P 



s 



-^ 



which is outside the field and which is con- 

 nected to an amplifier and a meter. 



Calibration curves (light emitted by 

 the tube B versus field strength) are 

 shown in Fig. (4-5)4. The transfer char- 

 acteristics are linear, but their slope 

 varies with the frequency of the rf field. 

 The system avoids metallic parts in the 

 field, the glow tube does not disturb the field appreciably, and the 

 power consumption of the tube is, in general, a fraction of a watt. 



1 R. W. Warren, Rev. Sci. Instr., 26, 765 (1955). 



2 K. 8. Lion, Helv. Physica Acta, 14, 21 (1941), and Rev. Sci. Instr., 13, 338 

 (1942); a similar system is described by J. F. Steinhaus, Rev. Sci. Instr., 27 

 575 (1956). 



Fig. (4-5)3. Electric-field trans- 

 ducer for measurement in 

 strong rf fields: B, electrode- 

 less discharge tube: «S, tubular 

 light shield; P, photoelectric 

 receiver. 



