0020-4412/03/4604- $25.00 © 2003 åAIK “Nauka /Interperiodica” 0505 Instruments and Experimental Techniques, Vol. 46, No. 4, 2003, pp. 505–507. Translated from Pribory i Tekhnika Eksperimenta, No. 4, 2003, pp. 81–84. Original Russian Text Copyright © 2003 by Alekseev, Gubanov, Orlovskii, Stepchenko, Tarasenko. Pulsed electron beams formed in an direct-action accelerator are widely used. However, difficulties emerge in measuring the amplitude of an electron beam with a low density and short duration. When a nanosec- ond electron beam is ejected through a metallic foil into the air, the plasma generated by the electron beam emits, and the electromagnetic noise makes the detec- tion of the beam current more complicated [1, 2]. When a voltage pulse with a steep edge is fed from a generator into a load in the form of an antenna, the generator becomes an efficient source of broadband microwave radiation [3]. The measurements present the most severe problem if an electron beam is formed in gas- filled diodes at elevated pressures (about atmospheric pressure) and is ejected into the air through a grid [4] or hole [5]. In this case, the plasma produced in the diode generates high-power electromagnetic radiation [5]. At a high pulse energy, the plasma penetrates through the grid and reaches the Faraday cup, producing additional pickup in the recording equipment. The electromag- netic noise builds up with an increase in the rate of change of the voltage across the gap and the current through the load. The objective of this work is to develop and test a technique for detecting electron beams of low ampli- tude and short duration. We propose to determine the characteristics of an electron beam by the measured parameters of a non-self-sustained discharge initiated by the same electron beam. In this case, the amplitude of the beam current may be found from the amplitude of the discharge current or the energy lost in the gas. An additional discharge gap is used to implement this technique (Fig. 1). When the voltage across the additional gap in the air is less than a half of the static breakdown voltage, a non-self-sustained discharge is formed, being initiated by an electron beam. The con- tribution of the collisional ionization to the balance of the electron density is comparable to the rate of ioniza- tion by the electron beam only at high field intensities (E/p > 23 kV/(cm atm) for the air, where E is the elec- tric field intensity and p is the pressure) [6]. At low volt- ages across the gap, the processes of collisional ioniza- tion may be ignored, and the current of a non-self-sus- tained discharge is proportional to the electron-beam current. In this case, the balance of the charge-particle transport in an ionized gas of the “expert” gap can be described as dn/dt = Ψ – βn 2 . Then, when n increases from 0 to 0.9n max , we have t = 1.47/ Ψβ, where β is the effective recombination coefficient, Ψ = ( j Ô / e)N 0 p〈σ〉, j Ô is the density of the beam current for the external ionizer, e is the electron charge, N 0 is the number of neutral particles per unit volume at atmo- spheric pressure, p is the gas pressure, n is the electron concentration, and 〈σ〉 is the average cross section for the ionization of the gas by fast beam electrons. For p = 1 atm, j b = 1–10 A/cm 2 , β = 10 –7 cm 3 [6], and 〈σ〉 = 5 × 10 -18 cm –2 [7], we have t b ~ 10 –9 s. This is a time interval in which virtually the highest possible electron concen- tration for the given j b value is achieved. It is possible GENERAL EXPERIMENTAL TECHNIQUES Measuring the Parameters of an Electron Beam S. B. Alekseev, V. P. Gubanov, V. M. Orlovskii, A. S. Stepchenko, and V. F. Tarasenko High-Current Electronics Institute, Siberian Division, Russian Academy of Sciences, Akademicheskii pr. 4, Tomsk, 634055 Russia Received November 29, 2002 Abstract—A technique for determining the amplitude and time parameters of pulsed electron beams is proposed. Using this technique, it is possible to measure weak currents. It is based on the non-self-sustained discharge initiated by the electron beam under investigation. The experimental results are presented for two electron beams formed in a gas-filled diode at the atmospheric pressure of air, nitrogen, a mixture of CO 2 : N 2 : He = 1 : 1 : 3, or helium and ejected through a foil or grid. C 0 U 0 C 0 1 2 34 5 6 6 7 Fig. 1. Block diagram of the experimental setup: (1) pulse generator; (2) cathode; (3) AlBe foil; (4) container; (5) pro- filed electrode; and (6, 7) ring shunts.