Effect of the Electron-Emitter Parameters of a Pulsed Source of Light Penning Ions on the Extracted Current
- PDF / 686,927 Bytes
- 7 Pages / 594 x 792 pts Page_size
- 79 Downloads / 138 Views
SCIENTIFIC AND TECHNICAL COMMUNICATIONS EFFECT OF THE ELECTRON-EMITTER PARAMETERS OF A PULSED SOURCE OF LIGHT PENNING IONS ON THE EXTRACTED CURRENT
D. S. Stepanov, A. P. Skripnik, and E. Ya. Shkol’nikov
UDC 533.9.072
Gas-filled neutron tubes generating pulses with specified parameters are needed for neutron well-logging. These parameters are affected most by the ion-pulse shape. The results of numerical modeling of the dynamics of the electron beam, secondary ions, and electrons generated in the volume of the Penning ion source with an electron emitter and their effect on the ion-pulse parameters are examined. The calculation was performed in an axisymmetric geometry with the aid of the KARAT software package. The physical processes occurring in the ion source and their influence on the growth of the ion current extracted from the plasma as a function of the maximum current of the injected electrons and the geometry of their source were analyzed. It was found that the steady-state ion current is independent of the electron current and is mostly due to the geometry and radius of the electron beam.
The present scientific and technical communication is the first part of a study devoted to a description of the dependence of the parameters of the pulsed ion current extracted from a Penning source on its internal structure. This dependence makes it possible to determine the conditions for realizing different neutron generation regimes ~1.5·108 sec–1 with the trailing and leading edges of the pulse being kept at the level required for neutron logging (~1 μsec) [1, 2]. The most efficient way to satisfy the indicated conditions is to introduce a thermionic cathode into the makeup of the ion source for gas-filled neutron tubes. The electronic emitter will make it possible to increase the injected electron concentration from 105 to 109 cm–3, which in turn will make it possible to reduce the leading edge of the neutron pulse from ~20 to ~1 μsec and less. Description of the model. The numerical model of the Penning ion source was constructed using the KARAT code and is shown in Fig. 1 [3]. With the exception of the presence of an electron emitter its geometry tree and parameters correspond to the neutron tubes used for logging [1, 2, 4, 5]. The inner surface of the source, including the cathode and anti-cathode, are under zero potential. A 2.5 kV voltage pulse with a 0.5 μsec leading edge is supplied to the anode. The equivalent extracting electrode with potential –2.5 kV creates inside the ion source the same electric field distribution as a full-fledged ion-optical system. The field inside the source is created by a hollow cylindrical magnet arranged around the outer surface of the ionsource chamber. Electrons are injected from a portion of the cathode that corresponds to a ring-shaped beam. Their current is determined from the condition that the normal component of the electric field on the surface of the emitter is equal to zero. The maximum current can be limited by some amplitude value corresponding to the operation o
Data Loading...
