Mechanism of electron current suppression in self-magnetically-insulated ion diode

Abstract

140Planar diodes with explosive emission cathodes are widely used for the generation of pulsed electron beams with current densities above 20 A/cm 2 . After the application of voltage to the diode and the forma tion of an anode plasma, generation of the electron and ion currents proceeds simultaneously. According to the Child-Langmuir relation [1], the proton cur rent density does not exceed 2.3% of the electron cur rent density, while the density of heavier ions is still lower. Thus, in order to more effectively generate ion beams, it is necessary to suppress the electron compo nent of the total diode current. In 1973, Sudan and Lovelace [2] proposed an original design of a pulsed ion diode with external magnetic insulation. However, the additional energy source necessary to create a magnetic field significantly increased the energy con sumption of the ion beam generator. At an ion beam energy of 80-90 J per pulse, up to 500 J was consumed by the field source [3].In 1977, Humphries [4] suggested the first con struction of an ion diode with self magnetic insula tion. According to this, a transverse magnetic field in the anode-cathode gap is formed by the intrinsic diode current passing through electrodes and, hence, no additional magnetic field source is required. This technical solution significantly simplifies the design and increases the reliability of high power ion beam sources. Unfortunately, the efficiency of ion beam generation in self magnetically insulated diodes does not exceed 10-15%, which significantly restricts their use [5].The present study was aimed at elucidating the mechanism of electron current suppression in self magnetically insulated ion diodes.The investigations were carried out on a TEMP 4M accelerator [6], which operated in a double pulse regime. The first (negative) pulse had a duration of 300-500 ns at a voltage of 100-150 kV, and the second (positive) pulse had a duration of 150 ns at 250-300 kV. The beam consisted of carbon ions (60-70%) and protons at a total ion beam current density on a target of 25 ± 5 A/cm 2 . The ion beam in the TEMP 4M accelerator setup was generated using self mag netically insulated diodes [7,8]. The majority of experiments were performed with a planar stripe diode with dimensions of 22 × 4.5 cm. The potential elec trode was made of graphite and the grounded elec trode was made of stainless steel with 0.4 × 5 cm slits and a geometric transparency of 60%. Figure 1 shows a schematic diagram of the diode unit and the scheme of total current and ion current density measurements.In contrast to the electron diodes with explosive emission cathodes, the grounded stripe electrode in the ion diode with self magnetic insulation is con nected to the chamber only at one end. During the first (negative) pulse, electrons start from the potential electrode and move in the anode-cathode gap toward the grounded electrode. Then, the electron current flows in this electrode to the ground and forms mag netic field in the anode-cathode gap so that the mag netic induction vecto...

Hydrodynamic Effects on Surface Morphology Evolution of Titanium Alloy under Intense Pulsed Ion Beam Irradiation

“…The irradiation was carried out on a pulsed ion beam accelerator TIA-450. [18,19] The IPIB was generated by a passive magnetically insulated diode (MID) with a graphite anode and accelerating voltage up to 250 kV, pulse width of 80 ns (FWHM). The beam was approximately composed of carbon ions (70%) and protons (30%) and the irradiation was carried out with various beam currents and shot numbers.…”