High-Current Heavy Ion Beams in the Electrostatic Plasma Lens

Results are presented from studies of the focusing of wide-aperture low-energy (100–400 eV) and moderate-energy (5–25 keV) beams of heavy-metal ions by a high-current electrostatic plasma lens. It is found experimentally that, because of the significant electron losses, the efficient focusing of such beams can be achieved only if the external potentials at the plasma-lens electrodes are maintained constant. Static and dynamic characteristics of the lens are studied under these conditions. It is shown that, as the beam current and the electrode voltage increase, the maximum electrostatic field in the lens tends to a certain limiting value because of the increase in the spatial potential near the lens axis. The role of spherical and moment aberrations in the focusing of wide-aperture low-divergence ion beams is revealed. It is shown that, even when spherical aberrations are minimized, unremovable moment aberrations decrease the maximum compression ratio of a low-energy heavy-ion beam because of the charge separation of multiply charged ions in the focal region. At the same time, as the ion energy increases, the role of the moment aberrations decreases and the focusing of high-current heavy-ion beams by a plasma lens becomes more efficient than the focusing of light-ion (hydrogen) beams. This opens up the possibility of using electrostatic plasma lenses to control ion beams in high-dose ion implanters and high-current accelerators of heavy ions.

Here is a brief review some new results of development novel generation cylindrical plasma devices based on an idea of magnetic insulation electrons and an electrostatic plasma lens configuration. The axial-symmetric plasma lens is a well-explored device for manipulating high current, heavy ion beams, that has been used to preference to focus wide-aperture, heavy metal ion beams such as produced by vacuum arc ion sources (like MEVVA ion sources). The plasma lens configuration of crossed electric and magnetic fields provides an attractive and suitable method for establishing a stable plasma discharge at low pressure. Use of plasma lens configuration in this way was elaborated, investigated and developed some cost efficiency, low maintenance plasma devices based on this approach, for ion treatment and deposition of exotic coatings with given functional properties. These devices make using of permanent magnets and possess considerable flexibility with respect to spatial configuration. We describe the new results of theoretical calculations and experimental investigations physical mechanisms, determining optimal operation conditions cylindrical plasma devices based on plasma lens configuration.

Beam behavior under a non-stationary state in high-current heavy ion beams

Some features of the electrostatic plasma lens as applied to the focusing of moderate-energy, high-current, wide-aperture metal ion beams have been investigated and are described here. Static characteristics within the plasma lens volume have been explored. It is shown that when the potentials applied to the lens electrodes are held constant, the electric field increase with increasing ion beam current is limited by the increase of the potential in the paraxial region of the beam. Some estimates are made of the limiting static electric fields that can be obtained in the plasma lens, based on a quasi-neutral, two-component plasma model with anomalous electron mobility, where it is assumed that the anomalous mobility is caused by the absence of electron correlations when vortex-like structures appear in the hydrodynamically unstable plasma. The results of a theoretical analysis are compared with experimental data. Some results of experiments on the focusing of high-current, large area, heavy metal ion beams are also presented.

Summary form only given. In a heavy ion inertial fusion (HIF), the generation and transport of intense heavy ion beams are important issues, because the required beam parameters are far from those of the conventional particle accelerator system. Since there are no sufficient data for the system design, the study of space-charge-dominated beams is crucial in HIF. Especially, in the final stage of HIF driver system, the beam pulse must be longitudinally compressed from ~100 to ~10 ns. Induction voltage modulators, based on a pulsed power technology, are proposed for this purpose. The pulsed power technology serves a precise waveform controllability and repetition capability. A beam buncher by the induction modulator can apply bunching voltage so as to make a considerable velocity tilt between the head and the tail of the beam bunch. Although the whole HIF driver design is under construction, the final beam bunching is indispensable in all of the suggested HIF accelerators. We investigate the beam dynamics under a strong space charge effect during the longitudinal bunch compression in the final beam bunching by using the multi-particle numerical simulation. During the final beam bunching, a beam instability is induced by the strong space charge effect. The beam instability can cause the emittance growth. We study the emittance growth mechanism induced by the beam instability with two types of the transverse focusing lattice system, i.e. alternating gradient (AG) focusing and continuous focusing (CF) configurations. In the beam transport with the AG focusing lattice, the beam instability can be induced rather than in the case with the CF lattice configuration

Plasma shape control by pulsed solenoid on laser ion source

This report describes the results of a study of arrays of up to sixteen quadrupoles in a single cryostat surrounded by an induction accelerator that is used for accelerating high current heavy ion beams for fusion. Each quadrupole in the array can have a gradient of 72 T/m, when the quadrupole has a warm bore diameter of 90 mm. An array of sixteen quadrupoles can be made to fit into a round cryostat vacuum vessel with a diameter of 850 mm. If the number of quadrupoles in the array is reduced to nine, the outer diameter of the cryostat is 700 mm. It is proposed that the quadrupole array be conduction cooled using either a 4 K cryocooler or two phase liquid helium in pipes around the magnet array. The two-phase helium can be supplied to a string of multi-bore quadrupoles using a large refrigerator.

Influence of spherical and momentum aberrations of the electrostatic plasma lens (PL) on the focusing character of the intense ion beam on the target is analyzed. The beam form of a focused beam depending on the noncompensated space charge and finite azimuthal velocity at the PL exit is found. The minimum focused beam radii are calculated for different system parameters. These radii are shown to depend on the beam particles charge to mass ratio. The radial profile of focused multicharged ion beams having unhomogeneous stepped form was founded out experimentally. This effect is most distinct near PL focus and grows when the PL magnetic field does. This leads to the possibility of using the PL for ion charge state analysis.

Influence of spherical and momentum aberrations of the electrostatic plasma lens (PL) on the intense ion beam focusing and maximal compression is investigated experimentally and theoretically. It is shown that when spherical aberrations are absent unavoidable momentum aberrations affect the radial profile of multicharged ion beams. This profile appeared to be steplike. PL electric potential radial profile providing transformation of unhomogeneous (Gauss) profile of a beam at the PL entrance to homogeneous one on the target on the given distance from PL is obtained analytically. High-current PL efficiency for compensated ion beam setting on the quasi-Brillouin orbit is demonstrated to be experimentally and theoretically substantiated.

Summary form only given, as follows. Optimum plasma lens realized in experiments at weak magnetic fields, when the Larmor radius of electrons is comparable with radius of the lens, has been theoretically researched. The optimum lens is lens, in which the vortical perturbations are not excited and particle density in which is uniform in space. Three possible reasons not excitation of vortices in near axis area of the high-current plasma lens have been researched namely, finite time of ion movement through the plasma lens, finite time of electron renovating in it, proximity of the plasma lens parameters to optimum ones.

A brief review of investigations carried out in the Institute of Physics is presented along with an analysis of processes determining production of ions as well as formation and transport of intense ion beams in plasma-optical systems. All these systems have a common mechanism of space charge equipotentialization. When a noncompensated beam’s self-potential exceeds significantly an external potential, a charge compensation is mainly provided by secondary electron emission. Two systems are considered, a magnetically isolated diode gap (MDG) operating in the plasma-optical regime, and a high-current electrostatic plasma lens. It is demonstrated that a MDG can be used for production of hydrogen ions with unique parameters (current 6 A, density of current 6 A/cm2, energy up to 20 keV, pulse duration up to 100 μs). An effective method of intense ion beam low pressure transport is proposed.

Noble metal nanoparticles (NPs) are important in many applications, including light trapping of photovoltaic cells, photoelectrochemical applications, etc. The present study reports the formation of silver NPs from the as-deposited silver coatings on fused silica substrates by solid-state dewetting induced by high-current intense pulsed ion beam (IPIB) irradiation. We described the effects of IPIB irradiation with different ion beam current densities and numbers of pulses on NP morphology and compared the results with conventional rapid thermal annealing (RTA). IPIB irradiation enables superfast heating (higher than 109 K/s) and cooling, providing a superfast annealing solid-state dewetting mechanism. Our results demonstrate that the sphericity of silver NPs is enhanced after IPIB irradiation relative to RTA-annealed silver NPs. Our results suggest further possibilities of shape and sphericity control of silver NPs with very fast heating/cooling annealing rates.

Production, acceleration and diagnostics of high intensity beams

In this paper, we present the research results on focusing and transport of an intense (up to 100 A) nonrelativistic (up to 20 kV) pulsed electron beam using an axially symmetric device with a high-current plasma lens configuration. The electron source is based on electron extraction from the plasma of a hollow-anode vacuum arc discharge. The arc is initiated by a dielectric surface flashover. The emission hole is covered with a fine mesh grid. The beam is extracted and accelerated in a diode-type electro-optical system formed between the grid surface and an open anode plasma boundary. The anode plasma is produced in an electron beam transport channel through residual gas ionization by beam electrons and a plasma lens discharge. The plasma lens configuration of crossed electric and magnetic fields provides an attractive means to obtain a stable low-pressure plasma discharge. This geometry allows the compression of the electron beam in diameter from 6 to 1 cm with more than 100- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm A}/{\rm cm}^{2}$</tex></formula> beam current density at the collector.

The authors describe experimental investigations the dispersion characteristics of electron oscillations exciting owing to drift instability in a high-current plasma lens. The experiments were carried out using heavy metal ion beams of copper and carbon with energy up to 18 keV, beam current up to 0.5 A, duration 100 /spl mu/s produced by the MEWA kind ion source. It is shown that under the passing of an ion beam through a plasma lens, noise oscillations of electrostatic potential arise in the midplane of the lens. These noises have a regular structure with stationary amplitude in range of 0.25/spl divide/2 MHz depending on ion beam current value. It observed also on their background a path of comparable low-intensity high-frequencies noises in range 20/spl divide/50 MHz. They show that the regular structure is waves drift along the azimuth having a clear spatial localization on the essential radius from plasma lens axis. Experimental results are in accordance with theoretical analysis showing an appearance of nonlinear electron vortices structures for these conditions.