Spatial and temporal distribution of a 1-MeV proton microbeam guided through a poly(tetrafluoroethylene) macrocapillary

Abstract

We present computer simulations about the spatial and temporal evolution of 1 MeV proton microbeam transmitted through an insulating macrocapillary with the length of 45 mm and with the inner diameter of 800 μm. The axis of the capillary was tilted to 1° relative to the axis of the incident beam, which ensured geometrical non-transparency. The simulation is based on the combination of stochastic (Monte-Carlo) and deterministic methods. It involves 1) random sampling of the initial conditions, according to distributions generated by the widely used and freely available computer software packages, SRIM and WinTRAX, 2) the numerical solution of the governing equations for following the classical trajectory of the projectiles, and 3) the description of the field-driven charge migration on the surface and in the bulk of the insulator material. We found that our simulation describes reasonably all of our previous experimental observations, indicating the functionality and reliability of the applied model. In addition, we found that at different phases of the beam transmission, different atomic processes result in the evolution of the beam distribution. First, in a scattering phase, the multiple small angle atomic scattering dominates in the beam transmission, resulting an outgoing beam into a wide angular range and in a wide energy window. Later, in a mixed phase, scattering and guiding happens simultaneously, with a continuously increasing contribution of the guiding. Finally, in the phase of the stabilized, guided transmission, a quadrupole-like focusing effect is observed, i.e. the transmitted beam is concentrated into a small spot, and the transmitted protons keep their initial kinetic energy.

Guiding as a General Consequence of the Charged Particle Interaction with the Inner Surface of an Insulator Capillary—Guiding of 1 MeV Proton Microbeam through Polytetrafluoroethylene Macrocapillary

“…In addition, the whole beam could be deflected vertically with the electrostatic deflectors, which proves, that the beam avoided the charge-exchange with the target and kept its initial 1+ charge state. Our experimental finding was certificated by our recent theoretical works [34].…”

Electric field driven focusing and transport of plasma ion beams by micro-glass capillaries beyond the self-focusing limit

“…The observed reduction in beam size with an increase in V 2 indicates an enhanced self-focusing capability of the capillary at higher voltages. Previous investigations [27,28,30,42,43] have shown that the self-focusing capability and ion beam transport through the capillary are determined by the charge patch distribution (σ) on the inner wall of the capillary and the resulting distribution of equipotential lines. To better understand this process, we performed simulations to obtain the distribution of σ and the equipotential lines, as detailed in section 3.…”

“…Since Stolterfoht et al first observed the ion guiding effect in insulating materials in 2002 [1], the guiding capabilities of ion beams through capillaries made of various materials have been explored experimentally worldwide. These materials include polyethylene terephthalate (PET) polymers [25][26][27], poly(tetrafluoroethylene) (PTFE) [28], Al 2 O 3 [29], SiO 2 [6,7,30], mica [31,32], Teflon [33], and polycarbonate [5,34]. Furthermore, investigations on capillary guiding effects have been conducted using not only ions but also electrons [35][36][37][38] and negative ions [39,40] as projectiles.…”