The all-optical approach to particle acceleration has drawn much attention over the past two decades for its potential advantages in dramatically reducing accelerator size and cost. The laser generated plasma can maintain a very high acceleration gradient, whereby efficient acceleration of the charged particles can be achieved. Especially in recent years, the use of wake-wave field technology to achieve electronic acceleration has made significant progress, the resulting particle beam quality and the traditional accelerator can be completely comparable. At present, experiments on laser-driven ion acceleration are also being carried out experimentally, mainly through the so-called sheath field acceleration mechanism: the potential gradient in the sheath field accelerates the ions emitted from the surface of the laser irradiated foil. The ion beam produced by this mechanism has some unique properties, but it also has some limitations in the energy spectrum, energy dispersion and divergence angle, which seriously hinders their practical application.
In an article published in 2016 [Nat. Commun., 7, 10792 (2016)], Satya Kar et al. Experimentally achieved a laser-driven, Further acceleration of the ions exiting the target. The coil not only boosts the energy of the ions, but it also achieves ion collimation within a narrow energy range. In addition, by arranging coils and targets one after the other, a cascade accelerator with beam collimation and energy selectivity can be constructed. The authors of the paper, Dr. Satya Kar, of Queen's University Belfast, UK, argue that "this progress has laid the groundwork for building the next generation of ultra-compact, low-cost particle accelerators that help to miniaturize advanced accelerator technologies."
In the experiment, the coil mainly works by directing the ultrashort electromagnetic pulse to transmit along its helical path, while the laser-driven ions move along the coil axis. The radial component of the electric field generated by the electromagnetic pulse is strong enough to bind the proton around the coil axis while the longitudinal component of the electric field accelerates the guided ion. As reported in the above paper, using a lab-scale laser in a proof-of-principle experiment, an effective post-exon acceleration of the emitted proton was achieved with an acceleration efficiency of 500 MeV / m, much higher than conventional accelerator technology could achieve.
The success of this program relies heavily on the understanding of electromagnetic pulses and their propagation along the coil. In a paper published in the first issue of High Power Laser Science and Engineering in 2017, researchers from Queen's University Belfast in the UK and University of Dusseldorf in Germany used a self-probing solution using laser-driven Protons study the propagation of the electromagnetic pulse in the spiral coil in situ from both the horizontal and vertical directions respectively.
The researchers used lateral detection mode to characterize the time-domain distribution of electromagnetic pulses transmitted along the helical coil. The experimental results show that the characteristics are similar to those obtained from previous measurements of plane geometry, as shown in Fig.1. On the other hand, the longitudinal detection of the coil illustrates the effect of the ultrashort characteristic of the electromagnetic pulse on the proton beam generation, that is, the field generated by the electromagnetic pulse reduces the divergence of the proton beam flux, and this effect is energy-dependent. By increasing the length of the coil, the focusing field functions for an extended period of time, whereby a high degree of focusing of the proton beam can be achieved. These results help to understand the intrinsic mechanism by which helical coil targets selectively direct ions and are also of great benefit to the further development of this technique.
Caption: Proton Transverse Detection of Electromagnetic Pulse Transmission along a Helical Coil. (A) Schematics of experimental setup; (b) Front view of the target and coil; (c) (d) Images of the helical coil using a proton beam with energies of 5.5 MeV and 3.0 MeV.
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