The all-optical scheme to achieve particle acceleration has attracted a lot of attention in the past two decades due to its potential advantages in significantly reducing the size and cost of accelerators. The plasma generated by the laser pointer can maintain a very high acceleration gradient, thereby enabling effective acceleration of charged particles. In particular, in recent years, significant progress has been made in the use of coda wave field technology to achieve electron acceleration, and the quality of the particle beams produced by it is completely comparable to that produced by traditional accelerators. At present, experiments are also being carried out on laser-driven ion acceleration, which is mainly achieved through the so-called sheath field acceleration mechanism: the potential gradient in the sheath field causes ions emitted from the surface of the foil irradiated by the laser to be accelerated. The ion beam generated by this mechanism has some unique properties, but it also has some limitations in energy spectrum, energy spread, and divergence angle, which seriously hinder their practical application.
In an article published in 2016, by attaching a coil device to the rear surface of the foil, the ions emitted from the foil target driven by the laser were further accelerated experimentally. The coil can not only increase the energy of ions, but also achieve the collimation of ions in a narrow energy range. In addition, by arranging coils and targets in sequence, a cascade accelerator with beam dynamic collimation and energy selectivity can be constructed. Dr. Satya Kar, one of the authors of the paper and Queen University of Belfast, UK, believes that “this progress has laid the foundation for the construction of the next generation of ultra-compact, low-cost particle accelerators and provided assistance for the miniaturization of advanced accelerator technology.”
In the experiment, the coil mainly works by guiding ultrashort electromagnetic pulses to propagate along the direction of its spiral path, while the laser-driven ions advance along the direction of the coil axis. The radial component of the electric field generated by the electromagnetic pulse is strong enough to confine protons near the coil axis, while the longitudinal component of the electric field accelerates the guidance of ions. As reported in the above paper, the proof-of-principle experiment uses college-scale lasers to achieve effective post-acceleration of the proton emission, with an acceleration efficiency of 500 MeV/m, which is much higher than that achieved by traditional accelerator technology.
The success of this scheme relies heavily on an 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 2017, researchers from Queen’s University Belfast and Dusseldorf University in Germany adopted a self-detection technology solution using green laser pointer-driven protons studied the propagation of electromagnetic pulses in helical coils in situ from the horizontal and vertical aspects.
The researchers characterized the time-domain distribution of electromagnetic pulses transmitted along the helical coil through the lateral detection mode. The experimental results show that its characteristics are similar to the results measured before in the plane geometry situation, as shown in Figure 1. On the other hand, the longitudinal detection of the coil clarifies the effect of the ultrashort characteristics of the electromagnetic pulse on the proton beam, that is, the field generated by the electromagnetic pulse will reduce the divergence of the proton beam, which is energy-dependent. By increasing the length of the coil, the focusing field plays a role for a longer period of time, so that the proton beam can be highly focused. These results help to understand the inherent mechanism of the helical coil target selectively guiding ions, and at the same time, it is also beneficial to the further development of this technology.