The all-optical program to achieve particle acceleration has attracted a lot of attention in the past two decades with its potential advantage in dramatically reducing the scale and cost of accelerators. The plasma generated by the laser can maintain a very high acceleration gradient, whereby efficient acceleration of the charged particles can be achieved. In particular, in recent years, the use of tail wave field technology to achieve electronic acceleration has made important progress, the resulting particle beam quality and traditional accelerator produced can be compared. Efforts to accelerate laser-driven ion acceleration are also currently being experimentally achieved by the so-called sheath-field acceleration mechanism: the potential gradient in the sheath field causes the ions emitted from the surface of the 30mw laser pointer irradiated foil to be accelerated. The ion beam produced by this mechanism has some unique properties, but there are some limitations in terms of energy spectrum, dispersion and divergence angle, which seriously hinders their practical application.
By further attaching a coil device on the rear surface of the foil, a further acceleration of the burning laser-driven ion from the foil target is experimentally achieved. The coil not only improves the energy of the ions, but also achieves ion alignment in a narrow energy range. In addition, by sequentially arranging the coils and targets, a cascade accelerator with beam dynamic collimation and energy selectivity can be constructed. Dr. Satya Karr believes that this progress for the next generation of ultra-compact, low-cost particle accelerator laid the foundation for the advanced accelerator technology to provide a small increase.
In the experiment, the coil is mainly through the guidance of ultra-short electromagnetic pulse along the direction of its spiral transmission to work, while the laser-driven ions along the coil axis direction. The radial component of the electric field generated by the electromagnetic pulse is of sufficient strength to bond the proton to the vicinity of the coil axis while the longitudinal component of the electric field accelerates the conducting ions. As reported in the above paper, the principle validation experiment using the laboratory-scale high power laser, the realization of the effective proton after the acceleration, acceleration efficiency of 500 MeV / m, much higher than the traditional accelerator technology can be achieved.
The success of this program relies heavily on the understanding of the electromagnetic pulse and its propagation along the coil. The researchers used a self-probing technique to study the propagation of electromagnetic pulses in helical coils in situ from two aspects of transverse and longitudinal, respectively, using laser-driven protons.
The researchers used the transverse detection mode to characterize the time domain distribution of electromagnetic pulses transmitted along helical coils. The experimental results show that the characteristics are similar to those of the previous geometric situation measurements, as shown in Fig. On the other hand, the300mw laser pointer longitudinal detection of the coil illustrates 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 over a longer period of time, whereby the height of the proton beam can be focused. These results help to understand the intrinsic mechanism of helical coil target selective ionization, and it is also beneficial for the further development of this technique.
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