Dynamics of guided post-acceleration of protons in a laser-driven travelling-field accelerator

By directing the laser-driven electromagnetic pulses along a helical path, one can achieve a travelling-field accelerator arrangement for simultaneous beam shaping and re-acceleration of laser-accelerated protons. The dynamics of guided acceleration of the transiting protons was studied by varying the length of the helical coil. Experimental data shows that the protons co-moving with the field region exhibit stronger focussing while increasing the coil length, with an increase of kinetic energy due to simultaneous post-acceleration. The net energy gain for a coil of constant pitch however saturates eventually when the post-accelerated protons overtakes the accelerating field region in due course. 3D particle tracing simulation underpins the dynamics of beam transport inside the coil, which highlights the requirement for a variable pitch coil geometry in order to sustain the post-acceleration over an extended coil.