Proton probing of laser-driven EM pulses travelling in helical coils Download: 535次
1 Introduction
Ion beams generated via the target normal sheath acceleration (TNSA) mechanism posses remarkable characteristics such as high particle flux, ultra-low emittance and short pulse duration, but also exhibit large envelope divergence and broad energy distribution[1]. Although the two latter properties are advantageous in plasma radiography applications[2–6], these are generally undesirable in view of many other potential applications[7, 8]. Therefore controlling and optimising the laser driven ion beam parameters has been one of the intensively studied research topic over the past decade[8–13].
Fig. 1. (a) shows a schematic of the experimental setup for transverse proton probing of the EM pulse propagating along a helical coil (not to scale). (b) Shows the front view (looking from the detector side) of the target. (c) and (d) Show the radiographs of the helical coil obtained by 5.5 and 3.0 MeV protons, respectively. The dotted lines show the axes of the proton beam and the intersection point is the approximate centre of the proton beam. The spatial scale shown in (c) corresponds to the image plane.
TNSA-driven proton beams have been extensively used as a radiographic tool to study the dynamics of electric and magnetic fields generated by intense laser interactions[2–5]. The emission of an ultra-short burst of protons with a broad energy spectrum from a point-like source allows the implementation of point-projection probing schemes, while providing multi-frame snapshots of the probed object. The ultra-short burst duration enables a high temporal resolution (typically of a few ps), while the beam laminarity and small source size ensures a high spatial resolution[14, 15].
The propagation of an electromagnetic (EM) pulse generated by intense laser interaction with a solid target was recently studied by employing a self-probing arrangement[16–18]. The ultra-short EM pulses with peak electric field of the order of
It has been shown recently[16] that, by directing such a high amplitude EM pulse in a helical path around the proton beam, the spectral and angular properties of the beam can be controlled and optimized. This motivates the study of the propagation of the EM pulse in a helical geometry, which is presented in this paper. By following the spatial and temporal evolution of the electric field across the helical coil, probed transversely by the probe protons, the pulse profile was reconstructed with the help of particle tracing simulation. The characteristic parameters of the pulse, such as duration and amplitude, are broadly in agreement with those obtained previously in a planar probing geometry and similar interaction condition[17]. The effect of the EM pulse on the proton beam travelling through the helical coil was studied by probing a short helical coil longitudinally. Particle tracing simulations were employed to model this process, which are in agreement with the reduction in beam divergence observed for protons of a small energy range, travelling in synchronisation with the EM pulse. Furthermore, it is shown that the beam collimation is dramatically improved as the exposure of the protons to the EM pulse’s electric field is prolonged by extending the length of the coil.
2 Experiment
The experiment was performed using the TARANIS laser[20] at Queen’s university Belfast, employing the chirped pulse amplification (CPA) pulse of duration
3 Results and discussion
3.1 Transverse probing
Figures
By following the EM pulse through each winding of the helical coil at different probing times, as shown in the different RCF layers of the stack, the temporal profile of the travelling EM pulse can be reconstructed. In comparison to the square wave pattern used for the EM pulse characterization in the Refs. [16, 17], the helical coil geometry slightly complicates the data analysis as the different points on a given winding will be probed at different times even by protons of a fixed energy.
Fig. 2. (a) and (b) show the schematics (side and top view, respectively) of the arrangement for transverse probing of the helical winding. F and B represent, respectively, the front and back principal probing points on a winding chosen for analysis. The difference in probing times for the two principal points arises due to the different proton time of flights. (c) shows the temporal profile of the pulse travelling along the wire of the helical coil as obtained from the data shown in Figure 1 .
Fig. 3. (a) Image of the target used for longitudinal probing. The coil had ${\sim}900~\unicode[STIX]{x03BC}\text{m}$ diameter and 1.9 mm long and consisted of 8 windings with average pitch of ${\sim}260~\unicode[STIX]{x03BC}\text{m}$ . (b), (c) and (d) show experimental, spatially resolved dose profiles of the proton beam for energies 3.0, 4.4 and 5.5 MeV, respectively. (e) shows the percentage reduction of diameter of the central part of the beam with respect to proton energies (MeV), as obtained from the experimental (black) and simulated (red) RCF images.
The probing time at a given point,
After measuring the proton deflection in the data for a number of principal points along the helical coil, a linear charge density associated to the pulse was estimated at the corresponding probing time (
Fig. 4. (a) shows a schematic of the setup used for the shot taken with a 3.3 mm long helical coil with RCF placed at 70 mm from the interaction foil. (b), (c) and (d) show the raw RCF images for 3.0, 6.6 and 9.6 MeV protons respectively, where the pronounced focusing of the channelled beam of 3 MeV protons can be seen in (b), in contrast to the geometrical projection of the exit winding of the coil at the RCF plane shown by the red dashed circle.
3.2 Longitudinal probing
In order to control the inherent shortcomings of the TNSA-driven proton beams,
In order to understand more in detail the effect of the electric field on a beam of protons while transiting through a coil, longitudinal probing was carried out by employing a short coil of 1.9 mm long and radius of
By considering the projection of the exit ring of the coil over the RCF plane, the reduction in diameter of the central part was estimated, which is very pronounced for low energy protons, as illustrated in Figure
Although only a moderate reduction in beam divergence was obtained in this case (as compared for example to Ref. [16]), the observed energy dependance and its agreement with simulations underpin the underlying mechanism of the technique. One of the promising features of the helical coil lens is the transient nature of the travelling focusing field, which provides a flexibility to control the beam divergence of a given range of proton energies by carefully choosing the dimensions of the helical coil. Moreover, by increasing the length of the helical coil, while keeping the synchronization with the same slice of the energy spectrum, the focusing field can be applied for a longer time, which would result in a further reduction in the diameter of the channelled beam. In order to demonstrate this effect, a helical coil target of similar diameter and pitch (synchronizing around 3–4 MeV protons) as shown in Figure
4 Conclusions
The propagation of EM pulses in a helical coil, and their application towards improving the beam parameters of laser-driven ions were discussed. The temporal profile of an EM pulse travelling along a helical coil was characterized by transverse proton probing of the coil, which is in agreement with the previously reported measurements while probing the pulse-carrying wire in a planner geometry. By directing the transient EM pulses in a helical path around the proton beams, the angular and spectral properties of the proton beams can be controlled. The effect of the radial electric field inside a pulse-carrying coil towards focusing of the transiting protons was studied by probing a short coil longitudinally. Extending the coil length resulted in a highly collimated proton beam of less than
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Article Outline
H. Ahmed, S. Kar, A.L. Giesecke, D. Doria, G. Nersisyan, O. Willi, C.L.S. Lewis, M. Borghesi. Proton probing of laser-driven EM pulses travelling in helical coils[J]. High Power Laser Science and Engineering, 2017, 5(1): 010000e4.