Enhancement of the laser-driven proton source at PHELIX
1 Introduction
Within the past two decades, an extensive amount of work has been conducted on the topic of laser-driven proton acceleration, motivated by the unprecedented properties offered by such a particle source. Indeed the emittance and time duration of a laser-driven proton beam are many orders of magnitude lower than those for conventional accelerators[1]. Therefore, such a source holds promise for a broad range of new applications from biology[2] to novel energy concepts[3]. Many studies have focused on advanced acceleration mechanisms in the regime of relativistic intensities[4] or in the radiation pressure-dominated[5] regimes. However, the experimental results reported so far show that the most reliable acceleration mechanism is target normal sheath acceleration (TNSA), delivering good laser-to-proton conversion efficiencies[6]. Such a source is needed for many applications such as proton radiography, hadron therapy and the generation of neutron beams[7–9].
Even though there exists a large amount of research on TNSA, there is still a significant discrepancy in the experimental data reported by the laboratories working in this field[10]. An explanation for this is the variation in parameters for different lasers and uncertainties in the assessment of important experimental properties such as the on-target intensity distribution and the exact temporal profile of the driving laser pulse at a high dynamic range. These parameters may have constructive or destructive effects on proton generation: for instance, it is well known that thinner targets deliver better performance for a given set of laser parameters. However, increasing the temporal contrast of the laser to enable shooting thin sub-micrometre targets reduces the laser absorption and has a potentially negative impact on the efficiency of TNSA[11].
These parameters can be systematically controlled at the PHELIX laser (
On the way to generate reproducible proton beams, which can be used for further applications, we present a study at PHELIX comparing the influence on the ion acceleration when using a last focusing optic made of high-quality
2 Setup
The experiments were conducted during two different campaigns, using a similar setup as shown in Figure
Fig. 1. Top view of the basic setup used for laser-ion acceleration at which the laser is focused onto a thin target with varying incidence angle. The accelerated ions are captured by an RCF stack placed in the laser direction, together with a Thomson parabola, for the first setup and are rotated together with the target for the second setup.
Fig. 2. Comparison of the focal spot of the copper and glass parabolas, taken during alignment mode inside the target chamber, showing the improvement introduced by the new optic (upper half). The yellow curve corresponds to a lineout over the horizontal direction through the centre, which additionally shows the FWHM of the foci. The lower part of the figure shows the comparison of the encircled energy for both parabolas.
The accelerated ions are caught by a stack of radiochromic films (RCFs) layered in between Mylar foils. For the first campaign using an interaction angle close to normal incidence, the stack was placed in the laser direction at a distance of 10 cm. For the second campaign, the interaction angle was increased to
The targets, which were manufactured at the Technische Universität Darmstadt, consist of flat foils attached to small
To ensure similar conditions during all shots, the beam properties are monitored at different positions in the laser chain of PHELIX. The on-shot diagnostics, measuring near- and far-field, are placed behind the main amplifier and the compressor. Additionally, the wavefront is measured after the main amplifier, using a home-made Shack–Hartmann sensor and control software[17]. This device is part of the closed-loop wavefront control system at PHELIX that corrects the static wavefront distortions.
3 Overall laser performance
The laser parameters have a decisive influence on particle acceleration. Since the on-target laser intensity cannot be measured directly, it is always inferred from the measurement of the pulse energy, the pulse duration and the focal-spot fluence distribution. The latter is not measured on-shot but during alignment with the un-amplified beam.
The on-shot energy is measured at the output of the main amplifier using a pyroelectric detector located behind a leaky mirror. This detector is cross-calibrated with a full-beam-size calorimeter capable of measuring the energy of the full beam in a range from 20 J to 5 kJ, located before the leaky mirror. This calorimeter can be self-calibrated using resistive ohmic heating to deliver absolute energy measurements. Additional passive losses introduced by the compressor and transportation to the target chamber can also be measured by a second pyrometric detector located inside the target chamber. These measurements add up to a total uncertainty of the energy of
The focal spot is measured with a Plan Apochromat microscope objective, which is aligned in the laser direction, with the image relaying the focus to the centre of a 16-bit camera chip (Hamamatsu ORCA-flash4.0 LT) located outside of the target chamber. The imaging is in total carried out by two telescopes with a total magnification of 8. After the initial alignment, the camera centre and the focal position define the target chamber centre (TCC), onto which the targets are aligned later on. The exact magnification factor is determined by moving a sharp edge in the imaging plane by a known magnitude, typically
This measurement of the focus is additionally multiplied by an on-shot factor, which is obtained by dividing the maximum value in the far-field at the compressor diagnostic on-shot, by the value obtained prior to the shot in low energy mode, after normalizing them to the integral of the signal. This shows an increased area on-shot in which the energy is distributed, leading to an estimated on-shot intensity of only (
A larger impact on the intensity is given by the pulse duration, which was measured inside the target chamber, using a device based on FROG[18]. The measurement was carried out with low energy, only using the preamplifier, and a full energy shot as a comparison. Every measured FROG trace was reconstructed several times, while utilizing the bootstrap method. The resulting mean value of the pulse duration from measurement to measurement fluctuated by 8%, peak to valley, at a pulse duration of 500 fs. However, the reconstructions dominated the dispersion with
In addition, the knowledge of the temporal profile of the pulse on a high dynamic range is essential, and its characteristics such as the amplified spontaneous emission (ASE) background, the specific shape of the rising slope on a picosecond time scale and the possible presence of pre-pulses may alter the target condition at the time of the interaction and prevent the use of ultra-thin targets. For this reason, the temporal contrast has to be measured with a very high dynamic range, covering 12 orders of magnitude, and a wide temporal window of 2.5 ns prior to the peak intensity[19]. It is measured during the alignment mode prior to each experimental campaign to deliver the best knowledge of the experimental conditions. These measurements show that we reach a pre-pulse contrast of
All of these parameters have an influence on the maximum proton energy that is attainable, but only some of these can be improved with a reasonably small effort.
4 Influence of the focus quality on the proton beam
A well-known approach for enhancing the maximum proton energy and particle numbers is to increase the on-target laser intensity[10, 20]. Since a reduction in pulse duration is not easily obtainable, this can be done by either increasing the energy of the pulse or, due to an upper limit of 200 J for PHELIX, improving the focal spot. The sources of focal-spot degradation can be divided into two categories: static and dynamic. Beam degradation due to static aberration originates from alignment issues and the inherent quality of the optics. In general, flat optical surfaces at PHELIX are specified to have a
The situation is more complicated in and after the compressor, where the wavefront errors of the gratings (
To confirm the impact of the diamond-turned parabola compared to the performance with a high-end focusing element, we exchanged the copper parabola with a dielectrically coated high-quality parabolic mirror having the same geometrical properties. This parabola exhibits a better reflected wavefront error of
Quantitatively, one can see a strong reduction of the coma-like aberration, which distributes more energy in the outer region of the focal spot. This improvement can also be quantified by looking at the encircled energy for both parabolas (bottom). The energy within a radius of
To evaluate the effect of this improvement on the accelerated ion characteristics, a dedicated experiment was conducted, checking the scaling of maximum proton energy and laser intensity. This is done by focusing the laser onto a 300-nm-thin polystyrene target with an incidence angle of
By doing so, we quickly realized that the 300-nm-thin targets showed some signs of boring for the higher laser energies when the copper parabola was used. An indicator of this is an increase in the blurred electron background seen in the RCFs[21]. For this reason, the later shots were carried out with
Fig. 3. Scaling of the maximum proton energy in dependence on the laser intensity for different focusing optics and target thicknesses. The black dots correspond to the copper parabola with 300 nm target thickness, whereas the blue and red dots belong to the glass parabola with target thicknesses of 300 nm and , respectively. The black and blue lines correspond to a fit that is proportional to for the copper and glass parabolas, respectively.
In addition, one can see that the shots carried out with the glass parabola delivered the highest proton maximum energies. For an estimated intensity of
Fig. 4. Data of the PHELIX record shot showing proton energies of at least 90 MeV with a possible extension up to 93 MeV. Image (a) shows the signal at the last five EBT-layers, whereas the contrast has been enhanced to increase the visibility of the proton signal. The yellow arrow indicates the position of the highest energetic protons, close to the hole, which is used to obtain the Thomson parabola trace, shown in image (b). The lines and numbers correspond to the position and thickness in mm of the copper filters introduced in Section 2 . The spectrum that is extracted from this trace is shown in figure (c), showing a transition to the background level at 85–90 MeV.
Although this represents a new record for protons accelerated at PHELIX, the values obtained show only a slight improvement compared to previous data published by our group[24]. To analyse the difference, a square-root fit to the experimental data has been carried out, using a least-squares error merit function, which is shown in Figure
The first explanation for the deviation from the
The second reason for that could be related to the energy distribution, since the central spot of the focus gains energy, and therefore intensity when using the glass parabola, but its surrounding loses energy. This would lead to an increased contribution to the acceleration at the centre, and a reduction in the impact from the outer regions, only leading to a minor total increase.
Fig. 5. Scaling of the maximum proton energy in dependence on the laser power for different focusing optics for the same parameters as shown in Figure 3 . The lines correspond to the laser power scaling mentioned by Zeil et al. [20].
Since the scaling of the maximum proton energy with the laser intensity does not seem conclusive to draw a rule of thumb, a better picture can be obtained from a scaling law based on the laser peak power, as shown in Figure
The conclusion to this first study is that the use of a better optical focusing element brings a measurable although slight improvement to laser-driven ion acceleration, with a new record of
5 Enhancement of the proton source for day-to-day operation
As mentioned, it is of interest to reduce the amount of debris deposited on the focusing element and therefore the operation cost. Many of the usual methods relying on the use of a debris shield are hard to implement at PHELIX because of the combined difficulties arising from the short pulse duration, the large beam size and the incidence angle of
There are two strategies that can easily be implemented at PHELIX to increase laser absorption. One possibility is of increasing the preplasma formation[28] by decreasing the nanosecond temporal contrast of the laser, which was demonstrated by an earlier work of our group[29]. By doing so, a density of
Both approaches have been tested and compared to the regular setup with s-polarization, a high temporal contrast of
The first configuration is carried out with a nanosecond temporal contrast of
Fig. 6. Maximum proton energy depending on the laser peak power for different laser and setup parameters at incidence angle. The lines show the corresponding laser-power fit by Zeil et al. [20].
The dependence of the maximum proton energy on the peak power for the three different setups can be seen in Figure
Fig. 7. Proton spectrum for each configuration with comparable laser power on target. The exponential function is obtained by an iterative fit to the deposited energy in the RCF layers, whereas the circles are obtained by sequential deconvolution of the signal from the last layer from the previous ones.
We reconstructed the proton energy spectra based on the RCF stack available and plotted the results for the various configurations, which were obtained with similar laser powers, in Figure
Fig. 8. Comparison of the maximum proton energy scaling dependent on the laser peak power for the first beamtime (blue) and the p-polarization setup (red), both conducted with a high contrast. Despite the large angle difference of , the scaling is similar in both cases.
Figure
6 Summary
We have performed two experimental campaigns, focusing on the enhancement of laser-ion acceleration with the high-power laser PHELIX. The goal of these was to optimize the maximum achievable proton energy and particle numbers, while working under experimental conditions that minimize operation cost. This was done by improving the quality of the focal spot, which increased the expected intensity by a factor of 2.8 up to
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Article Outline
J. Hornung, Y. Zobus, P. Boller, C. Brabetz, U. Eisenbarth, T. Kühl, Zs. Major, J. B. Ohland, M. Zepf, B. Zielbauer, V. Bagnoud. Enhancement of the laser-driven proton source at PHELIX[J]. High Power Laser Science and Engineering, 2020, 8(2): 02000e24.