4 Proton-Acceleration Experiments
Proton beams accelerated at the rear surface of a thin foil irradiated by a high-intense laser pulse have attracted a great interest of research. The prominent beam characteristics, such as high particle numbers in a short pulse duration, directed almost laminar beam propagation and a low transverse emittance, make laser-accelerated proton beams attractive for multiple applications (see section 2.3). Some appli-cations, especially as a new generation ion source, require that the proton energy spectrum, the beam collimation and the transport capability are carefully controlled and tailored. In the past 10 years dif-ferent appoaches were made. Sub-millimeter targetry is a very common approach to reduce the initial beam divergence [28] or to enhance the efficiency of the acceleration mechanism [40, 41]. However, to achieve the precision of aligning on the "‘right spot"’ especially in high repetition rate experiments is a challenging task.
The experiments carried out in the scope of this thesis focuses on the optical control of the proton beam parameters by means of using dual laser pulses or changing the focal conditions. In the first part of the chapter studies on the effects of laser-driven front surface pre-plasma expansion on proton acceleration are presented. The experiment was performed at the VULCAN Petawatt laser system at the Central Laser Facility, Rutherford Appleton Laboratory, Didcot UK, and the results are published in [34, 178].
In the following part, studies of the influence of laser defocusing on the proton flux relevant for the fast ignition scheme are adressed. At the VULCAN Petawatt laser system conversion efficiencies from laser into proton energy of up to 7.9% were achieved. In the last part, x-ray Thomson scattering on laser-accelerated proton heated warm dense matter is presented as a possible application for this kind of particle source. Studies on target and diagnostic optimization were performed on the TRIDENT laser at the Los Alamos National Laboratory, New Mexico, USA.
light was used to drive the acceleration of protons. The focusing beam coming from the f/3 parabolic mirror is deflected by a plasma mirror [182, 183], at a position such that the intensity is in the range 1014-1015W/cm2. It is an antireflection coated glass slab, which can be used to enhance the con-trast of the laser on target by suppressing the ASE (Amplified Spontaneous Emission) pedestal to
∼1011 W/cm2. As soon as a suffciently intense laser pulse is incident on the slab, ionization takes place on the leading edge of the pulse and the peak of the pulse then interacts with a dense plasma. The mirror effect is achieved by the rapid change in reflectivity as the substrate evolves from a solid with a reflectivity of∼10−3into a plasma with reflectivity near unity. The glas is almost transparent for the ASE prepulse, while the primary short pulse easily ionizes the target and reflects off the plasma towards the target. A calibration yielded an energy reflection onto the target of 32%. The maximum energy on target was115 J, focused to a spot size of5µm(FWHM), leading to a peak intensity of 3×1020W/cm2. The angle of incidence onto the plasma mirror was 20◦ and onto the target 10◦.
The plasma formation at the target front surface, prior to the arrival of the main pulse, was induced by1054 nmlaser pulses of6 nsduration with lower intensities in the range of 0.5-5×1012W/cm2. The temporal profile has a fast rise time of0.2 nsand a slow decay from peak intensity to∼50% at6 ns. An f/10 lens was used to focus the beam to an approximately flat-top intensity distribution with a diameter of450µm, centered on the position of the short pulse focus. In addition, the delay of the short pulse beam with respect to the arrival of the long pulse on target was varied in the range of 0.5 to 3.6 ns with 0.2 nsprecision.
The targets were either25µmthick planar copper foils or25µmthick gold foils with a periodic groove structure (same targets as shown in figure 3.5). The spatial and energy distributions of the accelerated protons were measured with radiochromic films in stack configuration with alternate copper layers in between to increase the detection range. The stack was placed40 mm behind the target and aligned along the target normal axis. As shown in figure 4.1, the targets and the RCF stacks were mounted on rotation wheels to enable multiple shots before venting the target chamber. Thus, more shots were possible, because one vent and pump cycle lasted at least60 min.
In addition, a transverse interferometric optical probe (frequency doubled) was installed to study the front surface plasma expansion [184]. The target was probed at a fixed time of5 psafter the arrival of the short pulse. The snapshot was recorded by a CCD camera.
4.1.2 Results
Two different preplasma studies were carried out. First, the arrival of the long pulse beam was fixed to
∆t=0.5 nsbefore the short pulse reached the target and the proton beam parameters were measured for ablation pulse intensities from (0-4.9) TW/cm2). The second series was recorded at a fixed intensity of 1.2 TW/cm2 but as a function of the delay ∆t from 0 to 3.6 ns). The intensity scan leads to the results illustrated in figures 4.2 and 4.3. By the help of the radiochromic film imaging spectroscopy, the envelope-divergence and proton source size are calculated. Before discussing the results, the scale length of the plasma electron density is introduced. The interferometric probe provides a picture of the plasma expansion at the target front side. For each shot in the series, a picture of the electron density was recorded. The interferometry was limited in resolving higher densities. Hence, two different scale lengths are defined. Lo refers to the outer part of the preplasma, the underdense region. The inner region near the critical density is described byLi, which can not be resolved.Lois determined by fitting the relationne(x)∝ exp(−x/L0)to the electron density profile along the target normal extracted from the interferometric probe measurements.neis the electron density andxis the distance from the target surface.
48 4 Proton-Acceleration Experiments
Figure 4.1:Drawing and picture of the main experimental setup. The short pulse is deflected by a plasma mirror onto the target. Just before, the long pulse is focuses on the same spot to generate a preplasma. The targets and the RCF stacks are mounted on rotation wheels to enable multiple shots during one vent and pump cycle.
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Figure 4.2:Envelope-divergence of the VULCAN proton beam for the intensity scan of the long pulse beam.
The envelope-divergence clearly drops for an increasing scale length. But scaling all beams to their maximum proton energies, an almost similar behaviour can be observed apart from the shot withILP= 0.54 TW/cm2, where the scale length is in the optimum range to increase the proton number and the flux.
4.1 Proton beam manipulation by pre-plasma shaping 49
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Figure 4.3:Proton source size of the VULCAN proton beams for the intensity scan of the long pulse beam. It was not possible to calulate the source size for the proton beam up to the highest proton energy, because the line pattern in the detector disappears. However, if the proton beams are scaled to their maximum energy obtained by the envelope-divergence measurement no particular oberservation can be made.
It can be observed that up to an induced plasma scale length of60µm, the proton spectra is enhanced with much higher fluxes for all energies. Also a higher maximum proton energy up to∼25% compared to the case with no or very little pre-plasma is found. As the plasma scale length is increased to val-ues between 60 and 120 µm the maximum proton energy and the proton numbers at all energies are reduced to measured parameters without prepulse. Beyond plasma scale length of120µm the fluxes drop even more. As expected, the plasma expands faster when heated at higher intensities, creating a longer plasma scale length. An intensity dependent prepulse effect can also be seen in the envelope-divergence and proton source size illustration in figures 4.2 and 4.3. For scale lengths >60µm the envelope-divergence clearly drops for an increasing scale length. But scaling all beams to their maxi-mum proton energies, an almost similar behaviour can be observed. The shape of all curves is the same and they lie on top of each other apart from the shot withILP= 0.54 TW/cm2. For this case, the plasma scale lengthLo is∼38µmand is in the interval of 30-60 µm, where an optimum preplasma expansion exists for enhancing the proton flux, energy and conversion efficiency from laser light into proton en-ergy respectively. This same behaviour is observed in the timing scan. For a delay of0.5 ns, the plasma scale length was close to60µm. For the measured source sizes shown in figure 4.3, the intensity depen-dent behaviour is not to be seen as clear. For all shots including the long pulse beam, it is not possible to calulate the source size for the proton beam up to the highest proton energy, because the line pattern in the detector disappears. Up to now, there is no clear explanation for this phenomenon. But a possi-ble assumption is an initial distortion of the accelerating electron sheath at the rear side of the target, so that the beam laminarity is disturbed. Thus, the density modulations in the proton beam can not be transported up to the detector. For later times and lower energies respectively, the sheath form is rebuilt and the line pattern in the detector appears. If the proton beams are scaled to their maximum energy obtained by the envelope-divergence measurement no particular oberservation can be made.
An explanation for the drop in the envelope-divergence (flux respectively) can be found by comparing the transverse optical probe measurements for short and long plasma scale lengths, see figure 4.4. The propagation of the short pulse laser in the outer region of the expanding plasma changes significantly with increasing plasma scale length. At the optimum preplasma condition (enhanced proton flux and maximum energy), the laser propagation in the preplasma is observed as a single channel, figure 4.4(a), with a width smaller than the estimated focusing cone of the beam. This effect increases the laser
50 4 Proton-Acceleration Experiments
Figure 4.4:Example interferometric probe image showing channelling of the CPA laser beam in a short scale length preplasma (a) and filamentation of the laser beam in a long scale length preplasma (b). The laser pulses are incident from the top and self emission at the critical surface is observed as bright spot. Images taken from [34].
intensity and is know as relativistic self-focusing. For much longer scale length, figure 4.4(b), the laser propagation seems to break up and split in multiple filaments over a large area [185]. This reduces the laser intensity and agrees with the observed changes to the proton maximum energy, which scales with the square root of the short pulse laser intensity [179].
Figure 4.5: Comparison of identical RCFs of two different VULCAN shots. Layer energies: 4.3 10.4 -19.3 - 24.3 MeV.Top row: a reference beam without generated preplasma (∼170 J in a 30 µm spot leading to 1.1×1019 W/cm2) and in thebottom rowa proton beam, where a long pulse generates a preplasma 3.6 ns prior to the main pulse (long pulse: 7.3 J in a 500 µm spot leading to 1.3×1012 W/cm2, main pulse: same parameters as the reference shot). Significant improvements in the uniformity and circularity of the proton beam over the full proton energy range are observed.
Whereas there is an optimum scale length to enhance the maximum proton energy, the spectrum and flux respectively, improvements in the spatial-intensity profile of the proton beam are observed for all cases in which a preplasma expansion is produced by the ablation pulse. Figure 4.5 shows RCFs of two different proton beams. One is the reference beam with a sharp preplasma density gradient, because no prepulse was used (top row), and a proton beam, where the long pulse generates a preplasma3.6 ns prior to the main pulse (bottom row). For the first case, one can observe an uneven and asymmetric flux distribution across the beam, whereas with prepulse the profile of the resultant accelerated proton
4.1 Proton beam manipulation by pre-plasma shaping 51
Figure 4.6:Profiles for two different proton energies - RCF#1: 4.3 MeV and RCF#2: 10.4 MeV. The proton beam generated by VULCAN including a prepulse ( ) is more circular with a sharp edge and has a more uniform flux distribution at both energies compared to the reference shot without preplasma ( ).
beam changes. The proton beam becomes more circular with a sharp edge and has a more uniform flux distribution for all detected proton energies pointed out by two profile examples in figure 4.6. A total flux increase up to a factor of 3 can also be observed by the lifted profiles. Additional details, results and simulations are discussed in McKennaet al.[34] and Carrollet al.[178].