Laser Plasma Section

Ion acceleration from thin foil targets

Ion Acceleration in ultrashort ultrahigh intensity laser plasma interaction with thin foil targets

The interaction of ultrashort intense laser pulse with matter offers high brightness, low emittance, charge particle sources on a table top size and thus has the potential to be a viable alternative to the conventional particle accelerators. The dynamics of ultrashort laser pulse interaction with matter is primarily governed by electrons, which due to their lighter mass, instantly responds to the incident laser electric field and gains energy via various absorption mechanisms. The energetic electrons because of their high energy can break away from the plasma leading to formation of sheath field on both the surfaces of thin foil targets. The electric field in the sheath can be well above TeV/m. The protons and heavy ions are accelerated in this sheath electric field. Careful observation of ion emission characteristics reveal the dynamics of plasma evolution and thus provide vital clues towards understanding of the underlying processes in laser plasma interaction. In this direction, we have studied the ion emission from ultra-short moderately intense laser produced plasma. The experimental set up is shown in figure 1.The plasma is formed by irradiating 45 fs, 10 TW laser (Thales, Alpha-10) beam in f/7 focusing geometry to a focal spot size of 10 m. The ion emission from the freely expanding plasmas were monitored by using an in-house developed Thomson Parabola Ion Spectrometer (TPIS) which applies simultaneous electric and magnetic field to disperse the incident multispecies ions according to their charge to mass ratio on a parabolic trajectory onto a two dimensional position sensitive detector. A 16-bit EMCCD camera in tandem with a micro channel plate (MCP) detector has been employed to capture the ion energy spectra on single shot basis.

Fig. 1: The schematic experimental diagram is shown in (a). The Thomson Parabola Ion Spectrograph (TPIS) is used as primary diagnostics for ion emission studies. (b) The true arrangement of TPIS is shown in (b).
Fig. 1: The schematic experimental diagram is shown in (a). The Thomson Parabola Ion Spectrograph (TPIS) is used as primary diagnostics for ion emission studies. (b) The true arrangement of TPIS is shown in (b).

Radio chromic films (HD-810) and CR-39 sheets were used at a later stage to record the angular distribution of protons and heavy ions.

Fig.2
Fig. 2: (a) Typical TPIS trace observed in experiment. The inset shows the divergence of the entire proton beam recorded with radio chromic film. (b) The traces of protons recorded on CR-39 solid state nuclear track detector. The CR-39 sheets were appropriately developed in NaOH solution to make the proton tracks visible.

Ion emission from solid targets with various atomic numbers, mix-Z targets, nano-composites (nano particles, nanotubes, co-sputtered targets, fullerenes etc.), dielectric targets (Perspex, CR-39, Teflon, Black Polythene), semiconductors, nano-structures have been studied in detail. Preliminary analysis reveals that the maximum proton energy ~ 1 MeV was observed from Si which is 2-3 times more as compared to metal targets in the front direction. We have observed for the first time, highly repeatable, mono-energetic, multi-species, bunch gold ion acceleration from on gold, carbon co-sputtered nano-composite samples. Preliminary analysis indicates that the accelerated multi-charged gold ions possess same kinetic energy. Ion emission characteristics in the forward direction from few microns thick metallic foils of few microns thickness have been investigated extensively. The main accelerated species were found to be protons which have highest charge to mass ratio. The maximum proton energy was found to be close to 3 MeV. Data from RCF and CR-39 shows that divergence of proton beam > 1 MeV is about 10°. The effect of foil thickness, atomic number on ion acceleration has been monitored. As is well known that the “hot” electrons plays the crucial role for accelerating ion in terms of forming a sheath layer at the rear surface, understanding the transport of hot electrons is paramount importance. Double layered (High-Z / Low-Z) Sandwich targets composed metal and dielectric layers have used to investigate the role of hot electron transport on ion emission. We have observed for the first time high energy neutrals and negative ions in interaction of high intensity laser pulse interacting with transparent solids, semiconductor and thin metallic foils. The neutral atom emission was studied by conventional time-of-flight (TOF) signal from MCP with all the charged particles thrown out of the active detection area. By suitably adjusting the experimental conditions we have, recorded negative ion signals from transparent dielectric samples and thin metallic foils.
We have performed an initial study on optimization of proton energies by irradiation the moderately intense, ultra-short laser pulse on thin metallic foils. Our results show that by judicious choice of target material the maximum proton energy can be maximized for a given pre-pulse condition of the laser.

Fig. 3: Comparison of maximum proton energies obtained from thin metallic foils of different materials.
Fig. 3: Comparison of maximum proton energies obtained from thin metallic foils of different materials.

The accelerated ions have sufficient energy to induce nuclear fusion reaction, thereby offers the possibility to study laboratory nuclear physics with table top lasers. Figure 5 shows the neutron signal from deuterated plasma, recorded with plastic scintillator based neutron Time of Flight detector. A neutron yield of 104 per laser shot was observed with a table top laser system but one can expect a much higher neutron yield with 100’s of TW laser. The neutrons thus generated have very short pulse duration and small source size, therefore can be important radiography and pump probe type of experiments.

Fig. 4: Time-of-Flight signal of neutrons generated from moderately intense laser pulse interaction with deuterated carbon
Fig. 4: Time-of-Flight signal of neutrons generated from moderately intense laser pulse interaction with deuterated carbon.

Accelerated ion beam has been envisaged as a tool in radiation oncology because of its ability to impart localized energy deposition without collateral damage unlike photon and electron beams. The protons and lighter ions e.g. carbon, deliver most of their energy at the end of their path, at the so-called Bragg peak. This very property makes protons and lighter ions very suitable for highly localized energy deposition. The conventional hadron therapy facility demands large expense & a major part go to transport of the particle beam to the patient (40 – 50 %). The Laser based proton / ion sources offers a promising alternative. The beam transport cost can be drastically reduced. In our recent experimental campaign using 150 TW, 25fs Ti:sapphire laser, we have been investigating the possibilities of increasing proton and ion beam energies. With 0.4 μm thick Al foil we have now able to record protons up to 12 MeV and 18 MeV of C4+.

It is expected that with improvised target geometries and proper conditioning of the laser pulse, a considerable increase in proton and heavy ion energy and flux will be accomplished with strong relevance to the hadron therapy application. This application will get a major boost with the soon to be installed petawatt class laser at RRCAT.

Fig. 5: A typical TPIS image recorded in the recent experimental campaign with 150 TW laser at RRCAT. The inset depicts the derived proton and C4+ energy spectra derived from the recorded image.
Fig. 5: A typical TPIS image recorded in the recent experimental campaign with 150 TW laser at RRCAT. The inset depicts the derived proton and C4+ energy spectra derived from the recorded image.
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