System for fast ions generation and a method thereof

Abstract

The present invention discloses a system and method for generating a beam of fast ions. The system comprising: a target substrate having a patterned surface, a pattern comprising nanoscale pattern features oriented substantially uniformly along a common axis; and; a beam unit adapted for receiving a high power coherent electromagnetic radiation beam and providing an electromagnetic radiation beam having a main pulse and a pre-pulse and focusing it onto said patterned surface of the target substrate to cause interaction between said radiation beam and said substrate enabling creation of fast ions.

Claims

1. A system for generating a beam of fast ions, the system comprising; a sapphire substrate having a patterned surface, a pattern comprising nanoscale pattern features oriented substantially uniformly along a common axis; and a beam unit configured to receive a high power coherent electromagnetic radiation beam and to focus it onto the patterned surface of a target substrate to cause interaction between the coherent electromagnetic radiation beam and the substrate supporting creation of a flow of fast ions; and wherein an angle of polarization direction of the high power coherent electromagnetic radiation beam is controlled relative to the orientation direction of filaments of an oriented patterned target (OPT), such that by rotating polarization direction of the high power coherent electromagnetic radiation beam relative to direction of filaments of OPT orientation, energy of fast ions is decreased; and wherein the sapphire substrate is coupled to a cooling unit including a heat exchanger block coupled to a liquid nitrogen circulation system that pumps liquid nitrogen through the heat exchanger block to remove heat from the sapphire substrate.

2. A system for generating a beam of fast ions, the system comprising; a sapphire substrate having a patterned surface, a pattern comprising nanoscale pattern features oriented substantially uniformly along a common axis; and a beam unit configured to receive a high power coherent electromagnetic radiation beam and to focus it onto the patterned surface of a target substrate to cause interaction between the coherent electromagnetic radiation beam and the substrate supporting creation of a flow of fast ions; and wherein the sapphire substrate is coupled to a cooling unit including a heat exchanger block coupled to a liquid nitrogen circulation system that pumps liquid nitrogen through the heat exchanger block to remove heat from the sapphire substrate, wherein the sapphire substrate is sandwiched between bias electrodes connected to a power supply.

3. The system of claim 1 wherein a thickness of the sapphire substrate is 1 mm.

4. A system for generating a beam of fast ions, the system comprising; a sapphire substrate having a patterned surface, a pattern comprising nanoscale pattern features oriented substantially uniformly along a common axis; and a beam unit configured to receive a high power coherent electromagnetic radiation beam and to focus it onto the patterned surface of a target substrate to cause interaction between the coherent electromagnetic radiation beam and the substrate supporting creation of a flow of fast ions; and a power supply configured to apply a potential voltage between electrodes that generates a biasing electric field in the sapphire substrate, the electric field being parallel to direction of nanoscale pattern features, wherein the sapphire substrate is coupled to a cooling unit including a heat exchanger block coupled to a liquid nitrogen circulation system that pumps liquid nitrogen through the heat exchanger block to remove heat from the sapphire substrate.

5. The system of claim 1 wherein the high power coherent electromagnetic radiation beam is polarized and wherein polarization direction of the high power coherent electromagnetic radiation beam is substantially parallel to direction of orientation of filaments of oriented patterned targets (OPT) features.

6. The system of claim 1 wherein the nanoscale pattern features oriented substantially uniformly along a common axis comprise elongated clusters with characteristic size of 0.01-0.1 micron.

7. The system of claim 1 wherein the sapphire substrate is sandwiched between bias electrodes connected to a power supply.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1A-1C schematically show general block diagrams of the system for generating fast ions and of a method thereof in accordance with some embodiments of the invention;

(3) FIG. 2 graphically shows the interaction of different targets with the same radiation beam;

(4) FIGS. 3A-3C shows the interaction of targets with a radiation beam at different grazing angles;

(5) FIG. 4 schematically shows an example of the system for generating fast ions, in accordance with an embodiment of the invention;

(6) FIG. 5 schematically shows another example of the system for generating fast ions, in accordance with another embodiment of the invention;

(7) FIGS. 6A-6C schematically illustrate interaction of a polarized radiation beam with the target shown in FIG. 3, in accordance with an embodiment of the invention;

(8) FIG. 7 schematically shows another configuration of a system for generating fast ions in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) FIG. 1A schematically shows a block diagram system for generating a beam of fast ions 20 comprising an oriented, patterned target (OPT) 40 interacting with an electromagnetic radiation 32, in accordance with an embodiment of the invention. The OPT substrate 40 has a surface pattern with sub-resonant nanoscale features oriented substantially homogeneous along a certain axis indicated by 44 (as illustrated in FIG. 4; i.e. having a predetermined substantially homogeneous direction of orientation). The system 20 comprises a beam unit to be used with a high power coherent electromagnetic radiation source 92 configured and operable to receive a high power coherent electromagnetic radiation beam and to direct a radiation beam having a predetermined polarization direction onto the surface of the target substrate at a desired grazing angle . An angle between a polarization direction of the beam of electromagnetic radiation and the orientation axis of the pattern features of the target substrate, and the grazing angle are selected such that interaction between the radiation beam and the substrate provides an efficient coupling between the radiation beam and the substrate enabling creation of fast ions. In particular, the polarization direction of the radiation beam is selected to be have a predetermined orientation with respect to the orientation axis of the substrate such that interaction between the radiation beam 32 and the substrate 40 provides an efficient coupling between the radiation beam and the substrate enabling creation of fest ions. The beam unit 90 is adapted for receiving a high power coherent electromagnetic radiation beam and providing an electromagnetic radiation beam having a main pulse and a pre-pulse and focusing it onto the patterned surface of the target substrate to cause interaction between the radiation beam and the substrate enabling creation of fast ions. FIG. 1B illustrates a flow chart of the process used according to the teachings of the present invention. The method for generating fast ions comprises irradiating an OPT with a high power polarized coherent electromagnetic radiation beam (e.g. high power laser source e.g. having a power of at least 10 TW) and optimizing a relation between the pattern of the OPT and at least one parameter of the electromagnetic radiation by selecting/controlling at least one of an incident angle (i.e. grazing angle) for the beam of electromagnetic radiation, an angle between a polarization direction of the beam of electromagnetic radiation and the orientation axis of the OPT, a pre-pulse timing and pre-pulse intensity, such that interaction between the radiation beam and the patterned surface of the OFT provides an efficient coupling between the radiation beam and the substrate resulting in generation of a fast ions beam.

(10) As illustrated in the figure, in some embodiments, the beam unit is configured to control the intensity of the pre-pulse and/or the time period between the pre-pulse and the main pulse as well as the grazing angle for the beam of electromagnetic radiation, the angle between a polarization direction of the beam of electromagnetic radiation and the orientation axis of the OPT.

(11) FIG. 1C illustrates a flow chart of the process used according to the teachings of the present invention. As illustrated in the figure, in some embodiments, the high power laser source is configured to control the intensity of the pre-pulse and/or the time period between the pre-pulse and the main pulse. The beam unit is configured to control the grazing angle for the beam of electromagnetic radiation, the angle between a polarization direction of the beam of electromagnetic radiation and the orientation axis of the OPT.

(12) FIG. 2 graphically represents the resulting ions maximal energy of the interaction between a radiation beam and different laser-targets schemes, wherein the square, diamond, circles, X's and pulses are ions generated from solid and gas targets irradiated by high power short (>100 fsec) and ultrashort (<100 fsec) laser pulses and filled triangles are ions from an ultrashort laser and an OPT target.

(13) The proton energy is approximately scaled as the square root of the laser intensity (i.e. E.sub.protonsI.sup.0.5). As clearly seen in the figure, OPT target (triangles) provides about an order of magnitude above the results obtained by the other targets (square and circles, X's and plus marks).

(14) In a specific and non-limiting example, the OPT target is formed by H.sub.2O nanowires layed on a substrate of sapphire. The inventors have found that, when exposed, the target absorbs over 95% of incident light. The target also enhances the electric field associated with the interaction and acceleration of charged particles.

(15) In some embodiments, the surface pattern of the targets acts as a field concentrator for the electric field of the electromagnetic radiation (e.g. light pulses) interacting with the target. In particular, according to some embodiments of the invention, the surface pattern comprises a layer of filaments/wires characterized by a direction of orientation. In this case, the filaments may act as conductive needles concentrating and amplifying the laser electric field at their ends, like a macroscopic metal needle in an electric field generates an intense electric field at its point. The geometrical dimensions of the narrow tips at the end of the wires generate a large charge-separation when irradiated by the electric field. As mentioned above, the high intensity laser pulse ionizes the wires. The charge separation induced by the wire geometry is locally added to the electric field of the laser interacting with the individual particles (electron and protons).

(16) The main parameter for calculating the field enhancement is the geometrical ratio, g, which is the ratio between the diameter and length of a nanoscale feature.

(17) The field enhancement factor (FEF) scales with g linearly,

(18) $\mathrm{FEF}=\frac{E_{\mathrm{enhanced}}}{E_{\mathrm{laser}}}g.$
Here E.sub.laser is the corresponding electric field to irradiated laser pulse and E.sub.enhanced is the effective electric field that is involved in the acceleration process of the ions.

(19) Reference is made to FIGS. 3A-3C illustrating protons generated by from the interaction of an OPT with incident electromagnetic beam at different angles of incidence. In this specific and non-limiting example, the ions energies are measured by CR39 plates covered with aluminum sheets blocking protons below certain energy. The black dots represent ion marks in the CR39. FIG. 3A represents the background level of the system for reference purpose. FIG. 3B represents the interaction between the target and an incident beam hitting the patterned surface with an incident angle of 45. The protons energy cut-off is 0.5 MeV. The solid angle of the ions beam covered by the CR39 plates is about 34 (perpendicular to the target). FIG. 3C represents the interaction between the target and an incident beam hitting the patterned surface with an incident angle of 60 (i.e. grazing angle of 30). The protons energy cut-off is 5 MeV. The solid angle covered by the CR39 plates is about 5 (perpendicular to the target). Therefore, it is clearly shown that the use of the OPT allows for optimizing parameter(s) of the incident electromagnetic radiation, incident angle in the present example, to enhance the efficiency of the radiation coupling into the OPT (e.g. energy cut-off and solid angle) contributing to creation of fast ions with high kinetic energy. The figures illustrate the optimization of the variation of the grazing angle of the electromagnetic beam onto the OPT surface. The incident angle should therefore be higher than 45 (small grazing angle) being an angle between the beam propagation axis and the normal to the OPT surface. In this specific example, the irradiation of the OPT at a grazing angle of about 60 generates a quantity of fast ions (e.g. protons) by at least a factor of 36. The fast ions beam has kinetic energy higher by at least a factor of 10. According to the teachings of the present invention, the optimal angle may be determined by appropriately varying gradually the grazing angle and measuring the properties of the generated fast ions beams. It should be understood that the actual value of the grazing angle depends inter alia on the pattern features e.g. the height of the grooves.

(20) FIG. 4 schematically shows an example of a system for generating fast ions 20 comprising an oriented patterned target (OPT) 40 interacting with an electromagnetic radiation, in accordance with an embodiment of the invention.

(21) The radiation beam 32 is directed towards a target 40 at a desired grazing angle 8. The radiation beam 32 has a predetermined polarization direction indicated by an arrow 34. For example, the beam unit 30 is controllable to provide polarized laser beam pulses that are focused to a focal region in OPT 40 schematically indicated by a circle 60. In some embodiments, the beam unit 30 is controllable to provide a beam having a main pulse 32 and a pre-pulse 33.

(22) In this specific and non-limiting example, the surface pattern of the OPT 40 comprises oriented filaments formed on and supported by a target pedestal 50. An arrow 44 indicates a direction of orientation that characterizes orientation of nanoscale features 42 and OPT 40. In an embodiment of the invention, polarization direction 34 is substantially parallel to direction 44 of orientation of OPT 40.

(23) Pedestal 30 may comprise a sapphire substrate 51 coupled to a cooling unit 52 configured in accordance with any of various techniques known in the art. Optionally, cooling unit 52 comprises a Cu heat exchanger block 54 coupled to a liquid nitrogen circulation system (not shown) that pumps liquid nitrogen through the heat exchanger to remove heat from sapphire substrate 51. The substrate is sandwiched between bias electrodes 56 that are connected to a power supply 55. OPT 40 and pedestal 50 are located in a vacuum chamber (not shown).

(24) To produce OPT 40, in accordance with an embodiment of the invention, pressure in the vacuum chamber is reduced to between about 510.sup.4 mBar to about 10.sup.5 mBar and the cooling unit is operated to cool substrate 51 to about 80 K. Power supply 55 is controlled to apply a potential voltage between electrodes 56 that generates a biasing electric field in substrate 51, which is parallel to direction of orientation 44. Water vapor is then introduced into the vacuum chamber and condenses on substrate 51 in the form of elongated ice filaments 42. Because water is a polar molecule, as the molecules condense onto the substrate and grow ice filaments 42, the molecules, and the ice filaments tend to orient parallel to the electric biasing field and thereby direction of orientation 44. Other materials having the ability to be patterned, the pattern having nanoscale pattern features oriented substantially uniformly along a common axis, such as silicon, carbon or plastics (i.e. CH composites) can also be used to form the target substrate having a substantially uniform direction of orientation according to the teachings of the present invention.

(25) In some embodiments, the radiation beam 32 includes a beam pulse.

(26) In an embodiment of the invention, water vapor is introduced into the vacuum chamber for a period long enough to grow layer 41 of surface pattern to thickness sufficient to absorb substantially all the energy in pre-pulse 33 and pulse 32. The pre-pulse 33 and main pulse 32 may be provided by the beam unit 90 and/or by a coherent light source 92 of FIG. 1. Pre-pulse 33 energy would therefore be dissipated by ablating and ionizing a portion of layer 41 and leave in place of the ablated material a relatively thin, sub-critical density, plasma overlaying a remaining portion of layer 41 prior to pulse 32 reaching the layer. The sub-critical density plasma does not interact strongly with energy in pulse 32, and as a result, energy in pulse 32 couples efficiently to the nanoscale features 42 in the remaining non-ablated portion of layer 41.

(27) The presence of the electric field generated in substrate 51 would of course not result in all nanoscale features 42 that condense on the substrate being substantially aligned along direction 44. However, the electric field results in a density of aligned surface pattern (e.g. ice filaments) that characterizes layer 41 and OPT 40 with orientation direction 44. And it is expected that interaction of OPT 40 with pulse 33 of beam polarized in a direction, e.g. direction 34, parallel to direction of target orientation 44, in accordance with an embodiment of the invention, would be enhanced relative to interaction of the pulse with a non-oriented target T. Ion fluxes and energies provided by interaction of radiation beam (e.g. laser light pulse) with OPT 40 are therefore expected to be enhanced relative fluxes and energies provided by interaction of the light pulse with a T target.

(28) The inventors have conducted experiments with a T target comprising a layer of non-oriented ice filaments interacting with intense, 800 nm wavelength laser light pulses to produce fast ions. An experiment conducted by the inventors was reported in the article entitled Generation of Fast Ions by an Efficient Coupling of High Power Laser into Ice Nanotubes, referenced above. The experiments indicate that fluxes of 150 KeV protons are produced per laser light pulse having pulse width less than about 0.1 ps and moderate intensity of about 10.sup.16 W/cm.sup.2 incident on a 1 mm thick T ice filament target formed on a target pedestal similar to pedestal 50. To produce same energy protons from conventional interaction of a laser light pulse and a solid, non-filamentary target, the laser pulse typically requires intensity of about 10.sup.17 W/cm.sup.2, which is about an order of magnitude greater than that required using a T target.

(29) In some embodiments of the invention, beam unit 30 focuses beam radiation 32 (e.g. laser light pulse) to a maximum intensity about equal to or greater than at least one of the followings: 10.sup.16 W/cm.sup.2; 10.sup.17 W/cm.sup.2; 10.sup.18 W/cm.sup.2; 10.sup.19 W/cm.sup.2; 10.sup.20 W/cm.sup.2, 10.sup.21 W/cm.sup.2.

(30) FIG. 5 illustrates a configuration of an example of the system of the present invention in which, the beam unit comprise an arrangement of dielectric mirrors and of an off-axis parabola mirror (e.g. gold coated) configured and operable to focus the radiation beam to a focal region.

(31) FIGS. 6A-6C schematically illustrate a process of generating fast protons, in accordance with an embodiment of the invention. In this specific and nom-limiting example, fast protons having an energy of about 50 MeV are produced by the system 20 of the present invention in which a radiation beam 32 (e.g. laser light pulse) is assumed to have a wavelength of 800 nm, pulse width of about 0.1 ps, and an intensity of about 510.sup.19 W/cm.sup.2 in a focal plane (when focused to focal region 60 of target OPT 40). Assuming a contrast ratio (ratio of pre-pulse intensity to main pulse intensity) of maximum 10.sup.3, when focused to focal region 60, pre-pulse 33 has intensity equal to maximum 10.sup.16 W/cm.sup.2. It should thus be understood that the energy of the pre-pulse and the position of the focal plane should be appropriately adjusted to on the one hand provide interaction at the desired energy of the beam for efficient coupling and on the other hand the focal plane energy should not be too high to not destroy the pattern features.

(32) FIG. 6A schematically shows the system 20 of the present invention just before the interaction between the radiation beam and the OPT 20.

(33) FIG. 6B schematically shows the system 20 of the present invention, after pre-pulse 33 has ablated and ionized, and has created a burn off layer having patterned nanoscale features 42 in focal region 60, leaving a sub-critical density plasma, represented by a shaded region 62. Plasma 62 overlays a remaining, non-ablated region 64 of nanoscale features 42 in focal region 60. In the figure, laser pulse 32 is just entering focal region 60. Because plasma is sub-critical it does not substantially affect laser pulse 32.

(34) FIG. 6C schematically shows laser pulse 32 interacting with nanoscale features 42 in non-ablated region 64 (as illustrated in FIG. 6B) to produce a flux of protons schematically represented by a cluster of dot-dash arrows 68, in accordance with an embodiment of the invention.

(35) Because the surface pattern has sub-resonant nanoscale features 42 e.g. the width of the surface pattern is much smaller than the wavelength of light in pulse 32, the electric field of the pulse, at any given moment is substantially constant within and in the neighborhood of the surface pattern. Without being bound by any particular theory, as mentioned before, the inventors believe that the surface pattern therefore acts similarly to a conducting needle in, and parallel to, an electric field, and concentrates the field at its tips, and that the concentrated field of a plurality of oriented nanoscale features 42 is particularly advantageous for generating a relatively large flux of fast protons. An inset 70 schematically shows nanoscale features 42 in the electric field of a localized region of pulse 32 smaller than a wavelength of light in the pulse. A block arrow 72 represents the electric field of light pulse 32 near feature 42 and dashed field lines 76 converging towards a tip 74 of the feature schematically represent the concentrated field at the tip.

(36) Concentrated field 76 generates a plume of hot electrons, schematically represented by circles 80, that leave feature 42 near its tip 74 by ionizing hydrogen and oxygen atoms (not shown) in the feature. The plume of electrons and ionized atoms in feature 42 produce an intense double layer field (not shown) that accelerates hydrogen ions in the filament to relatively high energies producing the flux of protons represented by cluster of arrows 68.

(37) It is noted that efficacy with which light pulse 32 produces fast ions 68 by interacting with OPT 40 (FIG. 3) is responsive to direction 34 of polarization of light in pulse 32 relative to direction 44 of a nanoscale feature orientation in OPT 40 and/or to the direction of the plane of incidence. For example, as described above, the light pulse is particularly effective in producing a flux of fast ions, such as protons, when direction 34 and direction 44 of feature orientation are parallel or having a small angle between them. In some embodiments of the invention, magnitude and/or energy of ions produced by the system of the invention 20 is controlled by controlling the angle of polarization direction 34 relative to direction of feature orientation. By rotating polarization 34 away front the correct angle between polarization 34 and direction 44 of filament orientation, energy of protons is expected to decrease. Thus, an angle between the polarization direction and the orientation axis of the pattern can be appropriately adjusted to optimal value.

(38) FIG. 7 schematically shows polarization of pulse 32 rotated, in accordance with an embodiment of the invention, away from direction 44 of features orientation.