A protocol is presented for automated irradiation of thin gold foils with high intensity laser pulses. The protocol includes a step-by-step description of the micromachining target fabrication process and a detailed guide for how targets are brought to the laser's focus at a rate of 0.2 Hz.
Described is an experimental procedure that enables high-power laser irradiation of microfabricated targets. Targets are brought to the laser focus by a closed feedback loop that operates between the target manipulator and a ranging sensor. The target fabrication process is explained in detail. Representative results of MeV-level proton beams generated by irradiation of 600 nm thick gold foils at a rate of 0.2 Hz are given. The method is compared with other replenishable target systems and the prospects of increasing the shot rates to above 10 Hz are discussed.
High-intensity laser irradiation of solid targets generates multiple forms of radiation. One of these is the emission of energetic ions with energies at the Mega electron-volt (MeV) level1. A compact source of MeV ions has potential for many applications, such as proton fast-ignition2, proton radiography3, ion radiotherapy4, and neutron generation5.
A major challenge in making laser-ion acceleration practical is the ability to position micrometer-scale targets accurately within the focus of the laser at a high rate. Few target delivery technologies were developed to answer this challenge. Most common are target systems based on micrometer-scale thick tapes. These targets are simple to replenish and may be easily positioned within the focus of the laser. Tape target has been made using VHS6, copper7, Mylar, and Kapton8 tapes. The tape drive system typically consists of two motorized spools for winding and unwinding and two vertical pins placed between them to keep the tape in position9. The accuracy in positioning the tape surface is typically less than the Rayleigh range of the focusing beam. Another type of replenishable laser target is liquid sheets10. These targets are delivered rapidly to the interaction region and introduce a very low amount of debris. This system comprises a high-pressure syringe pump continuously supplied with liquid from a reservoir. Recently, novel cryogenic hydrogen jets11 were established as means to deliver ultrathin, low-debris, replenishable targets.
The main drawback of all of these replenishable target systems is the limited choice of target materials and geometries, which are dictated by mechanical requirements such as strength, viscosity, and melting temperature.
Here, a system able to bring micromachined targets to the focus of a high intensity laser at a rate of 0.2 Hz is described. Micromachining offers a wide choice of target materials in versatile geometries12. The target positioning is performed by a closed-loop feedback between a commercial displacement sensor and a motorized manipulator.
The target delivery system was tested using a high-contrast, 20 TW laser system that delivers 25 fs-long laser pulses with 500 mJ on target. A review of the laser system’s architecture is given in Porat et al.13, and a technical description of the target system is given in Gershuni et al.14. This paper presents a detailed method for making and using this type of system and shows representative results of laser-ion acceleration from ultrathin gold foil targets.
The Thomson Parabola ion spectrometer (TPIS)15,16 shown in Figure 1 was used to record the energy spectra of the emitted ions. In a TPIS, accelerated ions pass through parallel electric and magnetic fields, which places them on parabolic trajectories in the focal plane. The parabolic curvature depends on the ion’s charge-to-mass ratio, and the location along the trajectory is set by the ion’s energy.
A BAS-TR imaging plate (IP)17 positioned at the focal plane of the TPIS records the impinging ions. The IP is attached to a mechanical feedthrough to allow translation to a fresh area before each shot.
1. Target fabrication
NOTE: Figure 2 and Figure 3 illustrate the fabrication process of freestanding gold foils.
2. Alignment
NOTE: Figure 4 shows the target irradiation setup.
3. Irradiation sequence and automated target positioning
This target delivery system was employed to accelerate ions from the back side of 600 nm thick gold foils. When irradiated with a normalized laser intensity of a0 = 5.6, these ions were accelerated by the target normal sheath acceleration (TNSA) mechanism21. In TNSA, the lower-intensity light that preceded the main laser pulse ionized the front surface of the target foil. The ponderomotive force exerted by the main laser pulse drove hot electrons through the bulk matter. A charge separation on the back surface, induced by these electrons22, created an extreme electrostatic gradient that accelerated ion contaminants in the target-normal direction.
A time series of the target displacement along the focal axis is shown in Figure 6. The values are relative to the focal position setpoint. The green dots indicate when the target displacement was within a tolerance value of 1 μm from the setpoint; this is when a laser shot was taken.
Figure 7 shows TPIS traces from 14 consecutive irradiations of 600 nm thick gold foil targets. The energy spectrum derived from these traces is shown in Figure 8. The peak-to-peak stability of the maximum proton energy is within 10%.
Figure 1: A technical layout of the Thomson parabola ion spectrometer. Please click here to view a larger version of this figure.
Figure 2: A schematics sketch of the target wafer.
The front side, showing 300 gold foil targets ordered in three concentric rings (left). The back, showing roughened fiducial rings positioned between the target foil locations (right). Please click here to view a larger version of this figure.
Figure 3: An illustration of the wafer fabrication process. Please click here to view a larger version of this figure.
Figure 4: A schematic layout (left) and photo (right) of the interaction chamber. Please click here to view a larger version of this figure.
Figure 5: Target positioning PID LabView code (VI). Please click here to view a larger version of this figure.
Figure 6: Target displacement during a shot sequence of 20 targets. Please click here to view a larger version of this figure.
Figure 7: TPIS traces from 14 consecutive shots. The trajectories of ions and X-rays passing through the TPIS are illustrated. Please click here to view a larger version of this figure.
Figure 8: Ion energy spectra derived from the 14 traces shown in Figure 7. Please click here to view a larger version of this figure.
Figure 9: A TPIS trace recorded using a low dynamic range CCD imaging of a CsI(TI) scintillator. Please click here to view a larger version of this figure.
Step | ν [rps] | ramp [rps2] | Duration [s] |
1 | 500 | 500 | 10 |
2 | 4000 | 1000 | 45 |
3 | 0 | 1000 | 0 |
Table 1: Resist spin coat steps.
Step | ν [rps] | ramp [rps2] | Duration [s] |
1 | 500 | 500 | 10 |
2 | 4000 | 1000 | 45 |
3 | 0 | 1000 | 0 |
Table 2: Photoresist spin coat steps.
With some variations, the target fabrication process described in this protocol is common (e.g., Zaffino et al.23). Here, one unique step that is critical to the operation of automatic positioning is the addition of nanometer-scale roughening in ring-shaped areas on the back of the wafer (step 1.2.3). The purpose of this step is to increase the diffused scattering of light incident on the wafer in those areas. The ranging sensor shines a low-power laser beam on the wafer, collects the scattered light, and determines its displacement by triangulation.
The data shown above were taken at a rate of one shot per 5 s, with the rate-limiting factor being the translation time of the IP. Shown here is a preliminary result of a simple, inexpensive, online readout method that will increase the shot duty cycle. Online readouts have been traditionally made using either microchannel plates24 or plastic scintillators25,26. In the latter case, an expensive, image-intensified gated CCD was required to record the relatively low amount of scintillation light. The current system uses a simpler readout system based on a different scintillator material, Csl(Tl), which is bright enough to be recorded with an inexpensive, low dynamic range CCD. This choice of scintillator has been suggested and discussed by Pappalardo et al.27.
Figure 9 shows a sample image of a TPIS trace taken with a low dynamic range CCD image of a Csl(Tl) scintillating screen. These traces were taken with a relatively large aperture, to produce a high quantity of scintillation light. Further study is required to identify the optimal settings in terms of signal-to-noise ratio and energy resolution.
The image shown in Figure 8 was acquired using a 1.6 megapixel camera. At a 10 Hz rate and 8-bit pixel depth, the data stream would amount to about 130 Mbps. This data rate is supported by either a USB3 or GigE communication interface.
The mechanical stability of any replenishable laser target delivery system may be compromised by a higher delivery rate or by the higher impact induced by higher energy laser pulses. Table 3 presents a comparison between this work and various other target delivery technologies. The performance of this system at higher shot rates and higher energy pulses will be investigated in the near future.
Reference | Target type | Materials | Thickness | Repetition Rate | Laser Energy |
[6] | Tape | Mylar | 15 µm | 0.2 Hz | 5 J |
[10] | Liquid Sheet | Ethelyne Glycol | 0.4 µm | 1 kHz | 0.011 J |
[11] | Hydrogen Jet | H2 | 20 µm | 1 Hz | 600 J |
This work | Micro-machined Au foil | Au | 0.6 µm | 0.2 Hz | 0.5 J |
Table 3: Comparison of different target types.
The authors have nothing to disclose.
This work has been supported by the Israel Science Foundation, grant No. 1135/15 and by the Zuckerman STEM Leadership Program, Israel, which are gratefully acknowledged. We also acknowledge the support of the Pazy Foundation, Israel grant #27707241, and NSF-BSF grant No. 01025495. The authors would like to kindly acknowledge Tel Aviv University Center for Nanoscience and Nanotechnolog
76.2 x 127mm EFL 90° Protected Gold 100Å Off-Axis Parabolic Mirror | Edmund optics | 35-535 | |
MicroTrak 3 LTS 120-20 | MTI Instruments | ||
Ultrafast high power dielectric mirrors for 800 nm | Thorlabs |