Automated Delivery of Microfabricated Targets for Intense Laser Irradiation Experiments

Summary

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.

Abstract

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.

Introduction

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 ta....

Protocol

1. Target fabrication

NOTE: Figure 2 and Figure 3 illustrate the fabrication process of freestanding gold foils.

1. Back Side
   1. Use a 250 μm thick, 100 mm diameter, high-stress silicon wafer in a <100> crystal formation, coated on both sides with silicon nitride.
   2. Clean the wafer using acetone followed by isopropanol and dry with nitrogen.  Then spin coat a layer of HMDS to form an adhesive layer following the steps outlined in Table 1.
   3. Spin-coat the wafer with an AZ1518 photoresist layer foll....

Results

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 separa.......

Discussion

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 li.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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