Ion Acceleration from the Interaction of Ultra-Intense Lasers with Solid Foils

Thesis/Dissertation · Thu Jan 01 00:00:00 EST 2004

DOI:https://doi.org/10.2172/15011790· OSTI ID:15011790

Allen, Matthew M. ^[1]

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1. Univ. of California, Berkeley, CA (United States)

The discovery that ultra-intense laser pulses (I > 10¹⁸ W/cm²) can produce short pulse, high energy proton beams has renewed interest in the fundamental mechanisms that govern particle acceleration from laser-solid interactions. Experiments have shown that protons present as hydrocarbon contaminants on laser targets can be accelerated up to energies > 50 MeV. Different theoretical models that explain the observed results have been proposed. One model describes a front-surface acceleration mechanism based on the ponderomotive potential of the laser pulse. At high intensities (I > 10¹⁸ W/cm²), the quiver energy of an electron oscillating in the electric field of the laser pulse exceeds the electron rest mass, requiring the consideration of relativistic effects. The relativistically correct ponderomotive potential is given by U_p = ([1 + Iλ²/1.3 x 10¹⁸]^1/2 - 1) m{sub o}c², where Iλ² is the irradiance in W μm²/cm² and m_oc² is the electron rest mass. At laser irradiance of Iλ² ~ 10²⁰ W μm²/cm², the ponderomotive potential can be of order several MeV. A few recent experiments--discussed in Chapter 3 of this thesis--consider this ponderomotive potential sufficiently strong to accelerate protons from the front surface of the target to energies up to tens of MeV. Another model, known as Target Normal Sheath Acceleration (TNSA), describes the mechanism as an electrostatic sheath on the back surface of the laser target. According to the TNSA model, relativistic hot electrons created at the laser-solid interaction penetrate the foil where a few escape to infinity. The remaining hot electrons are retained by the target potential and establish an electrostatic sheath on the back surface of the target. In this thesis we present several experiments that study the accelerated ions by affecting the contamination layer from which they originate. Radiative heating was employed as a method of removing contamination from palladium targets doped with deuterium. We present evidence that ions heavier than protons can be accelerated if hydrogenous contaminants that cover the laser target can be removed. We show that deuterons can be accelerated from the deuterated-palladium target, which has been radiatively heated to remove contaminants. Impinging a deuteron beam onto a tritiated-titanium catcher could lead to the development of a table-top source of short-pulse, 14-MeV fusion neutrons. We also show that by using an argon-ion sputter gun, contaminants from one side of the laser target can be selectively removed without affecting the other side. We show that irradiating a thin metallic foil with an ultra-intense laser pulse produces a proton beam with a yield of 1.5-2.5 10¹¹ and temperature, kT = 1.5 MeV with a maximum proton energy > 9 MeV. Removing contaminants from the front surface of the laser target with an argon-ion sputter gun, had no observable effect on the proton beam. However, removing contaminants from the back surface of the laser target reduced the proton beam by two orders of magnitude to, at most, a yield of ~ 10⁹ and a maximum proton energy < 4 MeV. Based on these observations, we conclude that the majority (> 99%) of high energy protons (E > 5 MeV) from the interaction of an ultra-intense laser pulse with a thin foil originate on the back surface of the foil--as predicted by the TNSA model. Our experimental results are in agreement with PIC simulations showing back surface protons reach energies up to 13 MeV, while front surface protons reach a maximum energy of 4 MeV. Well diagnosed and controllable proton beams will have many applications: neutron radiography, material damage studies, production of medical isotopes, and as a high-resolution radiography tool for diagnosing opaque materials and plasmas. Well collimated and focusable ion beams may also prove beneficial for alternative inertial-fusion concepts such as proton fast ignition, a potentially viable method for achieving a controlled fusion reaction in the laboratory earlier than expected.

• Research Organization: Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: W-7405-ENG-48
• OSTI ID: 15011790
• Report Number(s): UCRL-TH-208645; TRN: US0501442
• Resource Relation: Other Information: TH: Thesis (Ph.D.); Submitted to the Univ. of California, Berkeley, CA (US); PBD: 24 Nov 2004
• Country of Publication: United States
• Language: English

Related Subjects

29 ENERGY PLANNING
POLICY AND ECONOMY
73 NUCLEAR PHYSICS AND RADIATION PHYSICS
22 GENERAL STUDIES OF NUCLEAR REACTORS
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
42 ENGINEERING
ACCELERATION
DEUTERON BEAMS
DEUTERIUM
ELECTRIC FIELDS
HYDROCARBONS
ION BEAMS
LASER TARGETS
LASERS
NEUTRON RADIOGRAPHY
PALLADIUM
PROTON BEAMS
REST MASS