Ion Acceleration
Introduction
For the proton acceleration experiments, a thin (several µm thick) solid foil is used as target. Because of their large mass, protons require very high light intensity to be accelerated directly. Instead, electrons are used in an intermediate step to transfer energy from the laser to protons. This could happen in a number of ways, yielding protons that originate from different parts of the target, e.g. at the front (laser side) of the target, in the bulk or at the back. The process thought be generating the most energetic protons, known as 'target normal sheath acceleration' (TNSA), is outlined below.
Target Normal Sheath Acceleration (TNSA)
The principle of TNSA is illustrated in the right figure above. Very energetic (hot) electrons are generated and driven into the target foil by the intense laser pulse. When they reach out to the rear surface of the target foil, the space charge separation results in a large, electrostatic field that will ionize a thin layer of water vapor that is present under the experimental vacuum conditions. The accelerating electro-static field will be directed along the rear surface normal, which explains why ion beams are observed in this direction, also when the laser is obliquely incident on the target. Protons have a higher charge to mass ratio and will be accelerated quicker than heavy ions. They will then shield the accelerating field, suppressing acceleration of heavy ions.
Detectors
A problem with these experiments is the extreme noise level produced simultaneously with the protons. Electrons are present in large numbers (some of whom accompany the proton beam, charge neutralizing it) with energies up to several mega electronvolt. The experiment chamber is flooded with electromagnetic radiation up to gamma rays (from scattering of the high energy electrons). This makes it a difficult task to discriminate the proton signal from the background. In particular, electronic detectors, such as CCDs (Charge Coupled Device, a type of camera), tend to get blinded by the electromagnetic radiation. Other detectors (for example scintillators) are sensitive to electrons as well as protons. The solution is a plastic material called CR-39. It is only sensitive to protons and heavy ions (and neutrons). When an energetic proton hits the plastic, it breaks the polymer chains as it slows down. After etching in NaOH, a pit becomes visible.