## 3 Pulsed Ion Beams (Protons and Carbon Ions): Laser-Driven Source Features
### Spectral Control (with Lasers and Targets)  
Most of the early work on laser acceleration of ions was conducted with thick aluminum targets (micron). Using ultrashort laser pulses (33 fsec duration) with a high contrast ratio (10\({}^{10}\)) at the Lund Laser Centre, [PERSON] and his team [35] showed that as the target thickness decreased from 30 \(\upmu\)m to 100 nm the maximum proton energy and energy conversion efficiency (on target laser pulse energy to full spectrum proton kinetic energy) increased. This is shown in Figure 6.  
Figure 5: Energy scaling of the accelerated protons as a function of the laser power on target, as well as pulse width. The many different contributions from various authors are given in [34]. Reproduced from [34]. Copyright 2010, IOPscience. Figure 6: The proton spectra at 0\({}^{\circ}\) (target normal) and 14\({}^{\circ}\)[35] as a function of aluminum target thickness. The contrast ratio is \(\sim\)10\({}^{10}\) and laser intensity is \(\sim\)10\({}^{19}\) W\(\cdot\)cm\({}^{-2}\). The maximum energy increases as the thickness decreases with a maximum near 100 nm. Reproduced from [35]. Copyright 2006, AIP Publishing LLC. However recent developments both theoretical and experimental in laser driven ion acceleration have shown considerable promise for increasing the maximum energy efficiency of the acceleration process taking advantage of the so-called transparent overdense regime [13, 36, 37, 38, 39, 40]. The transition to this regime occurs when the target is thin enough--typically tens to hundreds of nanometers--and the laser intensity and contrast are high enough, for all of the target electrons to be removed from the material during the laser-target interaction. Relativistic transparency enables the laser field to interact with the entire target while it is still overdense. As a result, the entire ion population can be accelerated in unison. This induced dynamic transparency is a feature of both \"Break-Out Afterburner\" (BOA) Acceleration and Radiation Pressure Acceleration (RPA). BOA has been explored extensively over the last three years at the Los Alamos National Laboratory both experimentally and in simulations where it has been shown that ions (protons and carbon ions) of significantly increased energy can be produced. C\({}^{6+}\) ions of 700 MeV for fully ionized carbon ions are recorded [40, 41, 42]. A C\({}^{6+}\) spectrum obtained in the BOA regime is shown in Figure 7.  
Ion acceleration by laser light pressure (Radiation Pressure Acceleration--RPA) has also received a lot of attention recently because this mechanism can potentially accelerate ions to even higher energies (than TNSA or BOA) with significantly reduced energy spread and much higher efficiency. We do not address the RPA regime in any detail and readers can consult the listed references [43, 44, 45, 46]. Most of this work is theoretical and although a few experiments have been attempted significantly, greater carbon ion energies must be reached in order to be clinically relevant (in this regime proton energies are reaching useful energies). Almost all of the laser-driven proton ion beam studies have used 0.8 \(\upmu\) wavelength photons and laser pulse durations of either 1 ps or 50-100 fs. However, recently very different laser parameters with another new mechanism have been applied to proton acceleration. Two groups, [PERSON] _et al._[47] and [PERSON] _et al._[48] used an infrared CO\({}_{2}\) with a 10 \(\upmu\) wavelength to investigate the collisionless shock-wave acceleration mechanism in a hydrogen plasma. A 60 J macropulse (which included a number of 3 ps micropulses) produced almost monoenergetic 20 MeV proton bunches (\(\sim\)1% energy spread) with very low emittance. Simulations indicate that \(\sim\)200 MeV protons could be produced with  
Figure 7: The measured average and maximum particle energy C\({}^{6+}\) ions in the “Break-Out Afterburner” (BOA) and TNSA acceleration regimes is shown as a function of target thickness.