## 3 Pulsed Ion Beams (Protons and Carbon Ions): Laser-Driven Source Features
### Spectral Control (with Lasers and Targets)
In the BOA regime \(\sim\)30 nm the particles have a factor of 5 higher energy. Reproduced from [PERSON] _et al._[42]. the current CO\({}_{2}\) laser technology. Moreover, as laser technology improves and high power high repetition rate lasers are developed, the use of a gaseous hydrogen beam target for proton acceleration can become more practical than solid targets which must be physically advanced between shots. This is shown in Figure 8. In general however, to realize the clear rep-rate benefit typically afforded by gas sources the acceleration efficiency will need to be improved by orders of magnitude. In 2008, [PERSON] [49] and his team studied theoretically the direct high power laser acceleration of ions specifically for medical applications and found that ions accelerated by radially polarized laser have generally a smaller energy spread than those accelerated by linearly polarized lasers of the same intensity. A radially polarized beam can be focused to smaller dimensions than a linearly or circularly polarized beam but unfortunately such lasers at the required power are not yet available. As described earlier laser-accelerated proton and heavy ion spectral profiles from thin foil targets are normally quasi-exponential. Quasi-monoenergetic spectra are desirable for ion beam radiotherapy. [PERSON] and his team produced quasi-monoenergetic protons by irradiating microstructured targets, an idea that had been suggested by [PERSON] _et al._ in 2002, [50]. The authors pointed out that the resulting proton spectrum has a strong correlation to the spatial distribution of the protons on the target surface as shown in Figure 9a. A terawatt (TW) laser beam is focused on the front side of a metal target generating electrons, which establish a Debye sheath on the back surface as described earlier. Applying a small hydrogen-rich dot (a polymethyl methacrylate, PMMA dot was used) on the back surface enhances the proton yield in the central part of the accelerating field, which is more homogeneous. This produces a noticeable peak in the proton spectrum as shown in Figure 9b. The figure also includes particle in cell (PIC) simulation results for the laser and target conditions used in the experiment. An inset in the figure estimates the anticipated proton spectrum at a laser intensity of \(10^{21}\) W\(\cdot\)cm\({}^{-2}\). A number of authors however (e.g., [PERSON] _et al._ 2006 [53], [PERSON] and [PERSON] 2007 [54]) pointed out this was not sufficient to explain quasi-monoenergetic features. They emphasized that a pure proton microdot target does not by itself result in a quasi-monoenergetic proton beam. Such a beam can only be produced with a very lightly doped proton target in the presence of more abundant inert and heavier ions, which generate an electrostatic shock accelerating the protons away from the surface. Their simulations suggest that beam quality in current experiments could be considerably improved by choosing microdot compositions with a 5-10 times lower proton fraction. A laser-induced plasma optics (microlens) approach was adopted by [PERSON] _et al._ 2006 [55]. As shown in Figure 10, two CPA laser pulses were used (CPA means chirped pulse amplification). The CPA1 pulse produced a broad spectrum of protons from the foil target. Protons were directed downstream through a hollow metal cylinder irradiated by a second CPA2 laser pulse. The second laser beam induces a transient focusing (radial) electric field in the hollow cylinder. For a given target-cylinder spacing the relative timing between the two laser pulses can be varied (tuned) to select a range of proton energies for focusing and transmitted energy selection. This technique addresses two of the current drawbacks of laser accelerated proton bunches namely their large energy spread and the angular divergence at the source.