The eﬀects of a short plasma density scale length on laser-driven proton acceleration from foil targets is investigated by heating and driving expansion of a large area of the target rear surface. The maximum proton energy, proton ﬂux and the divergence of the proton beam are all measured to decrease with increasing extent of the plasma expansion. Even for a small plasma scale length of the order of the laser wavelength (∼1 µm), a signiﬁcant eﬀect on the generated proton beam is evident; a substantial decrease in the number of protons over a wide spectral range is measured. A combination of radiation-hydrodynamic and particle-in-cell simulations provide insight into the underlying physics. The results provide new understanding of the importance of even a small plasma density gradient, with implications for applications that require eﬃcient laser energy conversion to ions, such as proton-driven fast-ignition of compressed fusion fuel.
- short scale length density gradients
- laser-driven proton acceleration