We report on a plasma optical shutter to reduce the intensity level of nanosecond-duration pedestal of the amplified spontaneous emission (ASE) using an ultrathin foil. The foil is ionized by the ASE prepulse and forms an expanding underdense preplasma, which enables the main laser pulse transmission, leading to an enhancement in temporal contrast. When such a plasma shutter is applied in front of an interested main target, the preplasma profiles are similar to that produced from single-layer reference target irradiated by highcontrast laser and can be finely tuned by varying the shutter thickness. Proton beam with significantly reduced divergence and higher flux density was measured experimentally using the double-foil design. The reduction in beam divergence is a characteristic signature of higher contrast laser production as a combined consequence of less target deformation and flatter sheath-acceleration field, as supported by the two-dimensional (2D) hydrodynamic and particle-in-cell simulations. The plasma shutter holds the promise to enhance the laser contrast and manipulate the preplasma conditions for applications in high-field-physics experiments.
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