Enhancement of keV X-rays from low-density cellulose triacetate (TAC) foam targets

Abstract

The interaction of a high-power laser with a low-density foam target can in some instances result in a significant enhancement in x-ray generation relative to that when the same laser is incident upon a homogenous solid. In this paper, we present x-ray emission studies from foam targets where the density is varied from under-dense to over-dense. The targets are irradiated with the first harmonic of Nd:Glass laser. The laser intensity on the target was approximately 2 × 1014 W/cm2 with the pulse duration of 500 ps. Mass-matched cellulose triacetate foam targets with densities of 2 mg/cc, 4 mg/cc, 7 mg/cc, and 20 mg/cc were used. The areal density presented by the targets on the laser beam axis was held constant at 0.2 mg/cm2 by varying the target thickness in inverse proportion to the density. The x-ray yield in the spectral range (5–8 keV) and (4.5–16 keV) was found to be enhanced by approximately 2.3 times in foam targets with the density of 2 mg/cc (under-dense) compared with foam targets with the density of 20 mg/cc (over-dense).

Dynamics of Laser Produced Plasma From Foam Targets for Future Nanolithography Devices and X-Ray Sources

“…Currently, one important issue for the interaction between laser and solid targets is that the latter is highly opaque, thus limiting the interaction region to the skin layer of the target surface. Furthermore, the electron density of the plasma produced by the leading edge of the laser will reach the critical density quickly because of the highdensity of the solid target, which causes a significant fraction of the laser energy to be reflected and a rather low laser absorption rate 19,20 . However, reducing the initial density of target used in LPP could effectively solve this issue .…”

“…Currently, some research related to the laser produced low-density tin plasma show that the complex double pulse laser scheme is expected to be replaced by using low-density foam targets in the future [26][27][28] . In addition, low density foam targets have been demonstrated to have higher x-ray radiation than solid targets in reports of the laser produced Au 23,29,30 , Cu 31 , V 31 , Ti 32 , Ca 33 , Bi 34 , and cellulose triacetate 20 plasma. Thus, low density foam targets are expected to be the most suitable candidates for the efficient LPP EUV and x-ray sources 20 .…”

“…The theoretical analysis and MULTI 1D simulation results show that the density contour of gold foam moved slower than that of solid gold. Kaur et al 20,29,36 investigated the interaction of high-power sub nanosecond laser pulses with lowdensity cellulose triacetate polymer and gold foam targets. They demonstrated that the low-density foam targets in LPP could enhance the x-ray yield and reduce the kinetic energy of ions.…”

“…It is well known that the laser can only propagate in the plasma below the critical electron density. During the interaction between laser and plasma, an ionization wave and a rarefaction wave will be created in the conduction zone and the interface between the plasma and vacuum, respectively 20,23 . If the velocity of ionization wave propagating into the bulk target is higher than that of sound in the already heated matter, significant hydrodynamic motion will not be generated in spite of pressure gradients are very large at the position of heat front 20 .…”

“…During the interaction between laser and plasma, an ionization wave and a rarefaction wave will be created in the conduction zone and the interface between the plasma and vacuum, respectively 20,23 . If the velocity of ionization wave propagating into the bulk target is higher than that of sound in the already heated matter, significant hydrodynamic motion will not be generated in spite of pressure gradients are very large at the position of heat front 20 . Various studies 20,23,30 related to laser produced underdense target (the ionized electron density lower than the laser critical density) plasma pointed out that the ionization wave generated by laser ablating under-dense target is supersonic and the rarefaction wave is subsonic.…”