Plasma-breakdown retropulse isolators for the infrared

Abstract

The design and performance of a passive plasma isolator for suppressing retropulses in high-power CO(2) laser-fusion systems are described. The device uses a gas-filled spatial filter designed to produce a plasma at the focal plane iris. General design criteria for these isolators are discussed, and the performance of a specific isolator is evaluated. For the unit tested, retropulse attenuations of 33 dB for focal plane intensities of 1.5 TW/cm(2) have been demonstrated.

Enhanced vacuum laser-impulse coupling by volume absorption at infrared wavelengths

“…The maximum C m obtained by using equation (15) Figure 4 where we have adequate value. The range over which this transition occurs is similar to that for IR fluence-limiting in 600 torr nitrogen (Figueira et al 1981), and in our thermal coupling measurements, as shown in table 3. For celluloid, taking the geometric mean between the table 2 predictions, we expected C m to decrease from a maximum value of 25 at / 0 = 3 x 10 7 W/cm 2 (/ 0 XVr = 17) during the transition from volume to surface absorption, to a value of about 1 at an orderof-magnitude higher intensity, I o = 3 x 10 8 W/cm 2 (/ 0 XVT = 170), taking ¥ « 1.…”

“…The films are an intimate mixture of Cr 2 O 3 and Cr crystallites, rather than discrete layers of the two constituents. Therefore, this structure well simulates the array of tiny, electrically insulated conducting spheres which has been predicted to exhibit extremely large, broadband absorptivity (Genzel & Kreibig 1980) for plasma reradiation and for the IR laser. The absorption coefficients are greater than 10 4 cm" 1 in the image of 0.4 to 10 /xm wavelength for Cr filling factors in the coating above 25%.…”

“…This density, n ec = 1.115 x 10 13 / X 2 cm" 3 , is 9.94 x 10 18 cm~3 for X = 10.59 ^m, corresponding to 47% ionization at 600 torr pressure, which were the conditions of this work of Figueira et al That is, above a threshold fluence $ p , the transmissivity for optical fluence decreases with increasing $0, leading to a nearly constant transmitted fluence $ r for some range above $ p . In Figueira et al (1981), for example, $ p = 10 J/cm 2 for both 2-and 60-ns pulses in 600-torr N 2 , and a precise fluence-limiting trend was observed for two orders of magnitude in incident intensity above this threshold.…”

“…It is well-known that (see, e.g., Figueira et al 1981), where laser intensity and gas density are sufficient to achieve a critical-density plasma, plasmas produced by IR lasers in a gas exhibit an optical-fluence-limiting characteristic. This density, n ec = 1.115 x 10 13 / X 2 cm" 3 , is 9.94 x 10 18 cm~3 for X = 10.59 ^m, corresponding to 47% ionization at 600 torr pressure, which were the conditions of this work of Figueira et al That is, above a threshold fluence $ p , the transmissivity for optical fluence decreases with increasing $0, leading to a nearly constant transmitted fluence $ r for some range above $ p .…”

“…In transmission, this effect has been used (Yablonovitch 1974) to generate laser pulses with a rapid decay, as well as to automatically protect delicate laser optics from powerful amplified reflections (Auerbach et al 1979;Figueira et al 1981;Sheheen et al 1982) and, in reflection, to generate laser pulses with a rapid rise (Alcock et al 1975, Max et al 1976Alcock et al 1979). The transmissivity of metallic plasmas produced by irradiating an aperture in vacuum can be as small as 2 x 10~4 for incident fluences of order 1 kJ/cm 2 at 10-/xm wavelength, for pulse durations ranging from 1 ns to 1 jzs.…”