Near-field measurements of subnanosecond-created laser plasmas

Abstract

The near field of expansion for plasmas created by a 40-psec 1.0–1.5-J laser has been probed to measure the electron density and local electric and magnetic fields. Holographic interferometry was used to measure the electron density in the range 1017 to 6×1018 cm−3 at delays of 2.5–22 nsec. Space-charge electric fields of up to 1900 V/cm were detected by deflection of a fast He+ ion beam probe as late as 26 nsec after irradiation. Spontaneous magnetic fields of up to 50 G, rising in 10 nsec, were seen 4 mm from the target, decaying rapidly and often reversing at 8 mm. The temperature in the blowoff estimated from the combination of local electric field and electron-density scale length is high compared to predictions based on a peak Te=160 eV inferred from a multichannel soft x-ray spectral analysis. An anisotropic fast ion group appears in the interferometric density profiles and ejection of ∼109 fast electrons is observed in the electric field measurements. These effects are compared to the early time ion collector signal and nonthermal x-ray spectrum, respectively.