ABSTRACT
Figure 1. a) Experimental set-up of the Fresnel bi-mirror X-ray interference microscopy, b) Typical interference pattern obtained with Al plasma under the pumping laser energy of 1 J.
Experiments were performed on the hybrid Nd-glass laser system of the Advanced Photon Research Center (APRC), JAERI. A 13.9 nm Ni-like Ag TCE (several picoseconds of pulse duration) X-ray laser beam was produced on the system and used as the probing beam [12]. The diagnosed plasma was formed by irradiating an Al slab targetwith 1.2 ns IR (1053 nm) pulse. A line focus system produced a plasma column of ~ 100 um in width and lmm in length (intensity within the line focus: 1010'11 Wcnr2). The X-ray laser probed the plasma along the column axis (defined as X direction). Under such configuration, the electron density of the plasma can be calculated from the fringe shift by a simple linear equation without Abel inversion. A typical interference pattern of an Al laser plasma is shown in Fig. lb. The plasma column was produced under the pumping energy of 1 J (intensity: ~ 1.4xlOn Wcnr2). The peak-to-peak fringe separation out of the plasma region is ~ 10 um horizontally (defined as F direction). Pumping laser irradiates from the left side (opposite to Y direction) and the X-ray probe beam propagates along the plasma column (X direction), which is normal to the paper plane. Intense plasma self-emission can be observed close to the target surface and mixed with the interference fringes. Because the information of electron density is only carried by
the fringe shift, some numerical processing should be adopted to extract the interference fringe from the self-emission and other CCD noises, under the assumption that the selfemission and other noises are linearly overlapped with the interference fringes. Figure 2a shows the temporal evolution of the phase of the Al plasmas probed by the X-ray laser at times comprised between 0 and 3 nanoseconds after the peak of the plasma heating pulse. The phase card and electron density are connected to the fringes pattern. Thus, the electron density, /7e, is related to the fringe shifts as /?e^l.583xl020Airinge [cm3], for the plasma length of 1mm. Due to the limitation of the CCD resolution under this interferometry configuration, the minimum detectable limit of the electron density is ~ 3xl019cnr3.
