A novel technique for measurement of self-generated magnetic fields and the plasma density in laser produced plasmas from the Faraday rotation using two color probes A.S. Joshi ⁎, P.A. Naik, S. Barnwal, Y.B.S.R. Prasad, P.D. Gupta Laser Plasma Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India abstract article info Article history: Received 26 March 2010 Received in revised form 8 June 2010 Accepted 30 June 2010 Keywords: Self-generated magnetic field Optical probing Interferometry Diagnostic information about the self-generated magnetic fields (SGMF) generated in laser produced plasmas is normally obtained by measuring the Faraday rotation angle (FRA) of a linearly polarized laser probe beam passing through the plasma. Simultaneous recording of the corresponding interferogram is required to get the density information necessary for estimating the magnetic field. The problem with this method is that the visibility of the fringes in the interferogram can be poor, and the SGMF cannot be calculated in the regions where the interference fringes are not observable. In this paper, we propose a new method to obtain the density distribution and the SGMF from two simultaneous measurements of FRA using two probe beams of different colors, which allows one to calculate the SGMF without the need of interferometry. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Self-generated magnetic fields (SGMF) of the order of 100 kG to few MG [1,2] are generated in high density, high temperature laser produced plasmas. The knowledge of the magnetic field is important as they can significantly inhibit the heat transport [3,4] in the inertial confinement fusion (ICF) plasmas. The first measurements of SGMF [5] in a low density laser produced plasma, away from the planar target, were obtained by inserting small coil (dia ~ 1 mm) probes in the radial direction in the expanding plasma to record the temporal derivatives of the axial and azimuthal magnetic fields [6]. The magnetic probe method gives the information about the SGMF only in low density plasma. Moreover, it does not give any information about the plasma density. The optical probe method for the measurement of SGMF using the Faraday rotation angle (FRA), determined from the rotation of plane of polarization of a linearly polarized probe laser beam passing through the dense plasma, was first demonstrated by Stamper et al. [1]. The optical probe method requires that along with the FRA, the plasma density in the region of the magnetic field also has to be measured. The density information is obtained from the interferogram formed due to the interference between a reference probe beam and the part of probe beam that passes through the plasma. A three channel polaro-interferometer [7] can be used in a single shot experiment to simultaneously record: a) polarogram, b) interferogram, and c) shadowgram. The polarogram gives the information about the FRA, the shadowgram gives a record of the reference probe beam intensity distribution, and interferogram gives the density profile. The main problem in interferometry is that the visibility of the fringes in the interferogram, which depends on the value of the relative intensity of the probe and reference beams, may be poor in some regions, which makes it difficult to get density information in all regions. It is not possible to control the relative intensity of the probe beam that passes through the plasma, as it gets partly absorbed by the plasma. Currently, there is also a great amount of interest in measurements of SGMF in femtosecond laser produced plasmas. The magnetic fields in ns pump pulse regime are explained in terms the thermo-electric source of the electron current in hot collisional plasmas when the gradient of electron temperature is not collinear with the electron density gradient. In contrast, the mechanism for the magnetic field generation using laser pulses of ~ 1 ps or less duration is due to the DC currents driven by spatial gradients and the temporal variations of the ponderomotive force exerted by the laser on the plasma electrons [8]. The high focused intensity of the femtosecond pump laser may also cause optically induced birefringence and Kerr effect that may make the input linear polarization of the probe beams elliptical. Hence, FRA cannot be correctly measured in the pulse duration of the femtosec- ond laser pump pulse. However, if one measures the FRA just after the pump pulse is over, the SGMF can be estimated. The magnetic fields generated by a 1.5 ps laser pulse focused to an intensity of ~5×10 18 W/cm 2 have been measured by determining the FRA after the pump laser pulse is over [9]. SGMF produced by a 100 fs laser pulse has been measured by Cotton Mouton polarimetry (when the magnetic field is perpendicular to the direction of propagation of probe beam) in the picosecond time scales [10,11] using the pump- probe method. Formalisms have been proposed to measure magnetic Optics Communications 283 (2010) 4713–4716 ⁎ Corresponding author. E-mail address: asjoshi@rrcat.gov.in (A.S. Joshi). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.06.103 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom