A vacuum-ultraviolet spectropolarimeter for an electron beam ion trap

We have developed a vacuum-ultraviolet spectropolarimeter for an electron beam ion trap (EBIT) to measure the linear polarization of emission lines from multiply charged ions around the Lyman-α wavelength. The main components for polarimetry are a rotatable MgF2 waveplate and a SiO2/MgF2 multilayer-coated fused silica plate that functions as a reflective polarizer. A grazing-incidence grating is mounted between them to provide wavelength dispersion. The polarization is determined from the intensity modulation of the spectral line as the waveplate is rotated. The performance of the spectropolarimeter was demonstrated by measuring the polarization of the 2s-2p3/2 transition in Li-like N4+ (124 nm) excited by a 1000 eV electron beam in an EBIT. Clear modulation of the line intensity was observed as a function of waveplate rotation angle. From the measured modulation amplitude, the degree of linear polarization was determined to be P=-(0.178-0.005+0.014), with the negative sign indicating that the emission is polarized predominantly perpendicular to the electron beam. This result demonstrates the capability of the present spectropolarimeter to determine polarizations with an absolute uncertainty ΔP on the order of 0.01. This instrument provides a useful tool for benchmarking magnetic-sublevel-resolved collision theories through polarization measurements of multiply charged ions excited by a unidirectional electron beam.