Magnetohydrodynamics control of capillary Z-pinch discharge by using a triangular current pulse for lasing a H-like N recombination soft x-ray laser

Abstract

In expansion cooling phase of pinched nitrogen plasma generated by fast capillary discharge, it might be possible to realize lasing a Blamer ␣ recombination SXRL, which requires a rapid cooling of nonequilibrium plasma. It is effective to decrease the discharge current rapidly in reducing the additional heating caused by the joule heating and the magnetic compression of plasma as quickly as possible. The shaping of discharge current waveform was demonstrated with a transmission line and its effect on expanding plasma dynamics were investigated through magnetohydrodynamics ͑MHD͒ calculation, and validity of the MHD calculation in the expansion phase was shown using the discharge photographs taken by using a high speed camera. As a result, strong radiation from the H-like N ion at the maximum pinch, which is in the current decay phase of the triangular current with peak amplitude of over 70 kA and pulse width of 60 ns, has been confirmed in x-ray photodiode signals at wavelength of less than 2.5 nm, to clarify the existence of the Lyman series and continuum of the H-like N ion. Without additional heating by the discharge current after the generation of the fully stripped nitrogen ions, it might be possible to generate the population inversion between the principal quantum number n = 2 and 3.

Observation of emission process in hydrogen-like nitrogen Z-pinch discharge with time integrated soft X-ray spectrum pinhole image

“…However, although the total soft-X-ray radiation measured by the XRD at the maximum pinch and in the expansion phase are about the same intensity, strong emission from the He and Lyman series only exists at the maximum pinch. 22 As shown in the following experimental results, the observed minimum radius of the pinched plasma is approximately <1/5 of the capillary inner radius, and therefore the photon density in the expansion phase is decreased by one order of magnitude. Thus, the on-axis spectrum is used to evaluate the plasma parameters at the maximum pinch.…”

“…Typically, a secondary pinch was not observed and maximum pinch occurs in the first decay phase of the triangular pulsed current, as shown in typical XRD signals. 22,27 However, in the experiment with initial gas …”

“…In the Z-pinch discharge, the radial gradient of fluid velocity is presumed to reach rv Fluid $ 10 5 /s in the expansion phase. 22 Thus, the Doppler shift of the angular frequency associated with the fluid motion is several times larger than that arising from thermal effects. 1,31 Using the expanding plasma radius of several hundreds of lm as the characteristic plasma length, K n¼1À2 > 1=2 yields a required ion number density of N þ6 n¼1 < 10 16-17 cm À3 .…”

“…1) at the wavelength of 13.4 nm (EUV) has been proposed, 20 and the observation of He and Lyman series radiation has been reported thus far. 21,22 Lasing of the Balmer a radiation requires a strong ionization, to NVIII at the maximum pinch, and subsequent rapid cooling. These requirements are needed to establish the population inversion between the principal quantum number n ¼ 2 and n ¼ 3; the population of the ground state should be low enough to suppress radiation trapping by the Ly a, besides collisional or radiative de-excitation rates from the higher states to n ¼ 3 state should be higher than the radiative decay rate from n ¼ 3 to lower states, while collisional excitation from the ground state to n ¼ 2 state should be suppressed.…”

Effect of the rate of rise in discharge current on the output of a 46.9-nm soft X-ray laser based on capillary discharge

“…It is worth mentioning here that directed EUV emission has also been reported from discharge plasma at shorter wavelengths ranging from 12 to 16 nm [23]. Efforts are presently going on in this direction in various laboratories worldwide to demonstrate X-ray lasing at a wavelength of 13.4 nm in nitrogen z-pinch plasma using a capillary discharge system [24][25][26][27]. Unlike the 46.9-nm X-ray laser, this is based on recombination pumping scheme which requires fast heating of the plasma followed by fast cooling of the hot plasma.…”