We have proposed and demonstrated successfully a new approach for generating high-yield K-shell radiation with large-diameter gas-puff Z pinches. The novel load design consists of an outer region plasma that carries the current and couples energy from the driver, an inner region plasma that stabilizes the implosion, and a high-density center jet plasma that radiates. It increased the Ar K-shell yield at 3.46 MA in 200 ns implosions from 12 cm initial diameter by a factor of 2, to 21 kJ, matching the yields obtained earlier on the same accelerator with 100 ns implosions. A new "pusher-stabilizer-radiator" physical model is advanced to explain this result.
A large diameter gas puff nozzle, designed to produce a radial mass profile with a substantial fraction of the injected mass on the axis, has demonstrated an increase in K shell yield by nearly a factor of 2, to 21kJ, in an argon Z pinch at 3.5MA peak current and 205ns implosion time [H. Sze, J. Banister, B. H. Failor, J. S. Levine, N. Qi, A. L. Velikovich, J. Davis, D. Lojewski, and P. Sincerny, Phys. Rev. Lett. 95, 105001 (2005)] and 80kJ at 6MA and 227ns implosion time. The initial gas distribution produced by this nozzle has been determined and related to measured plasma dynamics during the implosion run-in phase. The role of two gas shells and the center jet are elucidated by the inclusion of a tracer element sequentially into each of the three independent plenums and by evacuating each plenum. The implosion dynamics and radiative characteristics of the Z pinches are presented.
Structured 12-cm-diam Ar gas-puff loads have recently produced Z-pinch implosions with reduced Rayleigh-Taylor instability growth and increased K-shell x-ray yield [H. Sze, J. Banister, B. H. Failor, J. S. Levine, N. Qi, A. L. Velikovich, J. Davis, D. Lojewski, and P. Sincerny, Phys. Rev. Lett. 95, 105001 (2005)]. To better understand the dynamics of these loads, we have measured the extreme ultraviolet (XUV) emission resolved radially, spectrally, and axially. Radial measurements indicated a compressed diameter of ≈3mm, consistent with the observed load inductance change and an imploded-mass consisting of a ≈1.5-mm-diam, hot, K-shell-emitting core and a cooler surrounding blanket. Spectral measurements indicate that, if the load is insufficiently heated, then radiation from the core will rapidly photoheat the outer blanket, producing a strong increase in XUV emission. Also, adding a massive center jet (⩾20% of load mass) increases the rise and fall times of the XUV emission to ⩾40ns, consistent with a more adiabatic compression and heating of the load. Axial measurements show that, despite differences in the XUV and K-shell emission time histories, the K-shell x-ray yield is insensitive to axial variations in load mass.
Recently, a new approach for efficiently generating K-shell x-rays in large-diameter, long-implosion time, structured argon gas Z-pinches has been demonstrated based on a “pusher-stabilizer-radiator” model. In this paper, direct observations of the Rayleigh–Taylor instability mitigation of a 12-cm diameter, 200-ns implosion time argon Z-pinch using a laser shearing interferometer (LSI) and a laser wavefront analyzer (LWA) are presented. Using a zero-dimensional snowplow model, the imploding plasma trajectories are calculated with the driver current waveforms and the initial mass distributions measured using the planar laser induced fluorescence method. From the LSI and LWA images, the plasma density and trajectory during the implosion are measured. The measured trajectory agrees with the snowplow calculations. The suppression of hydromagnetic instabilities in the “pusher-stabilizer-radiator” structured loads, leading to a high-compression ratio, high-yield Z-pinch, is discussed. For comparison, the LSI and LWA images of an alternative load (without stabilizer) show the evolution of a highly unstable Z-pinch.
A bent-crystal Laue {or Cauchois [J. Phys. Radium 3, 320 (1932)] geometry} spectrograph is a good compromise between sensitivity and spectral resolution for measuring x-ray spectra (15<E<100keV) from large area x-ray sources because source-size spectral broadening is mitigated. We have designed, built, and tested such a spectrograph for measuring the spectra from electron-beam x-ray sources with diameters as large as 30cm. The same spectrograph geometry has also been used to diagnose (with higher spectral resolution) smaller sources, such as x-ray tubes for mammography and laser-driven inertial fusion targets. We review our spectrograph design and describe the performance of different components. We have compared the reflectivity and spectral resolution of LiF, and Ge diffracting crystals. We have also measured the differences in sensitivity and spectral resolution using different x-ray to light converters (plastic scintillator, CsI, and Gd2O2S) fiber optically coupled to an intensified charge-coupled device camera. We have also coupled scintillating fibers to photomultiplier tubes to obtain temporal records for discrete energy channels.
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