The geometry of the von Hámos spectrometer that recorded the axial Kr(L) spectrum is illustrated

<p><strong>Figure 3.</strong> The geometry of the von Hámos spectrometer that recorded the axial Kr(L) spectrum is illustrated. As described in the text, axial translation along the <em>Z</em>-axis controllably positions the focal point of the instrument so that the spectrum of the amplified Kr(L) beam can be faithfully recorded as a strong signal on the CCD in the plane of the instrument without significant interference from the two regions of isotropic emission that are associated with the formation and termination of the self-trapped plasma channel. (a) Focal point of spectrometer positioned within the isotropic emission zone at the exit of the plasma channel. (b) Focal point of spectrometer positioned outside of both isotropically radiating regions that are accordingly designated as off-axis, defocused zones. With this geometrical configuration, only the directed Kr(L) 7.5 Å beam can produce a strong signal on the plane of the detector, thereby selectively recording the spectrum of the amplified Kr(L) transition on the CCD.</p> <p><strong>Abstract</strong></p> <p>Experimental evidence demonstrating amplification on the Kr²⁶⁺ 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)<em>_n</em> cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.</p

Electron density profile of Xe self-trapped channel

<p><strong>Figure 5.</strong> Electron density profile of Xe self-trapped channel. (a) Section A–A from panel (b) of the electron density that reveals an extended plateau of ionization. The two brown lines show the expected profile from linear absorption of Xe(M) ~ 1 keV x-rays in surrounding neutral Xe. The existence of the extended peripheral zone signals saturated absorption. The peak of the Thomson signal is ~ 35-fold above that for the onset of the plateau. (b) Taken from figure <a href="http://iopscience.iop.org/0953-4075/46/18/185601/article#jpb469036f2" target="_blank">2</a>(a) and illustrates the location of the A–A section through the peripherally ionized region. (c) Taken from figure <a href="http://iopscience.iop.org/0953-4075/46/18/185601/article#jpb469036f2" target="_blank">2</a>(b) with the position of section A–A indicated.</p> <p><strong>Abstract</strong></p> <p>Comparative single-pulse studies of self-trapped plasma channel formation in Xe and Kr cluster targets produced with 1–2 TW femtosecond 248 nm pulses reveal energy efficient channel formation (>90%) and highly robust stability for the channeled propagation in both materials. Images of the channel morphology produced by Thomson scattering from the electron density and direct visualization of the Xe(M) and Kr(L) x-ray emission from radiating ions illustrate the (1) channel formation, (2) the narrow region of confined trapped propagation, (3) the abrupt termination of the channel that occurs at the point the power falls below the critical power <em>P</em>_cr, and, in the case of Xe channels, (4) the presence of saturated absorption of Xe(M) radiation that generates an extended peripheral zone of ionization. The measured rates for energy deposition per unit length are ~ 1.46 J cm⁻¹ and ~ 0.82 J cm⁻¹ for Xe and Kr targets, respectively, and the single pulse Xe(M) energy yield is estimated to be > 50 mJ, a value indicating an efficiency >20% for ~ 1 keV x-ray production from the incident 248 nm pulse.</p

Axial spectrum of Kr(L) emission and corresponding transverse x-ray channel morphology are shown

<p><strong>Figure 11.</strong> Axial spectrum of Kr(L) emission and corresponding transverse x-ray channel morphology are shown. Data correspond to file 520/14 June 2012. (a) Axially recorded Kr(L) spectrum taken with the focal plane of the von Hámos spectrometer positioned at <em>Z</em> = 1.25 mm, the location for the data presented in figures <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f9" target="_blank">9</a> and <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f10" target="_blank">10</a>. Spectral narrowing of the Kr²⁶⁺ 3s → 2p line at λ 7.504 Å is seen with a width δ_λ,1  3 eV, a width that matches the axial spectrum shown in figure <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f7" target="_blank">7</a>(a). (b) The transversely observed Kr(L) x-ray morphology of channel is shown. The centre of the nozzle is located at <em>Z</em> = 0 mm and the focal plane of the axial von Hámos spectrometer positioned at <em>Z</em> = 1.25 mm. The depth-of-field of the spectrometer is indicated by the shaded zone centred on <em>Z</em> = 1.25 mm.</p> <p><strong>Abstract</strong></p> <p>Experimental evidence demonstrating amplification on the Kr²⁶⁺ 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)<em>_n</em> cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.</p

Transversely observed Kr(L) spectrum and corresponding x-ray image of plasma channel morphology

<p><strong>Figure 5.</strong> Transversely observed Kr(L) spectrum and corresponding x-ray image of plasma channel morphology. These data represent file 677/18 May 2012. (a) Kr(L) transverse spectrum recorded at longitudinal coordinate <em>Z</em> = −0.9 mm as indicated in panel (b). The Kr²⁶⁺ 3s → 2p line at λ 7.504 Å exhibits a width δ_λ,l,t  4.5 eV. The Doppler width expected from a coulomb explosion of the Kr clusters is estimated to be ~4.4 eV; see text for discussion. The limiting spectral resolution of the instrument is estimated to be ~3 eV. (b) X-ray image of Kr(L) emission from the plasma channel recorded transversely with the x-ray pinhole camera. The (<em>y</em>, <em>z</em>) spatial resolution is estimated to be ~50 µm in both coordinates. The centre of the 2.65 mm diameter nozzle is located at <em>Z</em> 0.6 mm. The 248 nm pulse energy was 182 mJ, the plenum pressure was 141 psi, and the nozzle temperature was 262 K.</p> <p><strong>Abstract</strong></p> <p>Experimental evidence demonstrating amplification on the Kr²⁶⁺ 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)<em>_n</em> cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.</p

Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Kr(L) ~ 1.7 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Kr cluster target

<p><strong>Figure 4.</strong> Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Kr(L) ~ 1.7 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Kr cluster target. The channel was produced at a height of 1.60 mm above the opening of the nozzle. The x-ray camera utilized a pinhole with a diameter of 50 µm and had a spatial resolution estimated to be 75–100 µm. The Kr cluster target was produced by a cooled high-pressure pulsed-valve fitted with a circular nozzle having a diameter of 2.65 mm. These data correspond to pulse 519 (14 June 2012). The direction of propagation is left to right and the position of the nozzle is the same as shown in figure <a href="http://iopscience.iop.org/0953-4075/46/18/185601/article#jpb469036f2" target="_blank">2</a>(a); the coordinates (<em>Y</em>, <em>Z</em>) = (0, 0) correspond to the centre of the nozzle. The matching locations of corresponding features in these images are indicated by the vertical connection lines. The abrupt expansion of the signal in panel (a) at <em>Z</em> 0 mm signals the termination of the confined propagation. A weak halo surrounding the bright zone indicating peripheral ionization is visible. The channel termination at <em>Z</em> 0 mm in panel (b) that coincides with the Thomson image in panel (a) is manifest.</p> <p><strong>Abstract</strong></p> <p>Comparative single-pulse studies of self-trapped plasma channel formation in Xe and Kr cluster targets produced with 1–2 TW femtosecond 248 nm pulses reveal energy efficient channel formation (>90%) and highly robust stability for the channeled propagation in both materials. Images of the channel morphology produced by Thomson scattering from the electron density and direct visualization of the Xe(M) and Kr(L) x-ray emission from radiating ions illustrate the (1) channel formation, (2) the narrow region of confined trapped propagation, (3) the abrupt termination of the channel that occurs at the point the power falls below the critical power <em>P</em>_cr, and, in the case of Xe channels, (4) the presence of saturated absorption of Xe(M) radiation that generates an extended peripheral zone of ionization. The measured rates for energy deposition per unit length are ~ 1.46 J cm⁻¹ and ~ 0.82 J cm⁻¹ for Xe and Kr targets, respectively, and the single pulse Xe(M) energy yield is estimated to be > 50 mJ, a value indicating an efficiency >20% for ~ 1 keV x-ray production from the incident 248 nm pulse.</p

Spectral details of Kr(L) amplification are presented

<p><strong>Figure 7.</strong> Spectral details of Kr(L) amplification are presented. These data correspond to file 677/18 May 2012. (a) Composite spectra are shown illustrating the axial (red) and transverse (brown) spectra. The transverse Kr(L) spectrum corresponding to <em>Z</em> = 0.3 mm is practically null; the axial von Hámos spectrometer, with the focal plane located at <em>Z</em> = 0.3 mm, as shown in panel (b), reveals sharp spectral structure with a width δ_λ,l  3 eV that is particularly obvious on the dominant Kr²⁶⁺ 3s → 2p transition at λ 7.5 Å. The depth-of-field of the axial von Hámos spectrometer is estimated to be ≤ 300 µm, as indicated by the shaded zone centred at <em>Z</em> = 0.3 mm in panel (b). Compare these spectra to those presented in figure <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f6" target="_blank">6</a>(a). (b) Transverse x-ray camera image of Kr(L) emission profile with a spatial resolution estimated to be ~50 µm. The axial position <em>Z</em> = 0.3 mm corresponds to the location of the focal plane of the axial von Hámos spectrometer.</p> <p><strong>Abstract</strong></p> <p>Experimental evidence demonstrating amplification on the Kr²⁶⁺ 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)<em>_n</em> cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.</p

Transverse Thomson and x-ray images of the plasma channel

<p><strong>Figure 9.</strong> Transverse Thomson and x-ray images of the plasma channel. Data represent file 520/14 June 2012. The physical parameters are the following: 248 nm pulse energy 200 mJ, Kr plenum temperature <em>T</em> = 295 K, and Kr plenum pressure 185 psi. The centre of the nozzle is located at position <em>Z</em> = 0 mm. The arrows located at position <em>Z</em> = 1.25 mm in all panels indicate the position of the focal plane of the axial von Hámos spectrometer. (a) Thomson image of the electron density. A lateral extension is visible at <em>Z</em> 0 mm along with a narrow axial elongation well into the zone <em>Z</em> ≥ 0.5 mm. This axial extension is illustrated further in figure <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f10" target="_blank">10</a>. (b) Panel (a) represented as an isometric view. (c) The x-ray morphology of the plasma channel viewed transversely with the pinhole camera corresponding to the Thomson image shown in panel (a). The spatial region defined by <em>Z</em> ≥ 0.5 mm is completely dark. The shaded zone centred on <em>Z</em> = 1.25 mm indicates the depth-of-field of the axial von Hámos spectrometer.</p> <p><strong>Abstract</strong></p> <p>Experimental evidence demonstrating amplification on the Kr²⁶⁺ 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)<em>_n</em> cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.</p

Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Xe(M) ~ 1 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Xe cluster target are illustrated

<p><strong>Figure 2.</strong> Simultaneously recorded single-pulse images of (a) the Thomson scattered signal from the electron density and (b) the transverse Xe(M) ~ 1 keV x-ray emission zone (log scale) of a stable 248 nm channel produced in a Xe cluster target are illustrated. The channel was developed at a height of 1.45 mm above the orifice of the nozzle. The x-ray camera utilized a pinhole with a diameter of 10 µm, a size that gives a limiting spatial resolution estimated to be 20–30 µm. The direction of propagation is left to right and the centre of the nozzle corresponds to the coordinates (<em>Y</em>, <em>Z</em>) = (0, 0). The Xe cluster target was produced by a cooled high-pressure pulsed-valve fitted with a circular nozzle having a diameter of 2.50 mm. These data correspond to pulse 19 (24 January 2011). The locations of corresponding features in these images are indicated by the vertical connecting lines. The sharp expansion at the longitudinal position <em>Z</em> 0 mm in panel (a) signals the termination of the self-trapped channel. A broad elliptical halo of ionization with a diameter of ~ 1 mm is seen in the −0.5 mm ≤ <em>Z</em> ≤ 1.0 mm region. The abrupt expansion of the x-ray emission in panel (b) at <em>Z</em> 0 mm marking the end of the channeled propagation mirrors the corresponding morphology of the Thomson image in panel (a) and the localized zones of x-ray emission coincide with matching features in the Thomson image.</p> <p><strong>Abstract</strong></p> <p>Comparative single-pulse studies of self-trapped plasma channel formation in Xe and Kr cluster targets produced with 1–2 TW femtosecond 248 nm pulses reveal energy efficient channel formation (>90%) and highly robust stability for the channeled propagation in both materials. Images of the channel morphology produced by Thomson scattering from the electron density and direct visualization of the Xe(M) and Kr(L) x-ray emission from radiating ions illustrate the (1) channel formation, (2) the narrow region of confined trapped propagation, (3) the abrupt termination of the channel that occurs at the point the power falls below the critical power <em>P</em>_cr, and, in the case of Xe channels, (4) the presence of saturated absorption of Xe(M) radiation that generates an extended peripheral zone of ionization. The measured rates for energy deposition per unit length are ~ 1.46 J cm⁻¹ and ~ 0.82 J cm⁻¹ for Xe and Kr targets, respectively, and the single pulse Xe(M) energy yield is estimated to be > 50 mJ, a value indicating an efficiency >20% for ~ 1 keV x-ray production from the incident 248 nm pulse.</p

Experimental configuration used for the Thomson and x-ray imaging of the plasma channel dynamics of 248 nm femtosecond pulses interacting with Kr and Xe cluster targets

<p><strong>Figure 1.</strong> Experimental configuration used for the Thomson and x-ray imaging of the plasma channel dynamics of 248 nm femtosecond pulses interacting with Kr and Xe cluster targets. The pinhole camera used for imaging the Xe(M)/Xe(L) or Kr(L) emissions, the Thomson imaging system, and the cooled pulsed-valve are designated. Also shown is the transverse von Hámos mica spectrometer. The spatial resolution of the Thomson system was measured to be 22 µm with the use of the standard calibrated USAF resolution target.</p> <p><strong>Abstract</strong></p> <p>Comparative single-pulse studies of self-trapped plasma channel formation in Xe and Kr cluster targets produced with 1–2 TW femtosecond 248 nm pulses reveal energy efficient channel formation (>90%) and highly robust stability for the channeled propagation in both materials. Images of the channel morphology produced by Thomson scattering from the electron density and direct visualization of the Xe(M) and Kr(L) x-ray emission from radiating ions illustrate the (1) channel formation, (2) the narrow region of confined trapped propagation, (3) the abrupt termination of the channel that occurs at the point the power falls below the critical power <em>P</em>_cr, and, in the case of Xe channels, (4) the presence of saturated absorption of Xe(M) radiation that generates an extended peripheral zone of ionization. The measured rates for energy deposition per unit length are ~ 1.46 J cm⁻¹ and ~ 0.82 J cm⁻¹ for Xe and Kr targets, respectively, and the single pulse Xe(M) energy yield is estimated to be > 50 mJ, a value indicating an efficiency >20% for ~ 1 keV x-ray production from the incident 248 nm pulse.</p

Details of Thomson images shown in panels (a) and (b) of figure 9

<p><strong>Figure 10.</strong> Details of Thomson images shown in panels (a) and (b) of figure <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f9" target="_blank">9</a>. These data correspond to file 520/14 June 2012. (a) Isometric view of Thomson image showing extension into the dark x-ray zone <em>Z</em> ≥ 0.5 mm illustrated in figure <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f9" target="_blank">9</a>(c). The arrow at <em>Z</em> = <em>Z</em>₁  1.8 mm specifies the location of the endpoint of the visible Thomson signal that indicates the presence of ionized material. (b) Axial line-out of the Thomson data pictured in panel (a). A discernable extension of the ionization is visible out to <em>Z</em> = <em>Z</em>₁  1.8 mm, a distance that represents a penetration of ~1.3 mm into the x-ray dark region shown in figure <a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344f9" target="_blank">9</a>(c) and is estimated to be comparable to the linear absorption length of the Kr cluster medium. This observation agrees with an earlier estimate [<a href="http://iopscience.iop.org/0953-4075/46/15/155601/article#jpb468344bib13" target="_blank">13</a>] of the ability to reach saturation of the absorption in Kr by Kr(L) emission; the clear conclusion was that, with a saturation parameter <em>ħ</em>ω/σ 400 J cm⁻², linear absorption would necessarily govern.</p> <p><strong>Abstract</strong></p> <p>Experimental evidence demonstrating amplification on the Kr²⁶⁺ 3s→2p transition at λ 7.5 Å (~1652 eV) generated from a (Kr)<em>_n</em> cluster medium in a self-trapped plasma channel produced with 248 nm femtosecond pulses is presented. The x-ray beam produced had a spectral width of ~3 eV and a corresponding beam diameter of ~150 µm, properties that were simultaneously determined by a two-dimensional x-ray spectral image formed with an axially placed von Hámos spectrometer and a matching Thomson image of the spatial electron density generated by the x-ray propagation.</p