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RECENT ADVANCES IN SOFT X-RAY LASER EXPERIMENTS USING GERMANIUM AND
COPPER PLASMAS
R. Elton, T. Lee, E. Mclean
To cite this version:
R. Elton, T. Lee, E. Mclean. RECENT ADVANCES IN SOFT X-RAY LASER EXPERIMENTS
USING GERMANIUM AND COPPER PLASMAS. Journal de Physique Colloques, 1987, 48 (C9),
pp.C9-359-C9-362. �10.1051/jphyscol:1987962�. �jpa-00227380�
48,
RECENT ADVANCES IN SOFT X-RAY LASER EXPERIMENTS USING GERMANIUM AND COPPER PLASMAS
R.C. ELTON, T.N. LEE and E.A. McLEAN
U . S . Naval Research Laboratory, Washington, DC 20375-5000,
U . S . A .
Abstract. -- Soft x-ray gain coefficients of 1.7-4.1 cm-I are
.
measured on eight lines in the wavelength range of 196 to 286 A using 3p-3s transitions in neon-like Culgt and GezZtions. The lasant plasma is created by a high-power laser line-focused onto thin-foil and solid-slab targets. Measured wavelengths agree with calculations. A previously-reported anomaly in gain for a J=O upper level is small in these experiments.
Soft x-ray lasers operating in the 100-300 wavelength range at powers in the MW regime now exist [I]; and plans are already underway to use such a laser to explore new pumping concepts based on
innershell photoionization. The 3p-3s lasing transition in neon-like plasma ions has proven to be a particularly successful and robust approach which lends itself to isoelectronic extrapolations beginning in the near ultraviolet region. The 3p and 3s energy levels involved in the five particular transitions for which we are reporting gain are shown in Fig. 1 , and are labeled A-E here for convenience.
To date, the highest powers and shortest wavelengths for such transitions have been achieved with very high-energy ( " 4 kJ), short
Fig. 1. Lasing energy levels and transitions. A sixth low-output (3/2,3/2)0-(3/2,1/2)1 unobserved transition is not shown.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987962
C9-360 JOURNAL DE PHYSIQUE
wavelength (0.53 pm), sub-nanosecond pulsed lasers line-focused onto thin metallic Se, Y, and Mo foils [2,3]. One surprising result has been the low relative gain measured for transition A with a J=O upper level, which was predicted to have the highest gain. (This is
indicated in the last two columns of Table 1.) It is clear that we still do not fully understand the pumping and redistribution
mechanisms which determine the population inversion. Most likely there is some mix of electron-collisional excitation from the Ne-like 2s22p6 ground state, 2s-innershell or 2p-ionization starting from the
2s22p63s,p Na-like ground state, and dielectronic or collisional recombination from the 2sz2p5 F-like ground state. This remains a leading research challenge that impacts directly on the design codes.
To further understand this "J=O anomaly" and at the same time use a more modest system, we have successfully completed the first
measurements of gain on 3p-3s transitions in lower-Z (29 and 32) neon-like Cui9+ and GeZ2+ ions. Most of the data reported here are from the most recent Ge experiments, some of which, along with the copper data, are described elsewhere C41.
The experimental setup has been described E 4 . 5 3 . Essentially. a single-beam of the NRL Pharos I11 Nd-glass laser was operated at 1.05
prn
wavelength over an energy range of 350-480 J with a 2-ns fwhm pulse. This beam was focused to a line of width 2 0 0 p m and a length of 18 mm for a target irradiance of - 6 x 1012 W/cmZ. Copper and germanium targets were prepared with various lengths, all less than that of the beam b at least 2 mm. Most of the copper targets consisted of 1200 k-thick Formvar with a 1000 copper overcoating.(Some 1.3 rm-thick copper foils and slab copper targets were also tested successfully.) The germanium targets were all thick slabs.
The soft x-ray lasing data in the 200 region were recorded on Kodak type 101 film with a 1-m grazing-incidence spectro raph. A 5-pm entrance slit provided a spectral resolution of -0.04,A8, which is comparable to the expected Doppler width of lines. A 1200 A-thick aluminum,filter was used to reject higher-order radiation between 70 and 170
A.
The entrance slit was positioned close to the plasma and was oriented parallel to the driving laser beam and perpendicular to the target plane, in order to collect refracted radiation. Intensities were related to film density by a multiple-exposure HD-curve.In addition to the soft x-ray measurements, ;a bent-mica crystal, x-ray spectrograph was used to monitor resonance lines in the 6-17 A spectral region in order to establish the presence of the proper ionic species. Also, an x-ray pinhole camera, filtered to detect x-rays of energy 21 keV, photographed the hot plasma in both the axial and transverse directions E51.
The intensities on the gain lines has been sufficient to deduce gain coefficients for five lines of germanium and three lines of copper, on transitions A-E. Exponentiation of the measured intensities with increasing length for the Ge22+ ion is shown in Fig. 2 for the three J = 2 to 1 transitions and in Fig. 3 for the J = 0 to 1 and J = 1 to 1 transitions. The solid lines in these figures represent a best fit of the gain scaling relation 141 for amplified spontaneous emission through a plasma of variable length.
The resulting gain coefficients are indicated in the figures, listed in Table 1 (along with those obtained for Cu19') and discussed below.
The data plotted represent 90% of the shots taken, indicating very good reproducibility. No oher lines in the spectral region covered were observed to increase non-linearly with length.
e = 196.06A (A) J = 0 + 1
F 1
a = 247.32A (D) J = 1
-
I/ /-f
GAIN = 3.l/cm
4 8 12 16
PLASMA LENGTH (rnrn) PLASMA LENGTH (mm)
Fig. 2. Line intensity vs. Fig. 3. Line intensity vs.
length for J=2 upper levels. length for J=0,1 upper levels.
The relative-intensity data for gain coefficients shown in F i g ~ . ~ 2 and 3 are normalized in the stronger second order. One line (286.46 A) is taken from weaker first order data because of an overlapping line.
The data for this line is sparse at short lengths, But matches the same gain coefficient (4.1 cm-1) as for the 232.2: A line at longer lengths. The third G e J = 2 t o 1 line at 236.26 A had an overlapping line in both orders, and no gain curve was possible. However,
intensities relative to the 232.24
1
line in both orders for lengths>10 mm were comparable, so that the same gain coefficient (4.1 cm-'1 - is quoted in Table 1.
Table 1. Lasing wavelengths (wl) and gain coefficients (GI.
I
----
Cul 9 +---
ITrans.
g1
G(9.2)[A1 [cm-I J
a E ~ ~ t . , Ref. 3; bTheory, Ref. 2; CG(286.46) from first order spectra.
Among the three J = 2 to 1 Ge transitions (B, C and El, the gain coefficients appear to be consistently 4.1 cm-I. A gain coefficient of 1.7 cm-I for Cu (B,C) further suggests a 2-scaling, such as predicted from modeling. (However, to derive a true 2-scaling relation it is necessary to demonstrate consistently that all data are obtained at optimum conditions for each element.) Also, the gain coefficient of 2.7 cm-I for the J = 1 to 1 transition (D) in Ge is lower than for J = 2 to 1 by a factor-of-0.66; this is consistent with modeling [21 for selenium which gives a ratio of 2/3 (Table 1 , last column).
However, the same selenium modeling predicts that the J = 0 to 1 transition should be higher than that for J = 2 to 1 by a
factor-of-3.3, whereas experiments indicated [3] a very much lower ratio of <1/3.9 or <0.26 (Table 1, column 6). In our present Ge experiments the ratio is larger, i-e., 3.1/4.1=0.76 (column 5,
C9-362 JOURNAL DE PHYSIQUE
Table 1). Furthermore, our coppper data for transitions A, B and C (column 3, Table 1) indicate an even larger corresponding ratio of 2.0/1.7=1.2, which is reasonably close to expectations [2,61 of 1-2X for 2=29. Whether this improvement of the J = 0 to 1 gain coefficient is associated with lower-Z elements or with experimental differences remains a question.
A second J = 0 to 1 line at 192.04 A for Ge (not included in Table 1 , see caption) was predicted for Se 123 to have a gain coefficient only 1/10 that of the stronger transition (A), and indeed showed no measurable signal in Ge2Zt here. Likewise, two "2s-hole",
ls2 2s2p6 (3d-3p) transitions at 193.95 and 198.99 A did not display measurable emission, unfortunately since this could imply the presence of significant innershell pumping.
Output energies (from both ends of the plasma) of 3 and 1 uJ were measured for Cu and Ge, respectively, using a film calibration by Henke, et al. 171. These correspond to an overall efficiency of
In addition to the gain coefficients and output energy, absolute wavelengths of the eight lasing lines have been measured within s . 0 4 A. These measured wavelengths agree with available theoretical values as shown in Table 2, the best being for J = 2 upper levels.
Table 2. Measured (meas) and theoretical wavelengths (wl) in Angstroms Trans. I--- cul 9 +
---
I I---
Ge22 +--- ---
1~ l m e a s wla wlb wlc ~ l m e a s ~ 1 ' wlb
aRef. 8; "J.H. Scofield, LLNL, unpubl.; CRef. 9; dAlso 284.97 A [lo].
In summary, time-integrated 3p-3s gain coefficients of as large as 4 cm-1 in the 200 A region are now measured in plasmas formed from single-beam irradiated solid-slab targets. Also, the gain coefficient for the J = 0 to 1 line is comparable to that for J = 2 to 1 lines at lower 2 . This result tends to support electron-collisional excitation (where the cross section is known to favor the J=O level) as a primary pumping mechanism. Also, the measured wavelengths for the lasing lines are in good agreement with recent theoretical calculations and
extrapolations.
The authors gratefully acknowledge the cooperation of colleagues at NRL, the Univ. of Maryland, and the Lawrence Livermore Laboratory.
References
[l] Conference papers in J. de Physique, Colloque C6, 47 (1986).
121 MATTHEWS, D.L., et al., J. de Physique 47 (1986) C6-1.
[3] MATTHEWS, D.L., et al., J. Opt. Soc. Am. B, 4 (1987) 575.
[4] LEE, T., McLEAN, E. & ELTON, R., Phys. Rev. Letters (in press)
.
[5] ELTON, R., LEE, T. & MOLANDER, W., J.Opt.Soc.Am.B, 4 (1987) 539.
[6] FELDMAN, U., SEELY, J. & DOS<CHEK, G., J. de Phys. 47 (1986) 187.
[7] HENKE, B.L., et al., J. Opt. Soc. Am. B 1 11984) 828.
181 COGORDAN, J. & LUNELL, S., Physica Scripta (1986) 406.
[9] BUCHET, J.-P., et al., J. Phys. B: At. Mol. Phys. 20, (1987) 1709.
[lo] HARR, R.R., et al., Physica Scripta 35 (1987) 296.
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