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LASER-DRIVEN SOFT X-RAY LASING IN COPPER AND GERMANIUM PLASMAS
E. Mclean, T. Lee, R. Elton
To cite this version:
E. Mclean, T. Lee, R. Elton. LASER-DRIVEN SOFT X-RAY LASING IN COPPER AND GERMANIUM PLASMAS. Journal de Physique Colloques, 1988, 49 (C1), pp.C1-51-C1-54.
�10.1051/jphyscol:1988108�. �jpa-00227428�
JOURNAL DE PHYSIQUE
Colloque C l , Suppl6ment au n03, Tome 49, Mars 1988
LASER-DRIVEN SOFT X-RAY LASING IN COPPER AND GERMANIUM PLASMAS
E.A. McLEAN, T.N. LEE and R.C. ELTON
Naval Research Laboratory, Laser Plasma Branch, Code 4732, Washington, DC 20375-5000, U. S.A.
Abstract-Soft x-ray lasing has been achieved for the first time on 3p-3s tran- sitions in neon-like copper (Cu19+) and germanium (Gezz+). The elongated lasant plasma was produced using the NRL Pharos 111 laser operating at X = 1.05 pm, 300-480 J with a pulse duration of 2 ns. Lasing was observed in the wavelength interval 196 to 286 A for J = 2
-
1 and J = 0-
1lines, using both thin film and thick slab targets.
INTRODUCTION
For over 15 years an active search has been underway to produce soft x-ray lasers. In the last few years several groups [l] have now conclusively shown that they were indeed getting lasing at soft x-ray wavelengths. In this paper we report on soft x-ray lasing on 3p-3s transitions in neon-like copper (Cu19+) and germanium (GeZ2+) [2]. The goal of this work is to understand the critical variables and improve the effi- ciency of the laser to the point that it could be used in a manner similar to higher power lasers in the visible and the infrared, e.g., as optical probes in plasma diagnostics, to produce other plasmas with special proper- ties, etc.
Several groups [3,4] have calculated the gain that would be expected for these neon-like transitions if certain conditions of temperature and electron density are fulfilled. Generally speaking, to excite neon-like copper and germanium, a temperature of 400-700 eV is needed; and to get appreciable gain, an electron den- sity of a few X 102" electrons/cm3 [4] is needed. In the work reported here, we draw heavily on the experi- ence of the group at the Lawrence Livermore National Laboratory (LLNL) [3,5,6] that observed lasing in nwn-like Se, Y, and MO ions. They used both time-resolved and time-integrated x-ray spectrographs to prove incontrovertibly that lasing occurred in the Se plasma.
EXPERIMENT
Our experimental setup is shown in Fig. l(a). The NRL Pharos I11 laser at 1.05 pm wavelength, 350- 480 J pulse energy, and a 1-2 ns pulse duration (with a 0.8 ns rise time) was used as the driver. A combina- tion of aspheric and cylindrical lenses allowed a line focus about 200 pm in width and up to 18 mm in length to be placed on either thin film or thick slab targets, which are located in an evacuated chamber. To hold the driving laser power density constant (-6 X 10" W/crn2) for various target lengths, the line focus length was held constant and the target length was decreased to cover the desired range of the lasant plasmas. A l-m grazing incidence spectrograph, which could be placed in the target chamber: was aligned to view the elongated plasma end-on. The spectral resolution of this spectrograph is -0.04 A. The entrance slit of this spectrograph was oriented perpendicular to the plane of ihe target and parallel to the driving laser b e a ~ (This was done to allow refracted rays of the x-ray laser to be accepted by the spectrograph.) A 1200-A- thick aluminum filter was used to reject higher-order radiation with wavelengths from the 170
A
L-edge down to approximately 70 A.A bent crystal x-ray spectrograph, covering the 6-17
A
wavelength range allowed observation of the strong resonance lines of neon-like, and somewhat weaker fluorine-like and oxygen-like Ge and Cu ions.This allows one to make rough estimates of the electron temperature of the lasant plasma and to be sure that the neon-like ions were present in the plasma. An x-ray pinhole camera filtered to observe 1-1.6 keV radia- tion could be positioned to view the plasma end-on to observe the symmetry of the front and back plasmas; or the camera could be positioned in front of the target to observe the uniformity of the elongated plasma along the axis of the plasma.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988108
CI-52 JOURNAL
DE
PHYSIgUEIn Fig. lb, the target holder is shown with a Cu-Formvar foil mounted, allowing a clear, end-on, x-ray pinhole camera view of the plasma on the front and back surface of the foil. A typical tirne-integrated pinhole photograph shows that the plasma on the front side (the driving laser side) of the target foil has a large blowoff plasma, whereas that on the rear side is a relatively small jet, This demonstrates conclusively that we have an ablation type phenomena rather than an exploding foil target as described by LLNL [3,5].
On the basis of this result, we were encouraged to use thick targets as well as foils.
l
DRIVING LASER BEAM
BENTCRYSTAL
TARGET
,
I- 0.6 mm FOIL lI TARGET SURFACE
H'
LASER BEAM X-RAY
CAMERA
(a) @)
Fig. 1 . (a) The experimental setup, and
m)
an x-ray pinhole camera photograph showing the plasma formed on the front and the back side of a Cu-Fomvar foil.To calibrate the relative intensities of these EUV spectral lines recorded with the grazing incidence spec- trograph, multiple exposures were taken in situ. The resulting curve of photographic density versus exposure compared reasonably well with the published data of Henke et al. [7] for Kodak type 101 film.
RESULTS
Using targets composed of 1000.4 Cu deposited on a backing of 1200.4 of Formvar, lasing was recorded for the three 3p
-
3s Cu XX lines (from neon-like CuI9+ ions), as indicated in Fig. 2(a). Two of these lines originate on transitions from the 2p:123p312 and the 2 ~ ~ J ~= ~2 levels and one from the 3 p ~ ~ ~ 2p:123p112 J = 0 level, with line emissions at 284.67, 279.31, and 221.11 A (&0.04 A), respectively. The 284.67 A lasing line nearly coincides with the longer wavelength component of the sixth order of the 4f-
3d Cu XIX doublet in sodium-like CU"' ions at 47.442
A.
The net contribution due to the lasing line can be determined by subtracting the intensity of the non-overlapping doublet component, because both lines of the"
CU
J=2-1-
0=279.3.&= 284.7
- 2 -
0 4 8 12 16
PLASMA LENGTH
(mm)
@)
Fig.
-
1 transitions, an? (b) the relative intensity versus length for the neon-like Cu 2. (a) Time-integrated spectrum of Cu foil showing the lasing lines of the Cu XX J XX (J = = 0 2- -
1 and J 1) lines at = 2 279.3A and 284.7A. The gain coefficient for both of these lines is 1.7 cm-'.doublet are approximately equal in intensity when there is negligible amplification of the neon-like CuI9+ ion lines. The intensities of the two J = 2 to 1 lines were found to be approximately equal at all plasma lengths.
Likewise, the overall intensity of the J = 0 to 1 line was found to be comparable to that of the J = 2 to 1 lines.
Exponentiation of the resulting intensities with increasing length is shown in Fig. 2@) for the two J = 2 lines. The solid line in this figure represents a best fit of the gain scaling relation [exp (aL) - 113/' / [OIL (aL)]"' for amplified spontaneous emission [S] through a plasma of length L which extends from 0 to 16 mm, with a combined (for the data points of the two lines) gain coefficient at line center of ct = 1.7
*
0.2cm-'. Similar data for the J = 0 to 1 line gave a gain coefficient at line center of a = 2.0 0.2 cm - l ;
larger by a factor of 1.2X than that for the J = 2 to 1 lines. This is in reasonable agreement with a predicted [4,8] ratio of 1 - 2X.
It is intriguing that comparable gain was obtained by irradiating thick (1.3 pm) copper foils of various lengths. In addition, 3.2-mm-thick solid copper slab targets, also produced the three gain lines, intensities of which are comparable to those measured with thin-foil targets of the same length. Such thick targets have been avoided previously because it was thought that non-uniformities and radial gradients in density and tem- perature could inhibit lasing.
Amplified spontaneous emission (lasing) was recorded for the three 3p-3s Ge XXIII lines (from neon- like Ge2'+ ions) as indicated in Fig. 3(a), where the spectrum for a slab target -3 mm thick and 15 mm long is shown. Two of these lines originate on transitions from 2p:,, 3p3,' and 2p:,23p3,2 J = 2Jevels and :ne from the 2p:,23p,,, J = 0 level with line emis$ons at 236.26
A ,
232.24 A , and 196.06 A (*0.04 A), respectively. Unfortunately, the line at 236.26 A is blended with another spectral line and we did not deter- mine the gain for this line. The other two lines are relatively free of overlapping lines. F u F e r observation showed that two more lines show gain, one at 247.32A (J = 1 to 1) and the other at 286.46A (J = 2 to 1).A plot of the resulting intensities of the Ge XXIII line J = 2 to 1 with increasing target length is shown as a solid line in Fig. 3@). The gain coefficient for this line is a = 4.1 h 0.3 cm-'. Similar data for the Ge XXIII J = 0 to 1 line are plotted in the same figure as a dashed line, where a best fit is obtained with a gain coefficient at line center of a = 3.1 h 0.3 cm-'. For the multiple-shot data obtained at each length, the shot-to-shot variations shown are more significant than relative-intensity uncertainties (approximately
h 10%).
(a)
WAVELENGTH (A)- . -
@) PLASMA LENGTH (mm)
Fig. 3. (a) Time-integrated spectrum of germanium slab target showing the lasing lines of the Ge XXIII J = 0
-
1 and J = 2-
1 transitions, and @) the relative intensity versus plasma length for the neon-like Ge XXIII J = 0-
l line at 196.06A and the J = 2-
1 line at 232.24 A . The gain coefficient derived from the curves is 3.1 cm-' for the J = 0-
1 (dashed) curve and 4.1 cm-'
for the J = 2-
1 (solid) curve.The energy emitted in the Ge and Cu lasing lines was derived from the spectrographic data using film calibration by Henke, et al. [7]. The magnitude of the output from both ends of the longest lasing plasma was estimated to be -3
IJ
for Ge and-
1IJ
for Cu. This energy corresponds to an overall efficiency of 10-9, which is comparable to an LLNL value in Se for similar target lengths. 151. The data plotted in Fig.3@) represent 90% of the shots taken, indicating very good reproducibility in lasing.
Cl-54 JOURNAL
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PHYSIQUESUMMARY
In summary, we have seen soft x-ray lasing in neon-like copper and germanium ions, similar to that pre- viously reported by the LLNL group [3,5,6] in nwn-like selenuim. In our experiment, the laser conditions were quite different, namely, a lower pulse energy (450 J versus 2 H), a longer wavelength (1.05 pm versus 0.53 pm) and a longer duration laser pulse (- 2ns versus 450 ps) with a slower rise time. Although the gain coefficients for Cu were much smaller than those reported by LLNL for Se, the gain coefficients for Ge were approaching those of Se. It will be most interesting to better understand the role these parameters have on lasing in the soft x-ray regime. Also the general question of target design is most important. Certainly, it was surprising that the slab targets would allow the amplification of these lines to occur. Our future goal will be to reduce refraction losses by varying the target design and optimize parameters to achieve the laser out- puts that would make these lasers useful in many applications.
The authors gratefully acknowledge the advice and assistance of their colleagues at NRL. The research was supported by the Ofice of Naval Research, the U.S. Department of Energy, and the Strategic Defense Initiative Office.
REFERENCES
Key, M.H., Nature 316, (1985) 314 and references in that article.
Lee, T.N., McLean, E.A., and Elton, R.C., to be published in Phys. Rev. Len. (1987).
Rosen, M.D., et al., Phys. Rev. Lett. 54, (1985) 106.
Feldman, U., et al., J. de Physique 47, (1986) C6-1.
Matthews, D.L., et al., Phys. Rev. Lett. 54, (1985) 110.
Matthews, D.L., et d., J. de Physique 47, (1986) C6-187.
Henke, B.L., et al., J. Opt. Soc. Am. B1, (1984) 828.
Linford, G.J., et al., Appl. Optics 13, (1974) 379.
