Strong pump energy reduction in collisional X-ray lasers

15 October 1997

OPTICS COMMUNICATIONS ELSEVIER

Optics Communications

142 (1997) 257-261

Strong pump energy reduction in collisional X-ray lasers P.V. Nickles, V.N. Shlyaptsev Mu

‘, M. Schniirer, M. Kalachnikov, W. Sandner

Born Institute Berlin, Rudower Chtss~r Received 6 January

T. Schlegel,

6. 12489 Berlin. Grnncq~

1997; accepted 2 I May 1997

Abstract Lasing in a titanium XII plasma at 32.6 nm and _ 30 nm as well as in vanadium XIII at 30.4 nm using a short pulse driven transient inversion population has been realized. Gain performances and temporal characteristics of the X-ray signal output at only few Joules of pump energy are described. 0 1997 Elsevier Science B.V.

1. Introduction X-ray lasers (XRL) working on neon-like ions are nowadays the most investigated collisional XRL since in 1984 the first X-ray laser was demonstrated in neon-like Se [I ,2]. A dominant part of these activities were directed to enhance the efficiency, that is to reduce the necessary pump energy of these X-ray lasers. This point is of crucial importance, since only a more compact device, including a small sized pump laser, will bring the long-expected breakthrough in XRL laboratory application. Therefore in the meantime different collisional excitation schemes on neon-like ions were proposed and numerous experiments have been carried out for producing X-ray lasing using single, prepulse or multiple pulse techniques (see, for example. Refs. [3,4]) as well as travelling wave schemes [5] and various target structures. One milestone on this way was the introduction of a long prepulse pumping scheme which helped to reduce the necessary pump laser energy from the common level of several hundred Joules (and durations from subns to ns range) to values below 100 J [6]. Difficulties arise, in principle, from the fact that, depending on the lasing atom (transition) and the excitation scheme, pump intensities between 10”-10’8 W/cm’

’ Permanent address: P.N. Lebedev Physical Institute. Leninsky Prospekt 53. Moscow. Russia.

are necessary to create the gain medium. Such intensities, however. can only be delivered from small lasers if their pulse duration is reduced to the ps- and subps-range. Therefore. advanced CPA-lasers, just delivering such pulses. have a large option as XRL-drivers and, indeed, remarkable results from different short pulse small sized X-ray lasers have been published recently [7-l I]. It is interesting that, besides the energy reduction via prepulse technique, already in 1989 a proposal for an efficient transient population inversion in neon-like ions using a long and short pulse pumping [ 121 has been reported. Shortly afterwards the long and short pulse scheme, together with travelling wave excitation, was also mentioned as a way to reduce the pump energy requirements [ 131. Detailed calculations for the case of a 3p-3s, J = O-l Ti XRL and Fe-XRL using this excitation have shown that an effective lasing should be possible with only few Joules of pump energy [ 141. Finally, in 1995 we have reported the first experimental realization of such a neon-like XRL for the case of a 3p-3s lasing in Ti at 32.6 nm [ 15,161. Very recently, recalculations of our Ti-experiment and theoretical estimations using the LASNEX and X-RASER code were published [ 171, confirming our results. The main features of our excitation scheme are the following: A long ns pump pulse irradiates a solid target and creates a plasma with a high partition of neon-like ions and a smooth electron density distribution. The subsequent illumination with the short ps pulse offers the important

0030.3018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO30-4018(97)00288-5

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advantage of a very fast heating of the preformed neon-like plasma. As a result of this fast heating a transient inversion population appears on some transitions (e.g. 3p-3s. J = O-l and others) by electron collisions. Transient situation means here that the duration T,, of the short pump pulse fulfils the relationship rp I T,, T,,,. where r, is the characteristic time for ionisation and r,,, for relaxation of the excited transitions, respectively. Theoretically, the transient gain factors can be one or two orders of magnitude higher than those of the long pulse driven, quasi-stationary gain, which appears at T,, < or < 7,. Additionally, as a result of the limited life time of the transient gain regime the emitted lasing pulse is also very short. Since this transient long-short pulse scheme is also applicable to nickel-like and other ion species it appears as a promising avenue towards table top X-ray lasers. We report here results of our X-ray laser experiments performed with this new two-pulse excitation scheme.

142 (1997) 257-261

3p-3s 30.4 nm

-15

20

25

wavelength 2. Experiment

and results

The experiments were performed at the CPA Ti:SaNd:glass laser facility at the Max-Born-Institute, Berlin [ 161. The laser delivers two synchronized laser beams with different pulse durations. A 1053 nm, 1 ns (FMHW) front-end system, consisting of a Ti:sapphire start oscillator, followed by a stretcher and a regenerative amplifier (Spectra-Physics), is used for the linear glass amplifier chain. Before entering the power amplifier the stretched pulse is split into two beams, then being amplified separately. One pulse is recompressed to 0.7 ps, the second one is kept at 1 ns duration. The corresponding maximum pulse energies were 4 J and 7 J. The two beams are polarized orthogonally and are switched with a polarizer into the same axis in front of the cylindrical focusing optics. The cylindrical double lens focusing optics enables a line focus on the target with a width of 30 pm and an adjustable length of the focal line. Typical lengths of 1 to 5 mm have been used in experiments. The X-ray emission from the plasma was recorded by an on-axis transmission grating spectrograph consisting of a toroidal mirror at grazing incidence and a free-standing diffraction grating (2000 lines/mm, 5 X 5 mm’ aperture). The N&coated toroidal mirror produces a 1: 1 image of the plasma emission on the recording plane. To obtain time-resolved spectrograms an X-ray streak-camera (Kentech) with a CCD system was coupled to the spectrograph. The acceptance angle of the spectrograph was 15 mrad in both directions determined by the aperture of the free standing grating. Home designed photocathodes ( 100 nm CsI on 300 nm Al, supported by a 2000 lines per inch Ni-mesh) for the wavelength range A > 17 nm have been used in the streak-camera. The main advantage of this Al-foil-CsI cathode consists of a high absorption for wavelengths shorter than the L-absorption edge of Al at 17 nm, provid-

30

-

35

(nm)

Fig. 1. Recorded lasing lines obtained with a transmission grating spectrograph and a X-ray streak camera. Line intensity increases upwards and wavelength to the right. Ti-XRL: 3p-3s at 32.6 nm and 3d-3p at 30 nm: V-XRL: 3p-3s at 30.4 nm.

ing a good suppression of second order grating diffraction below 34 nm. Additionally, the relatively sharp edge at 17 nm and the corresponding second order diffraction signal at 34 nm were used to calibrate the detection system. The spectral resolution of the whole recording system was about 0.5 nm. The temporal resolution of the streak camera was I 10 ps. It is worth mentioning that the stability and reliability of the home-made cathode was not satisfying, and its efficiency was unknown. In Fig. 1 spectrally resolved records of the lasing signals of our Ti- and V-XRLs are given. In this case, flat massive titanium or vanadium targets were typically irradiated with a long pulse of 4-6 J and a 0.7-1.0 ps short pulse of 2-3 J. The plasma column was 5 mm long which corresponds to pump intensities on target of about 10” W/cm’ for the long pulse and about lOI W/cm2 for the short one, respectively. In both cases, shown in Fig. 1, the short - 1 ps pump pulse arrives on the preformed plasma 1.3 ns after the start of the long prepulse. The sharp L-absorption edge of the Al foil used in the streak-camera cathode is clearly visible. In the case of titanium two bright and short X-ray signals appear in the spectrum. The brighter one is identified as the 2p5 3p ’ S, (J = 0) 4 2p5 3s ’ P, ( / = 1) transition in Ne-like Ti XIII at A = 32.6 nm, as given in Refs. [18.19]. The vanadium lasing signal corresponds also to the 3p-3s (J = O-1) line at 30.4 nm. This result demonstrates after the proof of principle experiment of our novel scheme in titanium [ I.51for the first time the possibility to scale this 3p-3s excitation scheme to shorter wavelength using targets with a higher atomic

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359

Al -L absorption edge

E

C i G h z 5

-100

short pulse arrival

time,

ps

Fig. 2. Streak camera record of a part of the emission spectrum of the Ti XIII XRL. Both short iasing signals at 32.6 nm and 30 nm appear, when the short pump pulse irradiates the preformed plasma column

number :. The second, but weaker lasing signal in Ti at 30 nm is identified as a 3d-3p, J = l-1 transition [20,21]. Our wavelength calculation for this transition has resulted in A = 29.97 nm. An estimation, using the energy level values from Ref. [22] gives a wavelength of 30.12 nm. The difference is obviously a result of the uncertainty in the experimentally determined wavelength of both resonance lines 3s ‘P,-2p IS,, and 3d ‘P,-2p ‘S,. A more precise experimental determination of the wavelength requires a spectrometer with higher resolution than the detection setup used. Following our modelling [I61 this transition is excited by the same transient collisional process as the more common 3p-3s signal. Note that this differs from recently described results of a long pulse pumped Ar plasma, where the observed 3d-3p transition is assumed to be excited by resonant photopumping [23,24]. Fig. 2 shows a temporal window from the emission scenario of the long-short pulse excited Ti XIII-XRL showing the appearance of the 3p-3s and the 3d-3p Ti-lasing lines. Both signals are emitted immediately after the short pulse arrival. The curvature of the vertical line representing the moment of the short pulse arrival is due to the X-ray streak camera behaviour. Typically, the off axis electrons suffer a certain delay on the acceleration path from the cathode to the screen as compared to the electrons moving closer the axis of the streak tube. Therefore, the curvature has to be taken into account when making spectral scans for this instant. Temporal scans of both lines, given in Fig. 3, show that the lasing signal duration is shorter than 20 ps. This is the shortest pulse duration from a collisional X-ray laser using a solid target reported so far. The result is in good agreement with the calculated life time of the transient high gain period displayed in Fig. 4. showing results of our RADEX modelling [16] for the

local electron temperature distribution and the local gain of the 3p-3s transition in the Ti-plasma versus the distance to the target surface. Calculations were done for two different times, 1: (1 ps after the peak of the short laser pulse) and 2: (IO ps after the entrance of the short pulse) assuming that irradiation conditions met in the experiment. The electron temperature distribution has a well pronounced peak of 2 X lo3 eV near the target surface caused by the short laser pulse. The corresponding gain for the time 1 shows a strong modulation. The gain has a minimum in the critical density region at a distance of _ 20 pm to the target surface, where the temperature and density is high enough to overionize the most part of Ne-like ions. Then the gain is increasing since the heat wave forms a moving front in the ablative material and the gain reaches very high values. Note that the effective gain will be influenced by a strong refraction. Then, IO ps later, the temperature distribution is more extended and, most important, already

1.0

0.8

-.x=323.3

A (3p.3s)

-----h=301.5A(3d-3p) 5 i

0.6 :

-20

A

[Ne] - like Ti

0

time, Fig. 3. Temporal

20

40

ps

scan of 32.6 nm and 30 nm Ti-XRL signals

i0

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P.V. Nickies et al. / Optics Communications

0.0

50

100

150

position, p Fig. 4. Calculated gain and electron temperature versus distance to the target surface for two different moments, 1: 1 ps after the peak of the short pulse heating; 2: 10 ps later.

remarkably cooled down to values of 300-400. Correspondingly, the period of high gain is also strongly reduced to a value comparable to the measured XRL pulse. Investigation of the amplification behaviour of the [Ne]-Ti scheme has shown that no lasing signal was observed at all with a short as well as with a long pulse only. Under the especial experimental condition the pump threshold energy for lasing at 32.6 nm has been determined to be 4 J in the long pulse and about 2 J in the short pulse. In order to verify the amplification we have changed the length of the plasma column between 1 and 5 mm at constant pump intensities of Ilong = 10” W/cm’ and I short= lOI W/cm”. In Fig. 5. showing the lasing signal as function of the length of the plasma column, a nonlinear increase can be seen clearly. From this increase of the 32.6 nm signal a gain factor for the recorded Ti-XRL pulses of g= 19cm-’ was evaluated. For the target length of 5 mm a high gain length product gl = 9.5 was achieved. Our calculations with the RADEX code [ 161 have revealed that the gain saturation can take place at gl = 15.5. A possible way to realize this important value is to increase the gain factor by a proper choice of the electron density distribution or to prolongate the plasma length above 5 mm. However, for the latter case one has to consider that, as above reported, the duration of the 3p-3s Ti XIII-XRL pulse is very short, I 20 ps. and correlates with the calculated lifetime of the high transient gain regime. This short pulse duration corresponds to a propagation length through a plasma column of 5 mm as chosen in the experiment. Therefore, for targets being longer than this value the gain will be reduced. However, a travelling wave scheme for the short pump pulse [5,13], keeping the transient gain condition, should allow a necessary longer amplification path. Corresponding experiments using a 300

142 (1997) 257-261

I/mm-grating inserted in the short pulse pump beam are under consideration. It is worth mentioning that the target geometry and the alignment of the viewing angle of the spectrograph with respect to the XRL-laser beam play an important role. By changing the angle between the target normal and the observation direction we estimated the divergence of the 3p-3s Ti X-ray laser. It was found to be less than the 1.5 mrad acceptance angle of the spectrograph. In the case of the 3p-3s vanadium-XRL, which has a higher threshold, only for 5 mm target length bright signals were measured. Therefore, keeping the intensity values as given above, the present pump laser parameters did not allow for a more detailed study of the signal dependence on target length. Up to now the reliability of the Ti X-ray laser action was not yet satisfying, the signal intensities exhibit a large scatter. We believe that one reason could be the high sensitivity of our system against refraction and plasma inhomogeneities. Calculations have shown that high transient gain of lo-100 cm-’ appears only in a region with small lateral extension of only several tens of microns in front of the target (with highest values up to 10’ cm-’ near the critical desity surface), additionally characterized by a large density gradient [ 16,251. A ray tracing model has revealed that the large electron density gradient in the high gain region with g 2 100 cm-’ restricts the effective X-ray beam propagation in the plasma due to refraction to a length shorter than 3 mm. Density inhomogeneities produced by the unsmoothed intensity distribution of the pump laser will have additionally a large negative influence. In order to improve the amplification reliability one should enlarge the gain region in front of the target and simultaneously reduce the density gradient e.g. by an additional second short pulse preceding the “normal” short pulse [26] or a second low intensity long prepulse. Corresponding investigations are under consideration. The sensitivity of the hydrodynamical conditions on the lasing process was explicitly apparent when irradiating the

12

lasing (h

he

=32.6 nm)

1

x-ray wavelength, nm Fig. 5. Nonlinear growth of the Ti X-ray laser signal at 32.6 nm with the plasma column length of 2 (dotted). 3 (dashed), and 5 mm (solid line).

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Acknowledgements

We gratefully acknowledge the help of J. Nilsen and E. Fill for discussions. This work was partially supported by the European X-ray laser TMR network.

Fig. 6. Enhancement of the lasing signal on the neon-like 3p-3s Ti-X-ray laser at 32.6 nm with the shot number on the same target area at fixed irradiation conditions. Series start already from a

preformed groove, produced by a first shot on the plane target (corresponding intensity value I = l), then from left to right

I. l-4. I shot. same target area several times. As seen in Fig. 6, the lasing signal increases remarkably with the number of shots. In both cases the series were started from an already preformed groove (from one pump event). The difference in the signal values of the series. even if the irradiation conditions are kept constant, reveals however also here the mentioned sensitivity of the lasing to small changes of the plasma conditions. Electron microscopy of the corresponding target location has shown the formation of a groove-like structure on it (after IO shots a groove with a width of 100 urn and a depth of 30 km has been created by the pump laser irradiation on the solid target surface). Nevertheless, we believe this effect should be used for a modification of the electron density (important for refraction and gain) profiles and could result in favourable waveguiding effects due to the modified hydrodynamics.

3. Summary We have demonstrate for the first time that both a 3p-3s titanium XIII- and a vanadium XIV-X-ray laser at 32.6 nm, and 30.4 nm correspondingly, works well using our earlier predicted collisional X-ray laser concept utilizing transient gain. Additionally, in [Ne]-Ti a second short, bright lasing signal at 30 nm was measured and determined to be a 3d-3p, J = I- 1 lasing driven also by a transient inversion population. With only several Joules energy in both pump pulses a very high gain coefficient of 19 cm- ’ and a gain length product of gl = 9.5 were measured for the 3p-3s transition in neon-like titanium. The duration of the titanium lasing pulses were shorter than 20 ps. A further improvement of the system reliability and the signal output should be achievable by a second short pulse or a long prepulse, causing an additional smoothing of the electron density profile as well as by a travelling wave pumping. Finally, the gain performance indicates that the realized scheme might be a very promising way towards saturated short pulse table top X-ray lasers.

References

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