Professor Greg Tallents, BSc (New England), PhD (ANU), CPhys, MInstP

Final Report: X-RAY LASERS GR/L11809

Contact: Dr Greg Tallents, now at Department of Physics, University of York.gjt5@york.ac.uk

Abstract

The work undertaken on X-ray lasers by the UK X-ray laser consortium has lead to the development of saturated lasers with outputs at wavelengths as short as 5.8 nm, the optimisation of the pumping efficiency of the lasers both with longer driving laser pulses (» 80 ps) and shorter length pulses (» 1 ps), an understanding of the saturation behaviour with and without travelling wave pumping and an understanding of the plasma conditions produced within the X-ray laser medium. During the grant period, X-ray lasser efficiency was optimised by the use of multi-pulse driver laser irradiation. A pre-pulse incident onto a solid target produces a pre-plasma of typically 100 m m scale-length with which the main driving laser pulse interacts. The larger volume of initial plasma results in less refraction of the X-ray laser beam as density gradients are less. The volume of plasma with the correct conditions for gain is also increased. Both effects result in good propagation of the X-ray laser beam along the length and hence efficient production of the X-ray laser beam. In addition, the absorption of the driving optical laser pulse in the pre-formed plasma is greater than with a similar laser pulse interacting with a solid target. The increased efficiency of pumping has enabled X-ray lasing to be extended to shorter wavelengths. The current record shortest wavelength saturated laser output in Ni-like Dy at 5.8 nm was achieved with a two 75 ps pump pulses.

Experiments have also been undertaken with shorter duration pumping pulses (» 1 ps) superimposed on longer pulses. The short pulse pumping operates such that the lasing ion populations are in the transient regime, that is collisional excitation is not in equilibrium with radiative decay. This enhances the efficiency of the short pulse pumping. The consequent gain duration is short (5 Â- 10 ps) and so requires that the pumping along the gain medium is travelling wave. With travelling wave pumping, the excitation and gain moves along the medium at the speed of light and maintains a large value for amplification of the propagating X-ray laser beam. Saturation and the travelling wave pumping have been successfully modelled with a one-dimension ASE laser model developed by the Essex group. The transient gain approach is potentially scalable to pumping with the new generations of table top high power lasers now being developed.

The understanding of the plasma conditions in the X-ray laser medium were facilitated by modelling studies using the York developed hydrodynamic and atomic physics code EHYBRID. The optimum conditions for modelling of Ne-like Ge X-ray laser output were studied using the code and compared to a simple model calculation based on the optimisation of the energy content of the plasma. The EHYBRID code was also used to model resonance line emission for comparison to measured spectra as a check that the code is modelling the ionisation balance correctly. Other recent investigations undertaken at Essex have included a scaling study of Ni-like lasers to shorter wavelength and an investigation of the potential of shorter wavelength X-ray lasers for probing plasmas.

Over the duration of the EPSRC grant period (October 1996 Â- October 1999), six Essex PhD theses were completed on X-ray lasers and 28 papers on X-ray lasers published in refereed journal of which 9 had an Essex principal/first author. In addition, 31 papers were published in refereed conference proceedings.

1. Introduction

UK research on X-ray lasers (XRLs) has been at the forefront of the world effort. Theoretical feasibility studies lead to the experimental development of useful laser output and the current record shortest wavelength saturated laser at 5.8 nm. The programme originates from early work at Hull and Belfast studying recombination laser concepts. Although successfully demonstrated experimentally, this scheme has serious deficiencies and work transferred to the collisional approach about 1990. During the period of the research grant, UK research involved collaboration between the Universities of Essex, Oxford and York, QueenÂ's University Belfast (QUB) and the Central Laser Facility with separate EPSRC grants held by Essex, Oxford, York and QUB.

During the grant period, a method of optimising the efficiency of X-ray lasers has been developed and exploited. Principally, the efficiency has been optimised by the use of multi-pulse driver laser irradiation. A pre-pulse incident onto a solid target produces a pre-plasma of typically 100 m m scale-length with which the main driving laser pulse interacts. The larger volume of initial plasma results in less refraction of the X-ray laser beam as density gradients are less. The volume of plasma with the correct conditions for gain is also increased. Both effects result in good propagation of the X-ray laser beam along the length and hence efficient production of the X-ray laser beam. In addition, the absorption of the driving optical laser pulse in the pre-formed plasma is greater than with a similar laser pulse interacting with a solid target. The increased efficiency of pumping has enabled X-ray lasing to be extended to shorter wavelengths. The current world record shortest wavelength saturated laser output in Ni-like Dy at 5.8 nm was achieved with pumping by two 75 ps pulses.

Experiments have also been undertaken with shorter duration pumping pulses (» 1 ps) superimposed on longer background pulses. The short pulse pumping operates such that the lasing ion populations are in the transient regime, that is collisional excitation is not in equilibrium with radiative decay. This enhances the efficiency of the short pulse pumping. The consequent gain duration is short (5 Â- 10 ps) and so requires that the pumping along the gain medium is travelling wave. With travelling wave (TW) pumping, the excitation and gain moves along the medium at the speed of light and maintains a large value for amplification of the propagating X-ray laser beam. Simulations and modelling show that short duration X-ray laser output is efficiently produced. The transient gain approach is potentially scalable to pumping with the new generations of table top high power lasers now being developed.

2. Optimisation of Ni-like output

The pumping efficiency of X-ray lasing with 75 ps driving laser pulse was studied both experimentally and by simulation. The pulse configuration to optimise the output of Ni-like lasers was investigated with a systematic experimental study of the effect of the pre-pulse to main pulse irradiation ratio (the pulse ratio) and the delay between main pulse and pre-pulse (the pulse delay) on the X-ray laser output of Ni-like Sm (figure 1 - 3). The results of this study enabled the scaling of the Ni-like lasers to be investigated with the result that Ni-like Dy at 5.8 nm was made to lase (figure 4). With Ni-like Dy, there are two approximately equal gain lines present. Interestingly, the lower gain line reduces in output with increasing length as the higher gain line is driven further into saturation.

Figure 1. The measured Ni-like Sm X-ray laser output at 7.3 nm as a function of the ratio of pre-pulse to main pulse irradiance for various pulse delays between the pre-pulse and main pulse.

Figure 2. The optimum ratio R_opt of the pre-pulse to main pulse irradiance ratio for best X-ray laser output as a function of the time interval T between pre-pulse and main pulse. The error bars show the range of R_opt required to produce a factor two variation in X-ray laser output. The line is a best fit curve varying as R_opt ~ T-2.

Figure 3. The variation of Ni-like Sm X-ray laser output at 7.3 nm as a function of the length of the gain medium. The curve is a fit of the calculated ASE output for small gain gain coefficient 9.5 cm^-1.

Figure 4. The variation of Ni-like Dy output at 5.86 nm (higher gain) and 6.37 nm (lower gain) as a function of the length of the gain medium. The curves are fits of the calculated ASE output. The lower gain line decreases with increasing medium length in the saturation regime because of stimulated emission depletion of the common upper quantum state due to the saturation of the higher gain line.

3. Optimisation of CPA pulse pumping

A travelling wave pump has been demonstrated to achieve saturated operation of Ne- and Ni-like X-ray lasers pumped by » 1 ps laser pulses in a transient collisional excitation regime. This was the first unequivocal demonstration of travelling pump-wave operation of both Ne-like and Ni-like lasers and lead to saturated outputs at wavelengths as short as 7.3 nm. In the transient regime, collisional excitation proceeds at a faster rate than radiative decay and the gain is higher. Length scans for Ne-like Ge pumped with 1 ps pulses superimposed onto a 280 ps background pulse are shown in figure 7. The transient gain approach with short pumping laser pulses should scale to pumping with table-top laser systems as the laser energy requirements are considerably reduced.

4. Density well experiments

The far-field and near-field beam intensities of X-ray lasers have been measured using spherically curved multilayer mirrors to image the X-ray laser beam onto CCD detectors (figure 5). It is clear that the beam quality departs significantly from the ideal Gaussian shape. The principal beam distortion is in the direction parallel to the target surface and is caused by refraction effects. In the direction normal to the target, refraction is small as a result of the lower density gradients produced by utilising the pre-pulse irradiation technique. An experiment to reduce the transverse refraction was undertaken. The technique utilised a broadly focused (» 100 m m width) pre-pulse and a sharply focussed (» 25 m m width) main pulse. Simulation studies showed that this pumping would produce a dip in density at the plasma centre with the potential for refraction of the propagating X-ray laser beam towards the plasma centre to produce a one-dimensional waveguide. Experiments showed that the density dip was produced as an X-ray laser beam profile was modified by passing through a short section (2 mm) of plasma with the density dip. However, a copper plasma produced a similar effect with no gain as a germanium plasma with gain, indicating probably that the gain and density dip region did not correspond. Near field images of the X-ray laser output with only the backlighter germanium x-ray laser and with copper and germanium density dip amplifiers are shown in figure 6.

Figure 5. Far field image of a Ne-like Ge x-ray laser output measured experimentally a multilayer mirror imaging system (left) and computed for similar conditions (right) using a ray trace code developed at York for post-processing EHYBRID fluid code predictions.

Figure 6. Near field images of (a) the backlighter Ne-like Ge x-ray laser beam, (b) the x-ray laser beam exiting a 2 mm length of copper plasma and (c) the x-ray laser beam exiting an identically irradiated 2 mm length of germanium plasma.

5. Preliminary non-linear optics studies

Preliminary experimental studies to investigate a non-linear optical process were undertaken. An experiment where four-wave mixing occurs in Na-like argon was studied. The aim was to produce second harmonic X-ray laser output by mixing two X-ray beams and an optical beam in the argon produced using a gas jet. The experimental conditions to produce the Ne-like Ni laser and the Na-like Ar ions were investigated. It was also shown that the X-ray laser could be split and recombined using a Lloyds mirror and spherically curved multilayer mirror. The recombined X-ray beams showed interference fringes indicating a good degree of coherence between the two beams. Evidence for second harmonic X-ray production was not seen, but many of the difficulties involved were studied.

6. Detailed reports of developments by the Essex group

Six PhD students working on X-ray laser topics graduated from the University of Essex in the grant period. The Essex group, now at York, have interpreted x-ray laser output verses length of lasing medium data for both longer double pulse experiments and short pulse, travelling wave (TW) pumping with the extension of a simple ASE model. Measurements of X-ray laser output into saturation (where stimulated emission is a significant de-populating mechanism), in the small signal gain region and with different travelling wave pumping velocities are all well fitted by the model. The model was developed to explain the lasing behaviour of the shortest wavelength Ni-like Dy laser where two approximately equal gain lines are present. The lower gain line is shown experimentally and in the model to decrease in intensity in the saturation regime which is useful for applications where a single lasing wavelength is required.

Other recent investigations undertaken at Essex have included a scaling study of Ni-like lasers to shorter wavelength and an investigation of the potential of shorter wavelength X-ray lasers for probing plasmas.

4.1 The ASE model

A model for calculating homogeneous gain including saturation effects, but allowing amplification in one direction because of the traveling wave pumping can be obtained by integrating the equation of radiative transfer. We have for the relationship between small signal gain G_o and gain G including saturation effects

(1)

where GL is the effective gain length product, I_o is a measure of the spontaneous emission in the amplification direction and I_s is the saturation irradiance. The retarded time t is such that

(2)

where t is time and z is distance along the gain medium. The parameter a (GL) takes account of wavelength integration of the laser intensity over an assumed Gaussian gain profile and can be calculated using the Linford approximation

(3)

If the wave front of the pumping pulse travels along the target with a speed faster than the speed of the amplified X-ray laser beam (ie. light speed c), the local gain G_o sampled will vary with time. Results assuming the small signal gain G_o increases as a step function and then decreases exponentially with time and hence distance (depending on the traveling wave velocity are shown on figure 1. It is clear that small variations of gain duration can produce large changes in the saturated output of the X-ray laser when only the intrinsic travelling wave pumping is present.

Figure 7. The experimental Ne-like Ge X-ray laser output at 19.6 nm as a function of target length with a traveling wave c and with only the intrinsic traveling wave at velocity 2.9c. Model fits are shown as continuous curves for travelling wave velocity c and for 2.9c with different assumed durations of gain (as labelled).

4.2 Computation of Ne-like Ge X-ray laser output

A study to optimise the output of Ne-like Ge x-ray lasing with different delays between the pre-pulse and main pulse and different ratios of the irradiances of the pre-pulse and main pulse was undertaken assuming 75 ps duration pulses. The simulations show that the peak X-ray laser output is obtained with pulse delay between pre-pulse and main pulse of 2 ns and a pulse ratio of 0.1. A broadly similar behaviour is obtained experimentally for Ni-like Sm (figure 1). The relationship between the peak x-ray laser output and the pulse ratio and delay can be explained in terms of a simple model based on the idea that the maximum x-ray laser output is produced when the energy content of the plasma is at an optimum value. The energy content is approximated by the ratio of the peak electron temperature to the volume of plasma produced by the pre-pulse irradiation. The best fit on figure 2 is a prediction of the simple model.

Publications over the Grant Period

Refereed Journals

1. G F Cairns et al [13 authors includ. G J Tallents, and A Demir] 1996 Optics Commun. 123, 777-89. "Using low and high prepulses to enhance the J = 0-1 transition at 19.6 nm in the Ne-like germanium XUV laser ".

2. M H Key et al [29 authors including L Dwivedi, M Holden and G J Tallents] 1996 Opt Quant Elect. 28, 201-8. "Development of XUV lasers at the RAL Central Laser Facility".

3. J Zhang et al [16 authors including G J Tallents, L Dwivedi and M Holden] 1996 Lasers and Particle Beams 14, 71-9. "Characteristics of rapidly recombining plasmas suitable for high-gain X-ray laser actionÂ'.

4. P Zeitoun et al [15 authors including P Zeitoun, A Demir, M Holden, J Krishnan and G Tallents] 1996 J. Electron Spect. 80, 255-8. "Reduction of X-ray laser driver energy".

5. J Zhang et al [15 authors including M Holden, G J Tallents, A Demir and P Zeitoun] 1996 Optics Lett 21, 1035-7. "High-gain X-ray lasing at 11.1 nm in sodiumlike copper driven by a 20 J, 2 ps Nd:glass laser".

6. C G Smith et al [10 authors including L Dwivedi, J Krishnan and G Tallents] 1996 Optics Commun. 130, 69-74. "Study of beam aberrations in a germanium XXIII XUV laser amplifier".

7. K Nazir, S J Rose, A Djaoui, G J Tallents, M G Holden, P A Norreys, P Fews, J Zhang, F Failles 1996 Appl. Phys. Lett. 69, 3686-8. "X-ray spectroscopic studies of hot, dense iron plasma formed by subpicosecond high intensity KrF laser irradiation".

8. J Zhang et al [18 authors including A Demir, J Lin, R Smith and G J Tallents] 1996 Phys. Rev. 54, R4653-6. "Saturated output of a Ge XXIII X-ray laser at 19.6 nm".

9. A G MacPhee et al [15 authors including A Demir, M Holden, J Krishnan and G J Tallents] 1997 Optics Commun. 133, 525-33. "The influence of prepulse level on the 3p-3s XUV laser output from Ne-like ions of Zn, Cu and Ni".

10. A Behjat et al [13 authors including A Behjat, J Lin, G J Tallents, A Demir and M Kurkcuoglu] 1997 Optics Commun. 135, 49-54. "The effects of multi-pulse irradiation on X-ray laser media".

11. A Demir et al [10 authors including A Demir, P Zeitoun, G J Tallents] 1997 Phys Rev E 35, 1827-35. "A detailed comparison of experimental and theoretical helium-like Ti and Ca satellite line spectra emitted from a laser-produced plasma".

12. D H Kalantar et [20 authors including A Demir, J Lin, R Smith and G J Tallents] 1997 Rev Sci Instrum 68, 802-5. Â'Extreme ultraviolet probing of laser imprint in a thin foil using an x-ray laser backlighterÂ'.

13. D H Kalantar et al (21 authors including A Demir, J Lin, R Smith and G J Tallents] 1997 Phys Plasmas 4, 1985-93. Â'Measurements of direct drive laser imprint in thin foils by radiography using an X-ray laser backlighterÂ'.

14. J Zhang et al [15 authors including J Lin, R Smith and G J Tallents] 1997 Phys. Rev Lett 78, 3856-9. Â'Demonstration of saturation in a Ni-like Ag X-ray laser at 14 nmÂ'.

15. J Zhang et al [13 authors including J Lin, R Smith and G J Tallents] 1997 Science 27
6, 1097-1100. Â'A saturated X-ray laser beam at 7 nanometersÂ'.

16. B. Rus, P. Zeitoun, T. Mocek, S. Sebban, M. Kalal, A. Demir, G. Jamelot, A. Klisnick, B. Kravilkova, J. Skala, G.J. Tallents 1997 Phys. Rev. A, Vol. 56, 4229-4241, Â'Investigation of Zn and Cu prepulse plasmas relevant to collisional excitation x-ray lasersÂ'

17. J Nilsen et al [16 authors including J Lin, R Smith and G J Tallents] 1997 Phys. Rev A56, 3161-5. Â'Near-field spatial imaging of a Ni-like Ag140 Å x-ray laserÂ'.

18. A Behjat, G J Tallents and D Neely 1997 J Phys D: Appl. Phys. 30, 2872-9. Â'The characterization of a high-density gas jetÂ'.

19. P J Warwick, C L S Lewis, S McCabe, A G MacPhee, A Behjat, M Kurkcuoglu, G J Tallents, D Neely, E Wolfrum, S B Healy and G J Pert 1997 Optics Commun. 144, 192-7. Â'A study to optimise the temporal drive pulse structure for efficient XUV lasing on the J = 0 - 1, 19.6 nm line of GeXXIIIÂ'.

20. J Zhang, A G MacPhee, J Lin, E Wolfrum, R Smith, C Danson, M H Key, C L S Lewis, D Neely, J Nilsen, G J Pert, G J Tallents, J S Wark and P J Warwick 1997 Phys. Lett. A234, 410-4. Â'Optimization of drive pulse configuration for a Ni-like Sn x-ray laser at 12 nmÂ'.

21. E Wolfrum, J Wark, J Zhang, D Kalantar, M H Key, B A Remington, S V Weber, D Neely, S Rose, J Warwick, A MacPhee, C L S Lewis, A Demir, J Lin, R Smith and G J Tallents 1998 Phys. Plasmas 5, 227-33. Â'Measurement of of single mode imprint in laser ablative drive of a thin Al foil by extreme ultraviolet laser radiographyÂ'.

22. J Y Lin, G J Tallents, A Demir, S B Healy and G J Pert 1998 J. Appl. Phys. 83, 1863-8. Â'Optimization of double drive pulse pumping in Ne-like Ge x-ray lasersÂ'.

23. P J Warwick, C L S Lewis, M P Kalachnikov, P V Nickles, M Schnurer, A Behjat, A Demir, G J Tallents, D Neely, E Wolfrum, J Zhang and G J Pert 1998 J Opt. Soc. Am. 15, 1808-14. Â'Observation of high transient gain in the germanium x-ray laser at 19.6 nmÂ'.

24. J Y Lin, G J Tallents, J Zhang, A G MacPhee, C L S Lewis, D Neely, J Nilsen, G J Pert, R M N OÂ'Rourke, R Smith and E Wolfrum 1998 Optics Commun. 158, 55-60. Â'Gain saturation of the Ni-like lasersÂ'.

25. S Sebban, F Albert, A Belsky, M Boussoukaya, A Carillon, S Hubert, P Jaegle, G Jamelot, D Joyeux, I Kamenskikh, A Klisnick, C L S Lewis, D Phalippou, A G McPhee, D Ros, B Rus, R Smith, G J Tallents, P Zeitoun, A Zeitoun-Fakiris 1998 Ann. de Physique 23, 81-88. Â'Collisional X-ray-UV lasers and their applicationsÂ'.

26. R Smith, G J Tallents, J Zhang, G Eker, S McCabe, G J Pert and E Wolfrum 1999 Phys. Rev. A 59, R47-50. Â'Saturation behaviour of two x-ray lasing transitions in Ni-like DyÂ'.

27. J Y Lin, G J Tallents, A G MacPhee, R Smith, E Wolfrum, J Zhang, G Eker, C L S Lewis, D Neely, R M N OÂ'Rourke, G J Pert, J Pestehe and J S Wark 1999 J. Appl. Phys. 85, 672-5. Â'Optimisation of double pulse pumping for Ni-like Sm x-ray lasersÂ'.

28. J Y Lin, G J Tallents, A G MacPhee, A Demir, C L S Lewis, R M N OÂ'Rourke, G J Pert, D Ros and P Zeitoun 1999 Optics Comm. 166, 211-8. Â'Travelling wave CPA pulse pumping for collisional excitation x-ray lasersÂ'.

Refereed Conference Proceedings

1. A G MacPhee et al [17 authors including A Behjat, M Kurkcuoglu and G J Tallents] 1996 X-ray lasers 1996 (IOP: Bristol) pp 250-4. "Improving the efficiency of collisionally excited X-ray lasers using multiple 100 ps pulses".

2. P J Warwick et al [17 authors including A Demir, J Lin, R Smith and G J Tallents] 1996 X-ray lasers 1996 (IOP: Bristol) pp 509-11. "Radiography measurements of direct drive imprint in thin Al foils using a bright XUV laser".

3. A Behjat et al [13 authors including A Behjat, J Lin, G J Tallents, A Demir and M Kurkcuoglu] 1996 X-ray lasers 1996 (IOP: Bristol) pp 247-9 "The effects of multi-pulse irradiation on X-ray laser media".

4. A Demir et al [10 authors including A Demir, G J Tallents, P Zeitoun] 1996 X-ray lasers 1996 (IOP: Bristol) pp 383-6 "Spectroscopic analysis of Li-like Ti and Ca X-ray laser media".

5. B Rus et al [9 authors including A Demir, P Zeitoun and G J Tallents] 1996 X-ray lasers 1996 (IOP: Bristol) pp 394-8. "Investigation of prepulse plasmas for collisional excitation X-ray lasers".

6. T Mocek et al [8 authors including A Demir, P Zeitoun and G J Tallents] 1996 X-ray lasers 1996 (IOP: Bristol) pp 399-401. "Spectroscopy of Zn and Cu line plasmas generated on slab target by irradiances 10¹⁰-10¹¹ Wcm^-2 ".

7. M H Key et al [47 authors including A Demir, M Holden, J Lin, G J Tallents] 1996 X-ray lasers 1996 (IOP: Bristol) pp9-16. "Development and application of ultra-bright laser and harmonic XUV sources".

8. A Klisnick et al [18 authors including G J Tallents, A Demir, M Holden and J Krishnan] 1996 X-ray lasers 1996 (IOP: Bristol) pp47-9. "Progress in the control and optimization of the 21.2 nm Zinc laser".

9. J Zhang et al [25 authors including A Demir, M Holden and G J Tallents] 1996 X-ray lasers 1996 (IOP: Bristol) pp122-9. "High gain recombination XUV lasers and efficient XUV harmonics from ps laser pulse interaction with solid targets".

10. G J Tallents et al [23 authors including G J Tallents, A Behjat, A Demir, M G Holden, J Krishnan, J Y Lin and P Zeitoun] 1996 X-ray lasers 1996 (IOP: Bristol) pp 372-79. "Spectroscopic investigations of X-ray laser media".

11. B. Rus, P.B. Holden, T.Mocek, M. Kalal, P Zeitoun, A. Demir, B. Krvilkova, S. Sebban, J. Skala, G. Tallents 1996 NATO ASI Series 3, Vol. 7 (High Power Lasers - Science and Technology), pp 199-216, (Kluwer Ac. Publishers, Dordrecht) Â'A high efficiency soft X-ray laser in the 25-30 nm spectral regionÂ'.

12. B. Rus, P.B. Hoden, T. Mocek, M. Kalal, P.Zeitoun, S. Sebban, A.Demir, G. Jamelot, B. Kravilkova, J. Krasa, L. Pina, J.Skala, G. Tallents 1996 Proc. SPIE 2767 (Iodine Lasers and Applications), 125-132, Â'Design of efficient soft X-ray laser using neonlike Fe driven by iodine laserÂ'.

13. M. Kalal, B. Rus, P. Zeitoun, T. Mocek, S. Sebban, A. Demir, B. Kravilkova, J. Skala, G. Jamelot, G. Tallents 1997 Advances in Laser Interaction and Inertial Fusion, G. Velarde, J.M. Martinez-Val, E. Minguez, J.M. Perlado, Eds., (World Publishing Co., Singapore), pp 487-490, Â'Studies of Phenomena Involved in the Prepulse Pumping of Collisional X-ray Lasers by Optical InterferometryÂ'.

14. G J Tallents et al [19 authors including G J Tallents, J Y Lin, A Demir, A Behjat, R Smith] 1997 Proc. SPIE 3156, 30-41. Â'X-ray laser enhancement with multi-pulsingÂ'.

15. T. Mocek, B. Rus, M. Kalal, P. Zeitoun, A. Demir, G. Jamelot, B. Kravilkova, J. Krasa, L. Pina, S. Sebban, J.Skala, G. Tallents 1996 Proc. SPIE 2767 (Iodine Lasers and Applications), 133-141, Â'Experimental investigation of line plasmas created by intensities 109 - 1011 Wcm-2Â'.

16. G J Tallents, J Krishnan, L Dwivedi., D Neely and I C E Turcu 1997 Proc. SPIE 3157, 281-90. Â'Film calibration for soft X-ray wavelengthsÂ'.

17. J Zhang et al [14 authors including A Demir, J Lin, R Smith and G J Tallents] 1997 SPIE 3156, 53-64. Â'Recent progress in nickel-like x-ray lasers at RALÂ'.

18. P V Nickles et al [14 authors including A Behjat, A Demir, G J Tallents] 1997 Proc. SPIE 3156, 80-85. Â'Transient inversion XUV lasers in Ti and GeÂ'.

19. S Sebban et al [12 authors including R Smith and G J Tallents] ] 1997 Proc. SPIE 3156, 11-20. Â'Large gain in Ne-like iron and Ni-like tin and silver plasmas produced by muliple-laser-pulse irradiationÂ'.

20. J S Wark et al [26 authors including G J Tallents, L Dwivedi and M Holden] 1997 Proc. Intern Conf on Atomic Physics (Amsterdam) in press. "Coherent ultra-bright XUV lasers and harmonics".

21. C L S Lewis et al [21 authors including G J Tallents, G Eker, J Y Lin, S J Pestehe, R Smith] 1998 X-ray lasers 1998 (IOP: Bristol) pp1-8. Â'Overview of x-ray laser research in the UKÂ'.

22. G J Tallents et al [14 authors including G J Tallents, J Y Lin, R Smith, S J Pestehe] 1998 X-ray lasers 1998 (IOP: Bristol) pp59-66. Â'Characterisation of x-ray lasers at short wavelengthÂ'.

23. J Zhang et al [11 authors including R Smith, GJ Tallents] 1999 X-ray lasers 1998 (IOP: Bristol) pp67-70. Â'Experiments of the saturated Ni-like x-ray lasers driven by a double 75 ps laser pulseÂ'.

24. R Smith, G J Tallents, J Zhang, G Eker, S McCabe, G J Pert and E Wolfrum 1999 X-ray lasers 1998 (IOP: Bristol) pp71-74. Â'Saturation behaviour in Ni-like DyÂ'.

25. A G MacPhee et al [13 authors including J Y lin, A Demir, G J Tallents] 1999 X-ray lasers 1998 (IOP: Bristol) pp75-78. Â'Transient gain in tavelling wave pumped Ni-like x-ray lasersÂ'.

26. J Y Lin et al [12 authors including J Y Lin, G J Tallents, A Demir, R Smith] 1998 X-ray lasers 1998 (IOP: Bristol) pp79-82. Â'Gain saturation and travelling wave effects for collisional excitation x-ray lasersÂ'.

27. P Zeitoun et al [16 authors including R Smith and G J Tallents] 1998 X-ray lasers 1998 (IOP: Bristol) pp115-118. Â'Obtention (sic) of high gains in the 3s-3p J = 0-1 transition of neon0like iron by an improved control of the ionization balanceÂ'.

27. A Demir, G J Tallents, Y Bektöre, C L S Lewis, J Lin, A MacPhee and G J Pert 1998 X-ray lasers 1998 (IOP: Bristol) pp467-470. Â'Spectroscopic studied of the Ne-like and F-like Ge resonance line spectra emitted from x-ray laser mediaÂ'.

28. J Zhang et al [11 authors including R Smith and G J Tallents] 1998 X-ray lasers 1998 (IOP: Bristol) pp471-474. Â'KeV spectroscopic diagnostic for the performance of Ni-like x-ray lasersÂ'.

29. E Wolfrum et al [17 authors including G Eker, J Lin, R Smith and G J Tallents] 1998 X-ray lasers 1998 (IOP: Bristol) pp657-654. Â'X-ray laser radiography of hydrodynamic perturbations due to laser imprintÂ'.

30. G J Tallents et al 1999 Proc. SPIE 3776, 75-82. Â'Saturated X-ray lasersÂ'.

31. C L S Lewis et [including G J Tallents, S Dobosz, S J Pestehe, F Strati] 1999 Proc. SPIE 3776, 282-291. Â'Progress with saturated soft X-ray lasers pumped by the Vulcan laserÂ'.

Department of Physics