Using low and high prepulses to enhance the J=0−1 transition at 19.6 nm in the Ne-like germanium...

1 February 1996

OPTICS COMMUNICATIONS

ELSE?AER Optics Communications 123 (1996) 777-789

Full length article

Using low and high prepulses to enhance the J= O-l transition at 19.6 nm in the Ne-like germanium XUV laser

G.F. Cairns a, C.L.S. Lewis a, M.J. Lamb a, A.G. MacPhee a, D. Neely d, P. Norreys d, M.H. Key bvd, S .B . Healy ‘, P.B. Holden ‘, G.J. Pert ‘, J.A. Plowes ‘, G.J. Tallents e,

A. Demir e a Department of Pure and Applied Physics, Queen’s University of Belfast, Belfast BT7 INN, Ireland

b Clarendon Laboratory, University of Oxford, O.@ora!, OXI 3PlJ. UK

’ Department of Physics, York University, York, YOl5DD, UK ’ Central Loser Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 OQX, UK

e Department of Physics, University of Essex, Colehester, CO4 3SQ, UK

Received 10 April 1995; revised version received 7 August 1995

Abstract

We report a study of the effect of prepulses on XUV lasing of Ne-like germanium for an irradiation geometry where = 20 mm long germanium slab targets were irradiated at = 1.6X 1OL3 W cm-* using = 0.7 ns ( 1.06 p,m) pulses from the VULCAN glass laser. Prepulses were generated at fractional power levels of = 2 X 10e4 (low) and =2 X lo-* (high) and arrived on target 5 and 3.2 ns respectively in advance of the main heating pulse. For both the low and high prepulses the output of the 3p- 3s, .I = O-1, line at 19.6 nm was enhanced such that the peak radiant density (J/St) for this line became greater than that for the normally stronger J= 2-l lines at 23.2 and 23.6 nm. The J=O-1 line, whose FWHM duration was reduced from ~450 ps to = 100 ps, delivered = 6 x more power ( W) than the average for the combined J= 2-l lines, whose FWHM duration was =: 500 ps for both levels of prepulse. The higher prepulse was more effective, yielding = 2 X more radiant density and = 7 X more power on both the .I = 0- 1 and J = 2- 1 transitions compared to the low prepulse case. The most dramatic observation overall was the = 40 x increase of power in the J= O-l line for the high prepulse ( = 2%) case compared with the zero prepulse case. These observations, coupled with measurements of beam divergence and beam deviation through refractive bending, as well as general agreement with modelling, lead us to conclude that, for germanium, the main influence of the prepulse is (a) to increase the energy absorbed from the main pulse, (b) to increase the volume of the gain zone and (c) to relax the plasma density gradients, particularly in the .I = 0- 1 gain zone.

1. Introduction

Collisional X-ray lasers have been made to work for

Ne-like ions spanning atomic numbers Z= 2247 [ l- 81. Typically foil or stripe targets are irradiated with 0.5-l .O ns pulses at 0.53 or 1.06 p,rn laser wavelength. Lasing is observed on several J-J transitions with the J = 0- 1 line (%*2p, &?p$23pl 12 J= 0 t0 2S22pl 122p&2

3sJ= 1) andtheJ=2-1 lines (2s22p:,,2p:,,3p,,2J=2

to 2s22p:,,2p;,, J= 1 and 2S22p,,,2p;,,3p,,, J=2 to

2s22p,,,2p$,3s J= 1) being the main ones of interest. However, it remains one of the outstanding anomalies in Ne-like collisional XUV laser schemes that relatively weak lasing on the J = 0- 1 transitions [ 1 ] is observed compared with that on the J = 2- 1 lines. Modelling consistently suggests that the gain on the J = 0- 1 transition

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778 G.F. Cairns et al. /Optics Communications I23 (1996) 777-789

should be markedly greater than on the J = 2- 1 transitions. In our previous germanium experiments [5,9] the 5=2-l transitions at 23.2 and 23.6 nm have been dominant, with the J= O-l transition at 19.6 nm being relatively weak. For the lower Z elements the J = 0- 1 line tends to be emitted only when a prepulse is applied. A number of possible explanations [ 81 have been proposed, but the most plausible from recent reported results [lo] is related to the adverse effects of refiac- tion due to high electron density gradients. Refraction deflects the lasing signal out of the gain region. Mod- elling predicts that the gain region for the J= 0- 1 line

lies in a higher density region [ 111 and for slab targets this corresponds to a region deeper into the plasma where gradients are steeper. However, in order to reproduce the low gain observed on the J = 0- 1 line in sim-

ulations, the monopole excitation rate into the upper lasing level is reduced to 59% of the value given by the R-matrix calculation [ 11,191.

One proposal to compensate for refraction effects was to use a low energy prepulse delivered early onto target to preform a low temperature plasma and then to reheat it with the main pulse at a suitable time in the

expansion. This in theory, can give a more relaxed expansion and shallower density gradients in the plasma when the gain coefficient is optimised.

Throughout this paper the prepulse level is expressed

as a fraction of the total energy in the main drive pulse arriving on target. We present results from two different experiments using prepulse levels which differed by two orders of magnitude. In one of our experiments the prepulse was 2X 10m4 (0.02%) of the main pulse which we define arbitrarily as “low”; the other experiment had a prepulse that was 0.018 (1.8%) of the main which we define as “high?‘. The purpose of our experiments was to observe the effect of different prepulse intensities when applied to the germanium XUV

laser and whether an enhancement of the J= 0- 1 output intensity could be achieved. This work supports our

overall aim of efficiently producing a bright, coherent and monochromatic beam suitable for application experiments. Preliminary modelling results [ 111 from the hydrodynamics-atomic kinetics code EHYBRID [ 18,191, using prepulses of l-7% delivered 5 ns early onto target, predict that the density gradients are indeed relaxed and the observable gain on the J= O-l and J= 2-l lines significantly improved. This is consistent

with modelling predictions from LASNEX [ lo].

Table 1 Summary of the main experimental parameters used, illustrating some of the differences between the experiments

Parameters Low PP High PP

Prep&e relative intensity (%) - 0.02 - 1.8

Target length (mm) 22kO.5 18f0.5

Target width (pm) lOOk 10 120* 10

Drive pulse duration (ps) 700 f 50 825 f 12.5

Average irradiance 1.850.4 1.6f0.5

( X 1013 Wcm-*)

Delay after prepulse (ns)

Detector

5 3.2

Ilford Q plates Kodak 10 1.04 film

We measured the intensity of the J= 0- 1 and J= 2- 1 lasing lines and their angular distribution relative to the target surface with and without the prepulse using

grazing incidence flatfield spectrometers for these two particular levels of prepulse. This allowed us to observe not only any variation in the output intensity but also to study refraction effects. We also measured the temporal history of the lasing lines with and without the

prepulse to observe any change in the gain kinetics or propagation characteristics within the amplifier. Res-

onance line spectra of the 3s-2p and 3s-2d lines were captured in time integrated and time resolved modes to

allow for diagnosis of the temperatureand density con-

ditions within the plasma. The two investigations were carried out on two dif-

ferent experimental runs at RAL, with a number of differences between them. Table 1 gives a summary of the main parameters used. With the low prepulse,

the profile was monitored with an optical streak camera showing a risetime of -250 ps and a FWHM of 700 f 50 ps. Since the drive pulse was not monitored

optically for the high prepulse case, we used the duration of the measured X-ray laser streaked data to scale between the two experiments. We concluded that the

effective FWHM of the drive laser in the high prepulse case was 82.5 f 125 ps. The diffraction gratings and

reflection filters were also different between the

experiments. Several groups have observed the effects of a pre-

pulse on the J=O-1 output [ 10,12-171 and a summary of the prepulse intensities and main pulses used is given in Table 2. All laboratories report significant enhancement of the J= O-l transition and the table includes estimates of the gains and enhancement factors. For the

Ti XUV laser it was concluded that resonant photo-

G.F. Cairns et al. /Optics Communications 123 (1996) 777-789 119

Table 2

Summary of prep&e details and enhancements from recently reported results where significant enhancement of the J=O-1 line took place. In

the table G denotes a Gaussian shaped pulse and T denotes a trapezoidal pulse with - 250 ps risetime. Also note ‘m’ subscript denotes main

pulse and ‘p’ subscript denotes prepulse. ‘E’ is energy in pulse, ‘I’ is irradiance on target and ‘Delay’ is the delay before the main pulse.

Elern. Driver E,,, (J) FWHM I, ( X 10’3) E,, (J)

A( pm) (PS) (Wcm-‘)

% Ip ( X 10”) Delay ./=0-l J=2-1 Enhan. Ref. (Wcm-‘) (ns) gain gain factor a

(cm-‘) (cm-‘)

LLNL Ti. Cr. 0.53 1100 600G 3.4 6 0.6 19 I - _ Ge-50 [lo]

Fe, Ni Ni>50 [13]

Zn. Ge 1141

Se 0.53 2200 600 G 6.8 6 0.3 19 I 2.5 3.3 -100 I131

cu 0.53 2400 600 G 6 I - I131

RAL Ge 1.06 280 700T 1.8 0.06 0.02 0.36 5 3.3 2.9 -9 ]121 Ni 1.06 280 IOOT 1.8 0.06 0.02 0.36 5 -

Zn 1.06 280 IOOT 1.8 0.06 0.02 0.36 5 -

Ge 1.06 270 700 1.8 5 1.8 32 3.2 3.2 3.6 -1

LULl” Zn I .06 450 600G 1.4 0.095 0.02 0.5 10 4.9 2.6 1151

MPlQ Ti,Fe I.315 -450 450 G 3.0 7 1.5 45 5 - I161 Ni, Cu 70 15 450

Ge, Zn

Zn 1.315 -450 450G 3.0 35-0.41 7.8-0.09 234-2.7 5 4.7 - Zn> 100 1161

1LE’ Ge 1 .os -250 100G 6 - 250 100 6000 0.4 - _ - 12 ]I71

” The nominal enhancement factor here is taken as the peak flux on the J= O-l line with the prepulse compared to that without the ptepulse.

h This experiment is now believed to have had a prepulse train, each separated by 10 ns, preceding the main pulse. Shown in the table is the

energy in the last prepulse before the main pulse.

’ These experiments at Osaka used a double pulse both of the same energy onto curved targets.

pumping was not a major mechanism in its operation as originally proposed [ 31 but rather that refraction out of the gain region was significantly reduced. The results

at RAL [ 121 were the first to show that a significant effect could be produced with a very low prepulse (0.02% ) in comparison to previously reported results

[ 10,131, using typically a 0.5% prepulse level. The LULI team now believe that their observed J = 0- 1

enhancement for Ne-like zinc was due to a low intensity prepulse train preceding the main pulse and previously undetected [ 151. It is interesting to note that the team

at ILE have produced significant J = 0- 1 enhancement in germanium using a double short pulse, with equal

energy in each pulse, but this also involved using curved targets [ 171,

2. Experimental details

The drive energy was supplied using three of the

Vulcan glass laser beams at 1.06 p,rn wavelength and

delivered on target using the standard line focus geometry at RAL [ 201. The line foci generated were typically 120 p,m wide and after appropriate overlapping

permitted uniform irradiation for target lengths up to

22 mm. The targets consisted of m 0.5 Frn thick ger-

manium stripes deposited onto 1.5 mm thick polished

glass substrates. For the low prepulse experiment, the

target stripes were N 100 km wide and up to 22 mm

long; for the high prepulse experiment, the target stripes

were - 120 p.m wide and up to 18 mm long. To allow

for gain estimates 9 mm and 14 mm lengths were also used. The alignment of the target stripe relative to the

main observation axis was set to an accuracy of + 1

mrad. Fig. 1 shows a schematic diagram of the setup used

in both experiments. Included in the inset is the pre-

pulser or “dogleg”, consisting of four mirrors, which

was used to generate the prepulse. It was inserted just before the pre-amplifiers in the Nd-glass laser chain.

The portion of the input beam transmitted through to

form the prepulse could be varied by changing mirrors

3

780 G.F. Cairns et al. /Optics Communications I23 (1996) 777-789

NORTH FLATFIELD SPECTROMETER/STREAK CAMERA

PREPULSER \

SOUTH FLATFIELD SPECTROMETER

Fig. 1. Schematic diagram of the experimental setup for both experiments.

1 and 2. Using the measured reflectivities of the four mirrors the prepulse out of the prepulser stage was estimated in the high prepulse case as m 1.1% of the main pulse. Using the calibrated gain curve for the final amplifier, which takes into account the saturation effects within the amplifier for high signal levels, the final ratio of prepulse to main was calculated at w 1.8%. For the low prepulse case, the prepulse to main pulse ratio after the prepulser was u 0.015% and the calculated ratio of the final output to target was -0.02%: this represents a difference of about two orders of magnitude between the two prepulse intensities. The uncertainties in each of the prepulse values were estimated to be no more than 50%. This covers uncertainties in the saturation effects of the amplifier and reflectivities of the optics in the prepulse generator. The low prepulse was delivered 5 ns early, whilst the high prepulse was delivered 3.2 ns early. Table 1 summarises the irradiation conditions on target.

Two grazing incidence spectrometers were used to examine the soft XUV emission on axis, one acquiring time integrated data, the other incorporating a streak camera with image intensifier to give time resolved data. Both spectrometers utilised aperiodic (1200 l/ mm) flat field gratings [ 211, as well as gold coated reflection filters operating at glancing angles of -7” [ 221 to eliminate higher orders of harder X-rays. The

reflectivities of the gratings used in each experiment were assumed to be similar for the purpose of analysis. The timing slit of the streak camera was set to observe 10 < A < 30 nm emission at 8 f 2 mrad off axis so that it intercepted at or close to the peaks of the lasing lines. The densities on the detecting media, 104 film and Q plates, were converted to photons/Pm’ using appropriate calibrations [ 231. Streak data was recorded on calibrated Ilford HP5 film which allowed the reduced data to be expressed in relative intensity units.

Resonance line emission in the wavelength range 7.0-10.5 8, was monitored on a spatially-resolving, time-integrating Bragg RAP crystal spectrometer which incorporated a CCD detector [ 241. Of particular interest were the 3s-2p F-like band at 9.439-9.553 A and the 3s-2p Ne-like lines at 9.762 and 10.01 A. This spectrometer gave spatial resolution along the length of the target and thus allowed for assessment of uni- formity of plasma conditions [ 91. A second KAP crys-

tal spectrometer coupled to an intensified X-ray streak camera (also with HP5 film recording), monitored the emission from a short length of plasma and was used to time resolve the resonance lines.

3. Results

The results for each experiment are presented sepa- rately in the next two sections and then a comparison is presented in the third section.

3.1. Low prep&e (0.02%)

A significant enhancement of the J = 0- 1 line relative to the J = 2- 1 lines was observed when the prepulse was applied. Fig. 2 shows a profile through a time integrated spectrum near the angular peaks of the lasing signals for a 22 mm target; the dominance of the 19.6 nm line is evident. The J=O-1 line consistently commenced lasing earlier in time compared with the J= 2-l lines, typically N 150 ps as deduced from the measurable signals above background. In both cases, with and without the prepulse, the peak of the J= 2-l lines corresponded closely to the peak of the drive pulse. This was in contrast with the J= 0- 1 line which peaked N 300 ps earlier in time compared to the peaks of the J = 2- 1 lines. Fig. 3 shows a time resolved spectrum where the zero corresponds to the estimated start of the continuum

2 1.6

f 1.2 .C g 0.8 E = 0.4

0

G. F. Cairns et al. /Optics Communications 123 (I 996) 777-789 781

17 19 21 23

-engthlm

Fig. 2. Spectrum obtained of the time integrated photon density near the peak of the angular distribution using the low prepulse ( - 0.02% of

main pulse)

1 Temporal resolution - 1 lops

H 6 19.6 /j

z _ , _. Drive beam profile

0.8 3

0.6 5 I’ ‘P

s 0.4 5

!z 0.2 z

-1 0 0 0.5 1 1.5

Time (ns)

Fig. 3. Time resolved profiles with the low prepulse ( - 0.02%). A

profile of the drive pulse is also included.

emission which we assumed corresponded closely to

the rising edge of the drive pulse: (the start of the

continuum emission was taken at the point where the

signal became measurable above the background noise

of the detector). The J= 0- 1 line had a FWHM much

less than that of the J = 2- 1 lines, ( - 120 ps and - 570

ps respectively). Consequently the intensity of the J = 0- 1 signal had fallen considerably by the time the

peak of the drive pulse arrived. The temporal lasing profiles were symmetrical whether the prepulse was

present or not. In the example illustrated in Fig. 3 the peak power of the J = O-l line was greater than 4 X the

peak power for the J = 2- 1 signal. The flattened profiles of the J = 2- 1 profiles were considered to be principally due to the shape of the drive pulse which had a flattened

profile itself; a typical drive pulse profile is included in

Fig. 3.

Fig. 4 shows angular distribution profiles of the pho-

ton densities on film in the lasing lines relative to the

target surface for the two cases, with (4b) and without

(4a) the prepulse. Comparing 4a and 4b it is evident

that the peak output photon density for the 19.6 nm line

-25 -20 -15 -10 -5 0 5 10

a) Deviation (mrad)

10

8

6

4

2

0 4.

-25 -20 -15 -10 -5 0 5 10

b) Deviation fmradl

Fig. 4. Typical angular profiles of the beam, (a) without any prepulse

and (b) with the low prepulse.

782 G.F. Cairns et al. /Optics Communications 123 (1996) 777-789

0.0 0.5 1.0 1.5 2.0

a) Time (ns)

12 0.4

0.0 0.5 1.0 1.5 2.0

b) Time (ns)

Fig. 5. Time resolved profiles of the Ne-like and F-like resonance

lines. (a) without the prep&e and (b) with the low ( -0.02%) prepulse. Also shown is the F/Ne ratio.

was increased to a level similar to that for the 23.2 and 23.6 nm lines. This is consistent with the integral over

time of the time resolved data (Fig. 3). Note the arbi-

trary units in both graphs are different. On the best 22

mm prepulse shot for J = 0- 1 output, the peak radiant density was enhanced to 380 f 90 p,J/st, similar to the

average of the J= 2- 1 lines which was 390 f 100 p,J/

st. On the same data shot the estimated average output

power on the 19.6 nm line was 0.6 + 0.2 kW,compared

to the combined output power on the 23.2 and 23.6 nm lines of 0.18 + 0.06 kW. The average powers were esti-

mated using the temporal FWHMs of the lasing pulses.

Due to the inherent fluctuations in output from shot to

shot and the variation of h-radiance on target, it was not

obvious whether the low prepulse had any significant

effect on the overall energy output in the J = 2- 1 lines,

though it did effect the pointing of the beam.

Relative enhancement factors due to the prepulses

were estimated from the most consistent data shots

using ratios of the peak radiant densities and powers.

These factors have large uncertainties due to the aver-

aging of data from a limited number of shots which inherently have a large spread in output intensity on a shot to shot bases. The peak radiant density on the 19.6 nm line was enhanced relative to itself by a factor of 11 f 8; the corresponding enhancement factor in the average power out was 50 + 40. The peak radiant density on the J = 0- 1 line compared to the average of that on the J= 2- 1 lines increased from being about one quarter to being about the same. The power on theJ= O- 1 line was enhanced from being about a fifth of the average of the powers on the J= 2- 1 lines to being - 7 X the latter. It is important to note that no account has been taken of small variations in the irradiance on target; it has been shown that the gain increases with increased it-radiance in this regime [ 91. Eventhough the difference in irradiances concerned were small ( u 0.2 X lOI Wcm-’ ) and within the uncertainties quoted for the it-radiance onto target, indications sug- gested that the prepulse shots might have some

enhancement due to this effect. We refer to this later

when comparing the two experiments. The gain coefficients were estimated using the inte-

gral over time of the streaked data for three different

lengths of target. Estimates of the gain coefficient for the 19.6 nm and 23.6 nm lines using the Linford formula [25] were -3.3 cm-’ and -2.9 cm-’ respectively. Fig. 9 shows the corresponding fits, where the

estimated average peak radiant density for 22 mm targets from the time integrated data recorded on film has been used to adjust the scale to appropriate units for comparison with results from the high prepulse experiment. As outlined in section 3.3, the radiant densities

were scaled for gain variation with irradiation to facil- itate a comparison. Values for the gain on these lines

have been measured previously [ 51 as 2.7 f 0.1 cm- ’ and 3.8 f 0.3 cm-’ respectively.

The angular pointing of the peak signal off-axis

tended to be reduced significantly when the prepulse was applied. With the prepulse, the J = 2- 1 lines peaked

at _ 2 mrad off-axis whilst the J=O-1 peaked at -4

mrad. Without any prepulse the peaks for all three lines were at m 9 mrad consistent with previous experiments

[ 5,9]. There were some indications of a decrease in the overall divergence of the J = 0- 1 angular profile when the prepulse was applied; there was no obvious change

for the J = 2- 1 lines. There is a correlation between the temperature of a

plasma and degree of ionisation possible; as the tem-

G. F. Cairns et al. /Optics Communications 123 (1996) 777-789 783

16 18 20 22

Wavelength (am)

24 26

b)

16 18 20 22

Wavelength (nm)

24 26

Fig. 6. Spectral profiles taken near the peaks of the angular distri-

bution of the photon flux from 18 mm targets for the J=O-1 and

J= 2- I lines. (a) without any prepulse and (b) with high prepulse.

perature rises we expect more F-like ions and conse-

quently more F-like emission. We therefore diagnosed the plasma state by using the ratio of the F-like band

(9.4-9.6 A) and the Ne-like lines (9.76 and 10.01 A), in the resonance spectra. Fig. 5 shows typical time resolved profiles for the Ne-like and F-like resonance

lines with and without the prepulse, with the arbitrary units being the same on both graphs. These shots, taken under similar illumination conditions, show that the

temperature was significantly higher with the prepulse as deduced from the F/Ne-like ratio. Modelling pre-

dicts that the F/Ne ratio will increase with the presence

of the prepulse, consistent with the experimental observations. This is due to increased efficiency in energy absorption from the main pulse when the prepulse is

applied.

3.2. High prep&se (1.8%)

When the high prepulse level was used we observed

that the J= O-1 signal at 19.6 nm was dramatically

enhanced compared to the J = 2- 1 lines at 23 nm. Fig. 6 shows two spectral profiles from time integrated data taken near the angular peaks, one with (6b) and the other without (6a) the prepulse for two comparable 18 mm length target shots. These profiles have not been corrected for filtering as is obvious from the Al L edge at _ 17 nm. Fig. 6a and Fig. 6b illustrate clearly the effect of the prepulse. The same arbitrary units for the time integrated photon density are used in Fig. 6a and 6b. In the two most directly comparable shots in this

data set the peak energy output for the J= 2-l (23.2, 23.6 mn) lines was increased by N 20% with the prepulse. Fig. 7 shows typical temporal profiles, without

the prepulse (7a) and with the prepulse (7b), where the same arbitrary units have been used in both graphs; (the lasing on the 23.2 and 23.6 nm lines are very similar to each other and only the 23.6 nm line is shown

here). As for the low prepulse case, lasing was observed to occur earlier in time for the J = 0- 1 transition; the peak of the J= O-l line occurred - 280 ps before the peak of the J= 2- 1 lines, with the J = O-l output falling considerably before the J= 2- 1 lines peaked. The J = 0- 1 line again lased for a much shorter

10 1 Temporal resolution - 75~s

a- t-l - 0.8 3 s

a) 0 0.5 1 1.5

Time (ns)

Temporal resolution - 75~~

H

0 0.5 1 1.5

b) Time (ns)

Fig. 7. Temporal profiles of the 19.6 nm and 23.6 nm lines, (a)

without any prepulse and (b) with the high prepulse.

3

784 C. F. Cairns et al. /Optics Communications 123 (1996) 777-789

-25 -20 -15 -10 -5 0 5 10

4 Deviation (mrad)

-25 -20 -15 -10 -5 0 5 10

W Deviation (mrad)

Fig. 8. Angular time integrated profiles for the photon flux in the J = 0- 1 and J = 2- 1 lines, (a) without and (b) with the high prepulse. A vertical line is used to reference the main axis.

time than the J = 2- 1 lines with a typical FWHM of _ 100 ps compared to a typical FWHM of N 550 ps for the J = 2- 1 lines. The temporal FWHM of the J= 2- 1 lines showed indications of being longer ( N 15%) with the prepulse. Without a prepulse the temporal profiles for all three lines tended to be symmetric with a rise and fall time ( 10% - 90%) of _ 250 ps for the J= 2- 1 lines. With the prepulse the temporal profile was notably different; the risetime for the J = 2- 1 lines was N 220 ps compared to a falltime of N 52Ops, whilst for the J = 0- 1 line the times were _ 50 ps and - 130 ps respectively. Using the commencement of the continuum as an indicator of the start of the drive pulse the lasing for all three transitions were estimated to start * 100 ps earlier in time in comparison to the no prepulse case.

Fig. 8 shows angular profiles of the photon density on film, corrected for filtering, for 18 mm targets with

(8b) and without (8a) the prepulse, where the same arbitrary units are used on both graphs. The peak photon densities for the J = 0- 1 line increased to a similar level as those of the individual J = 2- 1 transitions ( 8b). Estimates of the average output power on the different lines were made using the temporal FWHh4 measurements from the time resolved data. On the best prepulse shot for overall output with an 18 mm target, the peak radiant density for the 19.6 nm line was enhanced to 27 f6 JLJ/st. This was larger than the average peak radiant density for the J= 2- 1 lines which was’20 f 4 p.J/st. The average output power on the 19.6 nm line was estimated as 0.45 f0.30 kW compared with the combined power on theJ= 2- 1 lines of 0.14 f 0.05 kW. Relative enhancement factors were estimated using the average peak signals and powers from the most consistent data. The peak radiant density for the 19.6 nm line relative to itself was enhanced by a factor of 7 f 2, while the power was enhanced by a factor of 60 f 40. The prepulse produced a significantly more divergent beam. In this case the power output on the J= 2- 1 lines was increased by a factor of 2.5 f 1.0. The average of the peak radiant density on the J= O-l line compared to the average of that on the J = 2- 1 lines increased from being about one fifth to being about the same; the corresponding enhancement for the power was from being about a fifth to being almost five times the average power on the J= 2-l lines. Again small variations in output due to small changes in irradiance on target were not taken into account at this stage.

The peak radiant densities from the time integrated data were used to make estimates of the gain coefficient on the 19.6 nm line and the 23.6 nm line using the Linford formula [ 251. Gains for these lines were estimated as 3.2 f0.3 and 3.6f0.3 cm-’ respectively. These are shown in Fig. 9.

Deviation of the lasing emission off-axis for 18 mm targets was consistently at lo&2 mrad for all three lines without the prepulse as expected [ 5,9]. The deviation of the peak signal off-axis for the prepulse case was less reproducible, though the overall trend was towards smaller angles of deviation especially for the J= 2-l lines; the 19.6 nm line peaked at 9 i- 2 mrad, whilst the 5=2-l lines peaked from 5-9 mrad. The overall angular width of the beam (transverse to the target surface) was decreased for the J-O-1 line from u 24 mrad without prepulse to - 16 mrad with prepulse. The angular width for the J= 2-l lines was

G.F. Cairns et al. /Optics Communications 123 (1996) 777-789 785

pulse cases. It was significantly increased with the prepulse as can be seen in Fig. 6.

3.3. Comparison

We have made a comparison between the low prepulse and high prepulse results allowing for the differ-

ent target lengths employed and the gain variation with irradiance on target [ 91. This has been achieved by normalising the peak output observed on the J= 2-l lines without prepulse in each experiment. The average powers were normalised to those for the J = 2- 1 lines without prepulse. Relative enhancement factors again were found in terms of the averages of the J = 0- 1 and the J = 2- 1 signals.

It was assumed that the gain dependence on irradi- ante for the J = 0- 1 line was similar to that for the J = 2- 1 lines as characterised in Ref. [ 91. The output of each

shot was adjusted so that its gain corresponded to that which could be expected at an irradiance of 1.6 X 10L3 Wcm-*. This was achieved by calculating the change in gain ( Ag) for a corresponding change in it-radiance (AZ) using an expression of the form,

Ag= [5.6-6.3 sin(39.31)] AZ,

which was derived from a least squares fit to the data given in Ref. [9], (the units for ‘ ‘Ag" are cm- ‘, for “39.3Z”aredegrees,forZand“AZ”are 10’3Wcm-‘).

The averages of the peak radiant densities and power were then estimated for each experiment. Taking the

extra target length as 0.4 cm and a gain value of 3.0f0.5 cm-’ for both the J=O-1 and 5=2-l lines,

the output for the low prepulse case were further reduced to allow for direct comparisons. Finally the

0 !Am PP

U 1 2 3 a)

1000

s 3 100

2

8 P 10

! d 1

I

x *a PP

v Low PP

-1

3 2 b)

Fig. 9. These graphs illustrate estimates of the gain observed on the

19.6 nm and 23.6 nm lines for (a) the low prepulse and (b) the high

prepulse case.

observed to increase from _ 18 mrad without prepulse

to - 30 mrad with prepulse.

The continuum emission from the time integrated

data was compared between the prepulse and no pre-

Table 3

A summary of the normalised results for the low and high prepulse cases where scaling of gain with h-radiance and different target lengths have

been taken into account. A unit of power, as scaled to an 18 mm long target irradiated at 1.6X lOI Wcm-* with no prepulse corresponds to

0.035 kW. Peak radiant density for the same conditions are in units of 26 cJ/st. The large uncertainties are primarily due the inherent shot to

shot fluctuations in output coupled with a small sample number of shots for each case

Both no PP Low PP High PP Ratio PPlNoPP low Ratio PP/NoPP high Ratio high PP/low PP

Peak J = 0- 1 radiant density 0.2rto.1 0.5 f0.2 0.9f0.3 2.5 4.5 1.8

Peak J = 2- 1 radiant density 1.0 0.4kO.2 0.7 kO.2 0.4 0.7 1.8

Average (O- 1) power 0.2*0.1 1.4iO.8 8f5.5 7.0 40.0 5.7

Average (2- 1) power 1.0 0.2fO.l 1.7f0.8 0.2 1.7 8.5

Rad. Density ratio E(O-l)/E(2-1) 0.2 1.25 1.3

PowerratioP(O-l)IP(2-1) 0.2 7 5


values in each experiment were normalised to those for the J = 2- 1 lines. The normalised results from both experiments are summarised in Table 3.

The results show a significant enhancement of the J = 0- 1 output for high and low levels of prepulse. The J= 0- 1 peak photon energy was increased to a similar level as that on the J= 2- 1 lines. The average power on the J= 0- 1 line was increased to typically six times the average power in the J = 2- 1 lines in both cases. The overall enhancement of the J = 0- 1 line when compared

with itself was larger for the high prepulse case, with an estimated increase in peak energy of 4.5 f: 2.1 and in power of 40 + 34. The output power on the J = 2- 1 lines also increased with the high prepulse. This increase in power for the J = 2- 1 lines was most evident in the larger divergence observed in the output beams.

4. Modelling

Some preliminary modelling results using the EHY-

BRID code [ 18,191 are given here for comparison with the high prepulse experimental observations. A more detailed investigation into the effect of prepulse levels

ranging from -0.02% to lOO%, which includes an

improved equation of state calculation is given in Ref. [ 261. Typical results are illustrated in Fig. 10. These give the density and gain profiles at 500 ps after the start of the main pulse of peak intensity 1 X 1013

Wcm-’ when using a 2% prepulse delivered 5 ns early. Whilst these parameters do not reproduce exactly those in the experiment, for example the delay in the exper-

iment was 3.2 ns, we do believe that the modelling goes a long way in explaining the overall effects of the prepulse. Energy loss due to scattering mechanisms in the

experimental situation have to be taken into account. Hence, the irradiance used here actually corresponds to an experimental it-radiance of - 2 X 1013 Wcm-* [ 181. The model predicts overall enhancement of the absorbed energy into the plasma with the prepulse: the pre-plasma is a much more efficient absorber during the early stages of the main pulse. For example, in the

prepulse plasma - 90% of the energy supplied during the first 100 ps of the main pulse is absorbed, compared with 51% in the single pulse case. Similarly between

100-200 ps of the main pulse 88% of the energy is absorbed with the prepulse compared with 70% without. The overall absorption increases from - 80% to

0.0 0.2 0.4 0.6 0.6 1.0 1.2 1.4 1.6

'10'

6) radius(cm)

W

‘10’

radius(cm)

Fig. 10. Some results from the EHYBRID code at 500 ps after the start of the main pulse showing, (a) the electron density profile and (b) the gain region for the 19.6 nm line when a prepulse is applied. The solid line is for no prepule whilst the dashed line is with a prepulse of 2% delivered 5 ns early onto target at a peak irradiance of 1 X lOI Wcm-‘.

- 90% with the prepulse. This difference is due to the increased inverse Bremsstrahlung heating in the pre-

plasma. The prepulse consequently leads to a larger plasma mass than without the prepulse, i.e. the pre-

plasma mass represents -50% of the final useful plasma mass. The useful plasma mass is defined in this

case as that plasma with an electron temperature greater than 100 eV.

The prepulse tends to cause a relaxation in the pressure gradients within the plasma and thus a reduction

in the expansion rate. Consequently less main pulse


energy is expended in expansion cooling when the prepulse is applied. This is advantageous compared with the single pulse case where expansion is more explosive due to the very high pressures generated early on. As a result the prepulse leads to more efficient coupling into the electron thermal energy.

In general the model shows the electron density gradients to be smaller with the prepulse (lOa), and the

gains ( IOh) to be higher, particularly for the 19.6 nm line. (Although it is worth noting that the interaction between the slow moving plasma formed during the

prepulse and the plasma formed during the main pulse can result in fluctuations in the density profiles which are not apparent in the single pulse simulations.) From 3D raytracing calculations a significant enhancement across the FWHM of the far-field pattern for both the J = 0- 1 and J = 2- 1 lines was apparent. The intensity on the 19.6 nm line increased from - 2% of the 23.6 nm line without the prepulse to -50% of the 23.6 nm

signal with the prepulse. The code predicts little change in the deflected angle which is - 12 mrad with and

without the prepulse. However, it does suggest that the deflected angle will be reduced for shorter length tar-

gets. This is not unreasonable as those rays which expe-

rience the largest gain-lengths for shorter targets can be expected to deflect less in comparison to the corresponding rays in longer targets. Whilst the code suggests lasing slightly earlier in time for the J= O-l line, it does not predict it to occur as significantly early, nor to lase for as short of time, as observed experimentally.

Modelling of very low prepulses ( -0.02%)

requires a more sophisticated equation of state which

is currently being developed. More detailed modelling results will be presented in a future publication [ 261.

5. Discussion

It is perhaps surprising that similar enhancement effects should occur for prepulse levels differing in intensity by two orders of magnitude. The time resolved

profiles are strikingly similar with lasing tending to be

initiated earlier in time by - 100 ps in both cases for

all three lines. The J= O-l line lases earlier in time

relative to the J = 2- 1 lines and for a considerably shorter time for both prepulse levels, with similar esti-

mates for their FWHMs. However, one apparent dif-

ference was the asymmetry in the temporal profiles when the high prepulse level was used.

The time integrated data in both cases shows the photon flux output on the J= O-l line increased to a similar level as that observed on the J= 2-l lines. In terms of average power out, the power on the J = 0- 1 line was enhanced relative to that on the J = 2- 1 lines

to exceed it by approximately a factor of - 6 X in both cases. The results also show that the integrated output energy on the J = 2- 1 lines was larger with the high

prepulse. The increased divergence of the beams for the J = 2- 1 lines with the high prepulse, typically from

18 mrad to 30 mrad, is probably due to the relatively large pre-plasma formed. According to the modelling this leads to increased absorption of energy and con-

sequently to a larger plasmamass. Thus, whilst the peak energy observed may increase relatively little, a significant increase in the overall output is possible.

The high prepulse was more effective in increasing the absolute output on all three lines, with a dramatic increase in the power on the J = 0- 1 line of - 40 X It

has been shown in the case of the zinc laser that significant increases in output can be achieved by increasing the delay of the prepulse from 2 ns to 4.5 ns [ 271. Thus it may be possible to increase the output in the high prepulse case still further by increasing the delay beyond the 3.2 ns value used.

Modelling and experiments show similar trends with the modelling for the high (2%) level giving plausible reasons for the overall effect of that level of prepulse. It is not so clear for the low level case where a much

smaller pre-plasma is expected. However, as the experimental evidence suggests significant enhancement in

the energy absorption in both cases, it would appear that sufficient ionised plasma is produced even with the

ultra-low level to significantly enhance the initial absorption of the drive beam. Achieving the necessary conditions for observing significant gain on the J = O- 1 transition appears to be strongly dependent on the presence or absence of a prepulse; these conditions occur early in the drive pulse and for a significantly shorter time than those for the J = 2- 1 transitions. With

a single pulse it is likely that the high pressure generated within the plasma in the early stages causes a rapid expansion which destroys the necessary conditions for amplification before any significant lasing can occur. Later in time when the plasma is more relaxed the

densities and temperatures will have dropped below

3


those required. To recover the optimum conditions again, with the energy supplied later in the drive pulse, may well prove impossible, as this energy is absorbed over a greater plasma volume. Whilst serving to drive the gain on those transitions which require lower densities and higher temperatures (e.g. the J= 2- 1 transitions) , it would fail to regenerate the higher densities and lower temperatures required for the J = 0- 1 transition. If the prepulse leads to a more relaxed plasma earlier in time then the expansion velocities will be reduced giving a less rapid expansion and thereby producing a higher density region for a significantly longer

time. A corresponding relaxation in the density gradient would reduce refraction effects, and allow for greater amplification in the gain region. This is consistent with the experimentally observed reduction in the deviation

of the beams: it is not clear why this should appear most noticeable for the low prepulse case. The modelling also predicts significant enhancement of the J= 2-l

peak output energy with a prepulse but experimentally we observe little enhancement and at best, is still com-

parable to the output with zero prepulse. Due to the limited number of data shots and the inherent shot to shot fluctuations in output signal, we are unable to conclude whether this discrepancy is real or simply due

to a poor sample.

6. Conclusion

Brightness of the J = 0- 1 lasing line at 19.6 nm has

been considerably enhanced using high ( 1.8%) and low (0.02%) prepulse levels. This shows that the enhancement effect can be produced over a wide range

of prepulse intensities, greater than two orders of magnitude, and that there is likely some optimum level of prepulse and delay for this effect. Preliminary data from

a collaborative experiment to look for optimum conditions using the LULI laser supports this view [ 271.

The J = 0- 1 line lased earlier and for a considerably shorter time compared with the J = 2- 1 lines with both high and low prepulses. In both cases the average output power in the J = 0- 1 line was nominally - 6 X that of the average of the J= 2- 1 lines, and the peak radiant density on the J= O-l line increased to a level similar to that of the J = 2- 1 lines. Compared to the zero prepulse case, the output on the 19.6 nm line was increased

more with the higher level of prepulse. Its peak energy

was estimated to increase by a factor - 4.5 X and its

peak power by a factor of - 40 X . The output power for the J=2-1 lines was also increased with the high level of prepulse. There was - 2 X more radiant density and - 7 X more power for both J= 0- 1 and J= 2- 1 lines with the high prepulse compared with the low prepulse.

The observations suggest that the prepulse leads to (a) enhanced absorption of the drive pulse energy earlier in time, (b) increase in the volume of the gain zone, and (c) a relaxation in the electron density gradients. This produces the necessary conditions for more efficiently accessing gain on the J= 0- 1 line. These observations are consistent with preliminary modelling.

Acknowledgements

The authors gratefully acknowledge the technical and operational support given by the staff of the Central Laser Facility at the Rutherford Appleton Laboratory. This work was supported by the Science and Engi- neering Research Council through a number of research grants.

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Using low and high prepulses to enhance the J=0−1 transition at 19.6 nm in the Ne-like germanium...

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