S. Suckewer et al- Recent Progress in Soft X-Ray Laser Development at Princeton

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JOURNAL DE PHYSIQUE,

Colloque C6, supplement au na 10, Tome 47, octobre 1986

R EC EN T P R O G R ES S I N S O F T X-RAY LASER DEVELOPMENT AT PRINCETON

S. SUCKEWER, C.H. SKINNER, D. KIM, E. VALEO, D. VOORHEES and

A. WOUTERS

Plasma Physics Laboratory, Princeton University, princeton,

NJ 08544, U.S.A.

Abstract - Recent advances in research on soft X-ray lasers at Princeton are

described. A one-dimensional code has been constructed which is in good agreement

with the measured radial dependence of soft X-ray gain at 182 A in a magnetically

confined recombining plasma. Multichannel detectors have been installed in the

diagnostic spectrometers and spectra of the line emission in axial and transverse

directions are presented. Initial measurements of the relative divergence and, very

recently, absolute divergence measurements of the axial 182 beam have been made by

scanning the axial spectrometer across the beam.The absolute divergence was measured

to be in the range 5-10 mrad, depending on experimental conditions and the maximum

power of soft X-ray beam was 100 kW. Finally, a new two laser approach to create

gain at wavelengths below 100 A is briefly described.

I. INTRODUCTION

Progress in the development of soft X-ray lasers has been recently reported

by several 1aboratories.l At Princeton, an approach hased on a magnetically

confined recombining plasma column, cooled by radiation losses, has generated

amplification of stimulated emission of a 100 (a one pass gain length of

kll%6.5) .2 In this experiment a commercially available 1 kJ TEA COg laser

(duration 8n nsec, maximum gain was obtained at an energy 300 J) was incident

on a carbon disc target in a strong (90 kG) magnetic field. Rapid recomhination,after the laser pulse, created a population inversion between levels 3 and

2 in hydrogen-like carbon, (cvI). Installation of a soft X-ray mirror3 in

a double pass arrangement provided an additional demonstration of the

amplification of stimulated emission. With a measured normal incidence

reflectivity of 12% at 182 1, a 120% increase in axial stimulated emission

was ob~erved.~n work with axially oriented, thick (35~) arbon fiber targets

coated with a t&in layer of aluminum, gains of up to 6 cm-l were also generated

on the CVI 182 A transition4.

In this article, we will focus on recent progress in the theoretical

interpretation of our earlier results, measuremepts of the relatively small

divergence of the soft X-ray laser beam at 182 A with power %I00 kW, and on

the development of a system (two-laser approach) for the study of conditions

for lasing action significantly below In0 8.

11. COMPARISON OF GAIN MEASUREMENTS WITA A ONE DIMENSIONAL MODEL

A key element in the achievement of relatively large amplification of

stimulated emission was the realization, from measurements of the radial profiles

of the CVI line radiation, that the most favorable conditions for maximum gain

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986603

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http://dx.doi.org/10.1051/jphyscol:1986603

http://dx.doi.org/10.1051/jphyscol:1986603

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JOURNAL DE PHYSIQUE

should exist in the off-axis regions of the plasma column. Gain was measured

hy recording the amplification (enhancement), E, of the axial CVI 182 k emission;

that is the ratio of the 182 stimulated plus spontaneous emission in the

axial direction to the mostly spontaneous 182 tf emission in the transverse

direction. The enhancement is related to the one pass gain, G, by:

By varying the position of the laser focus on the carbon disc target with resDect

to the observation volume of the axial and transverse monochromators it was

possible to measure the gain as a function of radius r in the plasma column.

The results of one such scan were presented earlier.5 Although in that experiment

the C 0 2 laser energy was not optimal, there was a rapid rise of the enhancement

in the region r = 1.5 to 2.5 mm off axis. Further experiments2 with optimal.

plasma conditions led to measurements of an amplification of E %l00 near

r = 1.3 mm.

A one-dimensional hydrodynamic plus atomic physics model has been developed

to aid understanding these result^.^ In this model a single mean flow velocity

was used to describe the ion mass motion. After solution of time-dependent

equations for the ion density, momentum and electron energy, the gain was

calculated by a post processor code from the electron density and temperature

and the number density of the ground state populations of fully stripped and

hydrogen like carbon. Because the laser pulse length is longer than the

compressional Alfven transit time, radial pressure balance is quickly established

in the plasma. Strong heating on the cylindrical axis of symmetry (at the

laser focus) leads to a centrally peaked temperature profile with a corresponding

electron density minimum. On the other hand, off axis, strong radiative cooling

by CIV leads to low temperature, high density conditions conducive to a fast

recombination rate and high 182 gain. With the introduction of an ion

diffusion rate an order of magnitude greater than the classical value, totally

stripped ions were transported from the center to the cold off axis region

where fast recombinati.on generated high gain.

Fig. 1

PLASM A RADIUS (mm)

7;-_r

.zw

I

Radial profiles of CVI 182 % gain, k,,,,, and electron temperature, Te,versus radius in the plasma column as predicted by a lD code.

Figure I shows the predicted gain versus radius and it can be seen that

high gain occurs in a narrow %50-100p wide annulus at a radius of 1.4 mm.

This is in excellent agreement with the experimental results.

OO 0.5 1.0 1.50

2.0 2.5

-5

2 0 0 FaW[L

3

+[L

2- t o o 2

\ Corbon disc 8.\

\

- y~~t Time Of

\ kmox

\-- f' kmox\\

-- \

\ -

r\ Mognetic Fie ld

Radiatio n Cooling 8.

Diffusion -

Diffusion

CoefficientD = 2 . 5 ~ 1 ~ ~ c m ~ / s e c

- '\\

\\

I I -



111. AXIAL AND TRANSVERSE EMISSION SPECTRA

Recently, multichannel detectors, based on microchannel/reticon arrays,

were installed in the axial and transverse soft X-ray spectrometers. These

oermit the recording of emission spectra in the axial and transverse directionsin a single laser shot. One example is shown in Fig. 2. In the transverse

spectrum, the spontaneous CVI 182 A emission is weak compared to the strongest

lineoin the spectrum, OVI 173 L. However in the axial direction, the stimulated

182 A emission dominates the spectrum.

Some exciting results ohtained very recently were made by scanning the~os it ion of the axial spectrometer in the transverse direction (perpendicularto the axis of the plasma column). The axial emissions are imaged by a grazing

incidence mirror onto the entrance slit of the axial spectrometer (see Ref.

2 for experimental arrangement). The mirror is constructed by bending a glass

strip therefore the optical quality of the system is not ideal. Hence a

transverse scan of the axial spectrometer gives information of the relative

divergence of the stimulatedo 182 A emission in comparison to non-lasing lines.

As shown in Fig. 3 the 182 A emission was so intense on axis that the detectorwas saturated whereas the CVI 186 ? and OVI 173 A lines remained weak with

a flat spatial profile.The axial spectra presented in Fig. 2 were ohtained at a position of the

spectrometer corresponding to x = 2 5 0 ~ n Fig. 3. This horizontal positionof spectrometer was established for all earlier measurements2 by alignment

of the system i.e. C02 laser focusing mirror, target slot, and the spectrometerentrance slit using a He-Ne laser beam. However, from Fig. 3 we can see that

maximum gain is near x = 200u with a corresponding amplification of 182 1radiation ahout factor of 5 larger than presented earlier2. The sensitivity

of axial instrument for the data of Fig. 3 was higher than in Fig. 2 in order

to measure intensity changes of the non-lasing lines.

0

Fig. 2 Transverse and axial spectra in the region near 182 A from a carhon

disc target with four carbon blades. The laser energy was 500 J.



JOURNAL DE PHYSIQUE

To obtain information ahout the absolute divergence of the soft X-ray

laser beam, the grazing incidence bent mirror was removed. In its place a

1 mm wide and 10 mm high collimating slit was installed to block reflections

from the walls of the vacuum chamber. Figs. 4 and 5 show the result of a

shot-hy-shot horizontal scan of both the collimating slit and the axial softX-ray spectrometer. The principle of the experiment is presented schematically

on the right-hand side of Fig. 4, and on the left-hand side is shown the

intensity distribution of the CVI 182 1, CV 186 1, and OVI 173 lines for

a magnetic field R = 20 kG. For every shot the intensities of all three lines

were recorded simultaneously on the multichannel detector of the axial soft

X-ray spectrometer. One may see that the lasing line, CVI 182 W , is strongly

peaked on axis with a F W H M % ~ . ~m at distance 304 cm from plasma (target).

This corresponds to a horizontal divergence of the beam ofo%9 mrad. At the

same time the intensities of the non-lasing lines OVI 173 A and CV 186 % are

quite constant over the scan region %3 cm.

With increasing magnetic field (B = 35 kG and 50 kG) we observed further

narrowing of soft X-ray laser beam down to a 5 mrad (Fig. 5). This indicatesthat with a higher magnetic field maximum gain is created in a more narrow

plasma region (less than 50p transversely). Refraction of the 182 emissioni.n the plasma is negligible as the electron density (ne (L l ~ l ~ c m - ~ )s toolow and thus an estimate of the divergence of the stimulated emission in single

pass amplification can be obtained from ray tracing. With an annular width

of 50p as predicted by the one-dimensional code and a plasma length of 1 cm,

the angular range of rays that pass through the gain region is 10 mrad. With

a peak in the gain profile at a particular radius the divergence will be even

less and thus the 1D calculation provide a good understanding of the remarkably

low value of the measured divergence.

We also observe a slight shift of the peak intensity of the 182 1 radiation

at higher magnetic fields, which may be caused by a small tilt of the magnet

at high currents. Another interesting feature is the rise of the 182 8 line

intensity near the geometrical limits of scan indicated by open circles and

triangles in Fig. 5. The limits are determined by diameter of vacuum tubehetween the target and the soft X-ray spectrometer. In order to decrease the

effect of shot-to-shot line intensity fluctuations, the 182 % radiation was

normalized in Fig. 5 to OVI 173 8 line intensity (173 if line intensity was

quite uniform across the scanning distance, as can be seen in Fig. 4). Ourinitial interpretation was that this rising intensity of 182 k radiation near

the edges of the scan was due to diffraction effects on the edges of targetslot. However a more likely explanation may just be the reflection of the

182 1 radiation (grazing-incidence reflection) inside the vacuum tube. In

the future, we plan to reinstall the grazing-incidence mirror and relocatethe axial soft X-ray spectrometer with the entrance slit in the focal plane

of the bent mirror in order to measure the divergence of 182 if radiation inthe far-field.

Knowledge of divergence allowed us to estimate the total power of softX-ray beam 100 kW from measurements of the intensity of the 182 1 radiation

(a1-3 mJ) and pulse duratipn ($10-30 nsec).

IV. TWO LASER APPROACH TO X-RAY LASER DEVELOPMENT BELOW 100 HWe consider the lasing actiogs presently achieved by Livermore and Princeton

in the spectral region 100-200 A, to be the beginning of the rogd toward the

development of lasers in the more important spectral region -10 A. Therefore,

as a next step we are proceeding with a program for the development of soft

X-ray lasers at wavelengths below 100 i. We have begun constructing an

experiment in which a 1.5 kJ CO2 laser generates a highly ionized magnetically

confined plasma column in which a powerful (Id 1016 w/cm2) picosecond laserbeam will produce a population inversion and gain. The role of the C02 laseris to provide access to the high energy, short wavelength transitions of high

Z ions which are then excited by the picosecond laser via multi~hoton rocesses.



Scanning Axia lSpectrometer

EntranceS l i t

S l i t

3 0 4 c m

Plasma

Fig. 3 Transverse scan of CVI

182 1, CV 186 1, and

OVI 173 8 emission

showing a strong central

peak for the CVI 182 2stimulated emission.

In this figure, "Satur.indicates the level

above which the detector

is saturated. Intensi-

ties ahove this level

were obtained hy com-

paring the non-saturatedregion of the 182

spectral profiles.

Fig. 4 Absolute divergence

measurement: (31,9 mrad)

of the 182 A lasing

radiation for a magnetic

field B = 20 kG. For

comparison are shown

intensities of non-

lasing lineg OVI 173 1and CV 186 A, recorded

simultaneously with

182 8 line.

10 1 2 3 4 5

HORIZONTAL DISTANCE X (cm)



JOURNAL DE PHYSIQUE

Fig. 5

Element

Fig. 6 Wavelength of the 4s24p45s2 - 4 ~ 2 4 ~ 5 5 sransition in the KrI iso-

electronic sequence (from Ref. 8) .



The feasibility of exciting either one subvalence electron or two valence

electrons of an ion through multiphoton processes7 was studied for the argonand krypton isoelectronic sequences excited by a powerful picosecond KrF*

(2480 i) 1aserb8 The excitation of two valence electrons (e.g. 4s24p45s2 state)

is especially attractive because of the faster progression to shorter wavelengthswith increasin2 change of the target ion. One potential lasing transition

would be 4 ~ ~ 4 ~s2 + 4 ~ ~ 4 ~ ~ 5 snd Hartee-Fock values of the wavelength scaling

of this transition in the Kr isoelectronic sequence are s h o q in Fig. 6 from

Ref. 8. The potential lasing wavelength for e.g. cd12+ is 89 A. An additionaladvantage with the higher ionization stages is that competing processes such

as photoionization or autoionization are reduced or eliminated. Multiphoton

excitation is also expected to significantly increase the population inversion

and gain at 182 W in the current experiment.

As shown in Fig. 7 , the new experimental system incorporates two lasers;

a 1.5 kJ COq laser to produce the ionized medium and a powerful picosecondlaser to generate the population inversion. The target is placed in a solenoidal

magnet which will radially confine the plasma and produce the long, thin geometry

suitable for laser action. Primary diagnostics will consist of multichannelsoft X-ray spectrometers and an X-ray streak camera.

The powerful picosecond laser is expected to generate a 1 J, Ips, 2480 ilaser pulse. The main oscillator is a YAG-laser pumped dye laser at 6470 1which amplified in a three stage dye amplifier producing an energy output of

a few mJ. This is then drequency doubled and mixed with the 1.06~ YAG-laser

to produce a 1 ps, 2480 A pulse with an energy of a few hundred IJJ. In the

final stage, two KrF* amplifiers increase the pulse energy to the joule level

with an (unfocussed) power in the terawatt range. Focussing by a suitablelens will produce intensities in excess of 1016 w/cm2 and in addition to short

wavelengths laser applications, will produce new insights into the interaction

of radiation with ions in a regime where the laser field is comparable to the

Coulomb field between the electrons and the nucleus.

TWO LASER APPROACH TO X-RAY LAS ER DE VEL OP MEN T# 8 5 X ' 5 q 3

SYSTEM DIAGRAM

Syst em D.A.S. D.A.S.

Acquisition

Fig. 7 Diagram of new experimental setup "Two-Laser Approach" to X-ray laser

development below 100 2.



JOURNAL DE PHYSIQUE

We would like to acknowledge support and encouragement from H. Furth and

J.R. Thompson; assistance with the analysis of the multiphoton excitation scheme

by C.W. Clark, M.G. Littman, T.J. McIlrath, and R. Miles; significantcontributions by C. Keane, L. Meixler, C.H. Nam, J.L. Schwob, T. Srinivasan,

and W. Tighe; and technical assistance from L. Guttadora and J. Robinson.

This work was made possible by financial support by the U.S. Department

of Energy Basic Energy Sciences, Contract No. KC-05-01, and the U.S. Air Force

Office of Scientific Research, Contract No. AFOSR-86-0025.

See e.g. other papers in this Proceeding.

S. Suckewer, C.H. Skinner, H. Milchberg, C. Keane, and D. Voorhees,

Phys. Rev. Lett. 55, 1753 (1985).

T.W. Barbee, Jr., S. Mrowka, and M.C. Hettrick, Appl. Opt. 6,83 (1985).H. Milchberg C.H. Skinner, S. Suckewer, and D. Voorhees, Appl. Phys.

Lett. 47, 1151 (1985).

S. ~uck&er, C.H. Skinner, H. Milchberg, C. Keane, and D, Voorhees, PPPL

Report 2207 (March 1985); also Proc. Int. Conf. "Laser 85", Las Vegas,

NA (~ec. 985).

E.J. Valeo, C. Keane, and R.M. Kulsrud, Bull. Am. Phys. Soc. 30, 1600

(1985); also S. Suckewer et.al.; Proc. Int. Conf. "Laser 85", Las Vegas,

NA (Dec. 1985).

T.S. Luk, H. Pummer, K. Boyer, M. Shahadi, H. Egger, and C.K. Rhodes,

Phys. Rev. Lett., 2, 10 (1983).

C.W. Clark, M.G. Littman, R. Miles, T.J. McIlrath, C.H. Skinner, S.

Suckewer, and E. Valeo, J. Opt. Soc. Am. March 1986.

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S. Suckewer et al- Recent Progress in Soft X-Ray Laser Development at Princeton