Grazing Incidence Pumping and Short Wavelength Xray Lasers

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Grazing Incidence Pumping and Short Wavelength
X-ray Lasers

• Nicola Booth
• University of York

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Acknowledgements

• MH Edwards, Z Zhai, GJ Tallents
• University of York
• T Dzelzainis, R Ferrari, CLS Lewis
• Queens University, Belfast
• A Behjat
• Yazd University, Iran
• Q Dong, S Wang
• Institute of Physics, Beijing
• D Neely, R Clarke, P Foster, M Streeter
• CLF, Rutherford Appleton Laboratory

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Contents

• Introduction to Grazing Incidence Pumping
• Experimental arrangement
• Results
• X-ray lasing at shorter wavelengths
  experimental results
• Conclusions

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Introduction

• X-ray Lasers first demonstrated in 1984 with
  saturated lasing from 1994
• Continual process of improvement and development
• Slab targets, multi-pulse pumping, CPA pumping,
  which required few 10s J pumping
• Most recent advance Grazing Incidence Pumping
  now requires lt1J pumping

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Pumping Schemes
Traditional pumping scheme Both the pre- and main
pulses at normal incidence to the target with
100J in the main pulse
Short/main pulse incident at either 14o or 20o
focussed to a line width 50µm
Line Focus
Grazing Incidence scheme Pre-pulse normally
incident, whilst main pulse incident at an angle
(14o-23o) 500mJ Pre-pulse 250mJ Main Pulse
X-ray Laser
Long/pre- pulse focussed to a line width 50µm
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Grazing Incidence Pumping
Absorption approaching nc, optimum to create
population inversion
q
Target
Decreasing electron density in the pre-plasma
X-ray laser
ntnccos2q
Main (second) laser pulse
Energies, angle and timing are tuned to promote
absorption at (or close to) the turning point of
the pumping beam
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Experiment
Parabolic Mirror
Short Pulse 240mJ, 240fs
Crystal Spectrometer
Cylindrical and Spherical Lenses
Target
Long Pulse 600mJ, 500ps
X-ray Laser
Crossed Slit Camera
Flat Field Spectrometer / Streak Camera

• X-ray laser output parameters as a function of
• Target length
• Pre-pulse to main pulse time separation
• Grazing angle (changing angle impacts on line
  focus length and position)
• Pump energy

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Dual-order Flat-field spectrometer
CCD
A second order flat-field spectrometer was
employed in this experiment utilizing a flat gold
mirror which was angled to direct the second
orders of the two lasing lines to the second CCD.
Au Mirror at 52o
2nd Order
CCD/ Streak Camera
1st Order
X-ray laser
Grating
Calibration of first-order/second order XRL
output energies. Enables energy data to be
recorded when streak camera in use Ratio 0.03126
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Flat-field Spectrometer
The flat-field spectrometer was employed to image
the X-ray laser onto both first and second order
Andor CCDs and also in the second half of the
experiment onto the streak camera coupled to
the back of the first order flat-field.
4d-4p
Molybdenum 4d-4p lasing line at 18.9nm clearly
visible Fainter 4f-4d 22.6nm second lasing line
also visible
4f-4d
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Crossed-slit Camera Images
Both Pulses
2.1mm
Short Pulse only
2.1mm
7mm
The image of the short pulse shows that it is
possible to use a parabolic mirror at grazing
angles to produce a good line focus
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X-ray Laser Output
X-ray laser output as a function of target length
and parabola angle
14o Parabola Angle gives a value of g0 of 85cm-1
17o Parabola Angle gives a value of g0 of 59cm-1
20o Parabola Angle gives a value of g0 of 80cm-1
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Linford/Pert Fitting Procedure
Linford Formula
Pert Approximation
Equation for Intensity as a function of length
with saturation
I0 is the value for intensity of spontaneous
emission Is is the value of the saturated
emission intensity
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Fitting to GRIP data
The accuracy of fitting this formula to the data
has been investigated Our GRIP data does not
contain information at lower target lengths Makes
it difficult to assess the onset of saturation
Graphs a. and b. contain the same data for the
17o grazing angle, but fit to different curves
b.
a.
a. g037cm-1, R10-3
b. g080cm-1, R10-5
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Effect of Long to Short Pulse Delay Variation
Comparing the delay scans for all parabola
angles, for similar target lengths, shows that
the optimum parabola angle for these conditions
is 17o
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Time Resolved Measurements
Axis-Photonique Streak Camera used to measure the
temporal output of the XRL
Parabola Angle 17o LP-SP separation 673ps
4d-4p 18.9nm lasing line
3.0ps
18ps
4f-4d 22.6nm lasing line
3.3ps
Both images show the 18.9nm lasing line
however, in the second image this line is
saturated, stretching the pulse. This image also
shows the second lasing line at 22.6nm, which is
not saturated.
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Streak Camera Calibration
Fleischman et al., Nuclear Fusion 5 (1965) show
?T I1/2
A weakest signal detectable B the onset of
tube saturation Dynamic range IB / IA 6.4
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Short Wavelength X-ray Lasing

• Recent experiment carried out on Vulcan TAW, CLF
• 2 initial pulses with up to 120J, frequency
  doubled to 500nm
• Grazing Incidence no help in higher Z
• Frequency doubling beneficial as it enables
  second, CPA pulse to penetrate target to a higher
  density
• When CPA pulse arrives it interacts where the
  density gradients are shallower but the
  temperature hotter

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Ni-like Sm X-ray Lasing
Strong lasing observed in both the first and
second orders for the transitions at 7.3nm and
6.9nm
3/2 3/2
3/2 1/2
3/2 3/2
3/2 1/2
3/2 3/2
3/2 1/2
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Higher Z Lasing

• Attempts were made at lasing in higher Z targets,
  including Gd, Dy and Ta
• Gadolinium observed to lase weakly
• No lasing observed in Dysprosium or Tantalum
• Further investigation is required to solve the
  problem

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Conclusions - GRIP

• Results indicate that low laser pulse energy
  (lt1J) can pump saturated lasing at EUV
  wavelengths
• It has been shown that a Parabola Angle of 17o is
  optimum, whilst all angles have been shown to
  lase into saturation
• A pulse duration of 3ps was observed
• Develop high-rep rate x-ray lasers
• Cheap/small scale experiments should be possible

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Conclusions Short Wavelength

• EUV lasing seen in Samarium
• Weak lasing observed with Gadolinium
• Using the high VULCAN laser energy did not
  produce lasing lt6nm
• Further investigation required