An X-ray photoswitched XeC1 laser is described with an emphasis on
the physical processes with determine its behaviour. A short pulse
duration, 5 ns, has been used to phototrigger a 50 cm3 active volume
laser head. To gain insight into the avalanche processes, the breakdown
delay times have been measured and compared to the theoretical
values obtained from a code assuming a local field approximation. A
specific laser energy higher than 6 J/L with an effiency of 2 % is
reported. The duration of the laser pulse is 30 ns leading to a specific
extracted power higher than 200 MW/L.
The operation of a narrow-bandwidth Self Filtering Unstable Resonator
(SFUR) of magnification I M I = 10 applied to a UV-preionized XeC1 laser has
been demonstrated. An intracavity grating (2400 lines/mm)) at a grazing
incidence angle (80 ) has been employed to get a narrow-bandwidth SFUR. A
4.5 mm diameter output beam of 100 pJ, 19 ns long and with a linewidth of
0. 13 A has been obtained. Measurements of the spectral and optical laser
radiation characteristics are reported. The narrow-bandwidth SFUR operation
has been compared with the operation of a plane-plane cavity employing the
same grazing incidence grating and 1 mm diameter intracavity spatial
A 1 0 liter active volume, x-ray preionized XeC1 laser has been operated with both a
conventional spark-gap switched discharge and a x-ray triggered one. In the last case, 45 W
average output power was obtained. A simple model has been developed to study the minimum
conditions on the preionization source to drive a uniform switchiess discharge.
This paper describes part of the EUREKA Eurolaser project EU213, to build an excimer laser.
The emphasis is on control and monitoring systems. The performance of a test-bed laser built at
Salford will be described. In the design discussed, two voltage components are generated
separately and combined at the laser head to form a pumping pulse. A "magnetic switch" is used
to isolate the two parts of the transmission line'. A theoretical analysis of the sustainer section of
the line has been carried out and compared with measurements made using a dummy load in
place of the laser head. A control system is discussed that is being developed to monitor the
shape of each laser pulse at a high repetition rate. The control system is designed to protect the
laser from damage. The construction of various conventional probes, and the progress towards
various fibre probes will be reported with emphasis on measuring fast current pulses on the
various parts of the line.
Microwave discharges have been used to excite rare gas halide
mixtures like XeC1. For the realization of microwave excited excimer
lasers a minimum specific pump power density of 100 kW/cxn3 is needed.
The resulting power requirements are of the order of one to ten mega-
watts. This power has to be deposited in a discharge tube which is
housed in a hollow waveguide. For these purposes, a WR-650 waveguide
circuit was assembled. The pulsed microwave source was a magnetron
transmitter with a frequency of 1.35 GHz, a pulsed power of 2.5 MW and
a pulse length of 4 jis. The puise repetition frequency was 10 Hz. A
double ridge waveguide coupling structure was designed using the cornputer
code URMEL-T. This program is based on a finite difference discretization
method for the Maxwell field equations to compute electromagnetic
fields in waveguides and cavities. The laser gas mixture consisted
of He/Xe/HC1 = 1000/1/0.2 at a total pressure of 1000 mbar. A
laser pulse energy of 20 pJ with a pulse width of 65 ns (FWHM) was
measured yielding a peak power of 300 W.
The positive role of preionization in performance and characteristics
of excimer lasers is unquestionable. A brilliant confirmance
of it is the work of Taylor1 , in which phenomena, which havn't yet
found their' theorist, are described. A designer of concrete
instruments must follow one empiric rule: preionization must be
as powerful as possible and maximally homogeneous. The discharge
being most crucial of to gradients of density of electrons of
preionization across the field. It's no wonder, that the best results
were obtained when using exloying X-rays for exposition of discharge
gap. But X-ray sources are complicated and require good tech-
nology of performance, that's why most designers are have recourse
to UV preionization either from spark sources, or from surface dis-
Spectrally nantwed lasing of a KrF excimer laser has teen ahieved by a self-injection technique using abeam splitter for
power extraction aixi intravity etalons for spectral-narrowing. The laser cavity is divithi into an amplifying branch aix! a spectralnarrowing
branch. The spectral bandwidth was narrowed to <3pm FWHM with air-sed etalons placed in the spectral-narrowing
branch. A laser propagation model was intrOdUced for describing the laser intensity traveling in the laser cavity. The calculated
intensityincident onthe intracavityetalons wassmaller thanthat in theconventional Fabry-Perotcavity withplane-parallel mirrors.
The laser system ''oscillator-amplifier'1' was developed to provide the high
spectral brightness (linewidth < 0.02 cm ) and diffraction limited divergence of
radiation. Spectral narrowing was achieved by the use of dispersive intracavity
elements. The oscillator radiation was amplified by double pass amplifier with phase
conjugate SBS mirror. Oscillator and amplifier were separated with etalon, tuned on
Brillouin shifted wavelength reflection. As a Brillouin scattering medium both
liquid n-hexane and quartz-polymer fiber were utilized. In contrast with n-hexane
the use of fiber decreased the SBS threshold more than two order of magnitude.
A 10 x 10 cm2 aperture X-ray preionized discharge-pumped KrF excimer laser has been
constructed. A fast water pulse forming line is discharged via a rail-gap switch
into the laser chamber. Collimated X-ray preionization ensures a wide and uniform
discharge. The laser pulse energy exceeds 4.7 J in a pulse of 28 ns (FWHM). The
active beam cross-section obtained to date is approx. 10 x 8 cm2.
To increase the pulse repetition frequency, the average power, and the beam quality of excimer
laser systems, damping of the strong acoustic waves induced by the active medium excitation must
be solve. In order to achieve this goal different electrodes and acoustic damping configuration have
Excitation of active medium (energy deposition : 50 to 150 J/l) at high repetition rate
(up to 1 000 Hz) in a subsonic loop (flow velocity : up to 65 m/s) is achieved by means of a classical
discharge, through transfers capacitors. The discharge is preionized by X-Ray generated by a wire ion
Previous studies done at I.M.F.M. have shown that the density perturbations, induced by
successive excitations at high repetition rate (up to 1000 Hz), can involve output energy decrease.
Here, we will present and discuss the density or pressure perturbation maximum level which
don't involve this laser energy decrease; acoustic and thermal effects have been notably took account.
We will study the possibilities of fast pressure perturbations damping.
High efficiency X-ray Preioized discharge XeCl laser e,citation has been achieved with a
Ne/Xe/HCI mixture in a 50 cm" active volume (25*21 cm'), by combining double discharge
technique (spiker/sustainer) with a new fast ferrite magnetic switch. A fast high voltage spiker pulse of
low energy is applied across laser electrodes to initiate an uniform discharge. Peaking capacitors are
set in a very low inductance configuration between electrodes. Main discharge is energized by a 200ns
electrical time pulse charged double sided solid state PFL of much lower voltage which is
automatically applied to electrodes through a low inductance path after magnetic switch saturation by
the spiker.The magnetic switch is made of several coaxial structures in parallel, each of them
comprising many high frequency Ni-Zn ferrite tores set in serie. This laser has a very high pulse rate
frequency capability (> 1 KHz). An efficiency over 3% in energy and 4% in power at the maximum, with
110 mJ extracted optical energy in a 140 ns (FWHM) laser pulse length, has been demonstrated with
this not optimized device.A parametric study of this laser is presented. At the end, planned further
studies of the present advanced excitation system for conditions of very high PRF, in the frame of
Eureka EU205 Eurolaser program, are presented. The experimental part of these planned studies
would be made with LUX very high PRF high average power IMFM test-bed.
A wide aperture, X-ray preionized discharge pumped XeC1 laser, is described.
Before increasing the repetition rate, the laser performances are optimized without
any gas circulation.
The laser energy is given as a function of various parameters : delay time between
X-ray and laser pulses, repetition rate, gas mixture, X-ray dose, charging
voltage (Va), laser gas pressure and Vc/PL*D, (D is the interelectrode spacing).
The active medium is disturbed during about 3 seconds after the laser discharge
(confirmed by interferometry experiments). The laser energy per pulse is constant
with low fluctuations until a repetition rate of 0.3 Hz. The best efficiency, for
different pressures, is obtained for about the same Vc/PL*D value.
The laser energy and the efficiency increase with the buffer gas pressure. But
in this case, it is better to decrease the Xe and HC1 partial pressures. Laser
energy saturation versus X-ray dose depends on laser gas pressure and mixture.
This saturation is not reached for high pressures. Meanwhile more than 3 Joules
per pulse have been extracted from a 42*5*4 cm3 discharge.
We report a comparison between theoretical and experimental results of XeCl
selfsustained discharge laser, in particular emphasizing the role of the self
inductance of laser cell in affecting the theoretical results.
A compact kinetic code for theoretical modelling of discharge pumped XeC1 lasers has
been developed and has been used to model the laser performance and discharge characteristics
of the x-ray preionized lasers. The results are in good agreement with the experiments. The
effects of superelastic and electron electron collisions which are neglected in the compact code
on the modelled results are discussed in detail.
A numerical excimer laser model based on the Boltzmann-code developed by Rockwood1 was used to simulate several
discharge pumped XeC1-lasers. Modifying this code the amount of CPU-time necessary for the simulation of one
discharge pulse could be reduced for about 70%. Extending the reaction kinetics listed by Stielow, Hammer, B&-
ticher2 with electron molecule-ion recombination reactions a better agreement of calculated electron densities with
experimental values was obtained. The relative courses of exctited Xe population densities agree better with the
measured ones too, but the calculated number density values are a factor 2.2 too large, which may be due to the
simple xenon model taking into account two model levels for excitation only.
To reproduce the laser output energies measured by Cirkel3 and Steyer4 the values of the rate constants for XeCl*
and XeCl-quenching taken from literature have to be changed. A more correct description of these reactions and
of the stimulated emission cross section is in preparation. This extended model will be checked experimentally by
temporal and spectral resolved measurement of the small signal gain.
To obtain a high average laser output power and a good quality laser beam in high repetition
rate excimer lasers, it is necessary to swept the heated gas out of the laser cavity between two
electrical excitations and to damp density fluctuations induced by acoustic and shock waves. In this
aim, a numerical study of an unsteady two-dimensional flow inside the laser head has been
undertaken at I.M.F.M. This study is extended to the case when mufflers are added in the
surroundings of discharge zone. The work is achieved by means of a numerical model based on a
finite difference scheme with a flux corrected transport algorithm (Shasta. F.C.T. method). This method
allows to simulate the propagation of longitudinal (along the flow) and transversal (between the
electrodes) shock waves induced by the active medium excitation, and the effects of acoustic
dampers on the density fluctuations. Transversal acoustic waves are induced by the non-uniformity of
energy deposited between the electrodes. The model uses two dimensional Euler equations in the
laser flow channel and one dimensional Euler equations in the acoustic damper (grid and porous
material). Several comparisons between experimental and numerical results have been performed to
establish the model validity. The influence of the acoustic dampers on the transversal wave decay
process in a laser cavity is presented.
The behavior of gain versus peak power deposition and the gain-length product
versus total energ deposited are measured for devices with power deposition levels
from 1 to 13 MW/cm under a variety of gas mixtures. The temporal correlation
between the power deposition and gain shows that both the fluorescence and the gain
occur after the power deposition has concluded for excitation pulses in the range of
10-30 ns. We find that both B-X and C-A fluorescence takes place in the afterglow
and their temporal shapes are similar. This indicates that the C and B states are
tightly coupled. This tight coupling has two detrimental consequences for C-state
lasing. First is that the population of the B and C states are approximately equal
rather than 90 % in the C state believed to exist for short pulse electron beam
excitation. We believe this close coupling is due to the presence of electric fields
in the afterglow which keep the electron temperature relatively hot. The relative
populations of the B and C states are determined by a Boltzman distribution governed
by the electron temperature and their relative energy separation. Second is that
with the C state lifetime approximately the same as the B state lifetime the C-A
saturation intensity is very high and efficient energy extraction is substantially
A repetitively pulsed (5Hz) KrF laser-based X-ray source producing photons at
i-ru 1.1 keV (copper, L-shell) from a copper coated rotating target has been used to
study soft X-ray induced DNA damage effects in Chinese hamster cells. The source
was computer controlled for accurate delivery to the biological material of pre-set
doses. DNA damage was induced by exposures lasting 7s for V79 cells and 40s for AA8
cells. To minimise the debris from the laser-plasma source and for convenient
handling of biological specimens, the target chamber contained helium at 1
atmosphere with a slow flow. The X-ray yield of the source decreased by only at
most 10-20% compared to vacuum operation and a further 16% of X-rays were absorbed
in helium between target and the biological material placed outside the target
chamber behind a beryllium filter. The measured spectral and spatial distribution
of the copper X-ray emission was found to be largely independent of the ambient
helium pressure. The time resolved X-ray signal lasted for only 3 ns starting at
the beginning of the 2lns laser pulse and its shape was independent of helium
pressure in the target chamber.