Physical Sciences BARC HIGHLIGHTS BARC HIGHLIGHTS BARC HIGHLIGHTS BARC HIGHLIGHTS BARC HIGHLIGHTS 41 Lasers and Plasma Related Studies 4. LASERS AND PLASMA RELATED STUDIES INTRODUCTION Laser system development and employing them for basic investigations and technological applications formed the major thrust area in BARC. Ultra short lasers and related diagnostics are being developed for shock wave studies. Single axial mode and solid state tunable dye lasers will spur new applications. Laser cooling of atoms in traps in 100 micro Kelvin opened new areas of studies. Laser produced plasmas by resonant ionization and using intense lasers are of practical importance. Laser-based instrumentation was developed for applications in nuclear fuel cycle. Deeper understanding of non-linear dynamics of the arc and the hydrodynamics of thermal plasma sources have helped in building reliable plasma torches with long lives.
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Fast switiching electronics for thefemtosecond laser system..
Spectrum of the chirped laser pulse and effect offrequency filtering within the pulse stretcher
on the spatial profile of the laser beam.
. Ultra fast, high resolution, Laser and
Plasma Diagnostics
Scanning Second Harmonic Autocorrelator: Scanning Second Harmonic Autocorrelator: Scanning Second Harmonic Autocorrelator: Scanning Second Harmonic Autocorrelator: Scanning Second Harmonic Autocorrelator: An online
pulse duration monitoring set up with a sub-picosecond
temporal resolution has been developed.
Second Harmonic Autocorrelator set up
Laser pulse temporal profile
XXXXX-ray diagnostics for laser plasma : -ray diagnostics for laser plasma : -ray diagnostics for laser plasma : -ray diagnostics for laser plasma : -ray diagnostics for laser plasma : Several plasma
diagnostics with high temporal and spatial resolution have
been set up for laser-plasma interaction experiments. A Digital
X-ray pin hole camera has been developed to image the spatial
distribution of X-ray emission from laser plasma.
ns ina device called the stretcher that has been designed in this
laboratory. This 2ns pulse will be further amplified in a
regenerative amplifier (taking care that the spectral
characteristics remain as close to the original as possible) and
then further in a set of linear amplifiers before it is compressed
takes place due to dye degradation confined to the excited
region. Fig. shows the reduction in output power of the dye
laser, pumped by the second harmonic of a Q-Switched Nd:YAG
laser, with and “without” lasing, confirming that rate of
degradation is indeed slower in presence of stimulated emission.
The data for “without lasing” condition was taken by releasing
an intracavity shutter for a short time, to allow laser action
to take place for measuring the output power.
Alok K Ray, Sasi Kumar, Sucharita Sinha, S. Chattopadhya, K. Dasgupta,National Laser Symposium, Jan 2005, BARC, Mumbai.
4.8 DIFFERENTIAL WAVELENGTH METER:
AN IMPROVED TECHNIQUE
A differential wavelength-meter, is an instrument that was
developed in BARC for precisely determining small changes in
the wavelength of a highly monochromatic radiation source,
such as a frequency tunable laser, whose output frequency
may vary with time, either on its own or by design. It can also
be used to analyse the spectral components of a light source,
for example, between the axial modes of a laser. In order to
make precision measurements one uses a Fabry-Perot
Interferometer (FPI).
In earlier designs, it was necessary to accommodate a complete
circular fringe pattern, formed with a FPI and a short focal
length lens, in the CCD camera, in order to determine the
fringe radii of different orders that were used to compute the
change in wavelength. In that case, for very sharp spectral
lines analysed by a high resolution FPI, the fringe width becomes
comparable to the CCD camera pixel size, thereby limiting the
accuracy of the system. To overcome this resolution limit, a
new technique has been developed, in which, large diameter
FP fringes with proportionally large fringe-widths are produced
by using a long focal length lens. The CCD camera now
accommodates only a part of the fringes. However, the radii
of the fringes can still be determined from a measurement of
the fringe separations, which is possible because of a special
functional relationship between the diameters of the fringes
formed by a FPI.
The results of a test measurement of the separation (Δν)
between the axial modes of a He-Neon laser are shown in the
figures below. Fig. A shows the raw video signal across the FPI
fringes. Each fringe shows the He-Ne laser spectrum with
decreasing resolution from the fringe center (not seen in fig.).
A digital FFT based noise filter applied on the video line signal
brings out the laser spectrum clearly (Fig.B), from which the
mode separation is computed. Fig. C shows the results of a
large number of measurements, confirming a narrower spread
in data obtained, and a hence a higher accuracy using the new
technique.
Raw video signal (A) and filtered video signal (B) of FPIfringes recorded with a CCD camera and acquiredthrough a Digital Storage Oscilloscope in the PC,
showing axial modes in He-Ne laser spectrum.
Reduction in out put power of a solid state dye laserwith and “without” lasing, as a function of the
Generation of thermo physical properties of arc plasmas for
different plasma generating gases is a basic necessity for process
characterization and device design. A major problem in
computation of high temperature thermo physical properties
is the availability of elemental free energy data at extremely
high temperatures (>30,000 K). A code to compute composition
and other thermo physical properties of arc plasmas has been
developed using the partition function approach. Figures show
a few results of computation with a typical plasma gas namely
argon.
Composition of plasma consisting of (Ar(75% by weight)+Fe(5% by weight)+Al(5% by weight)+Cu(5% by weight)+Ti(5% by weight)+Zr(5% by weight) at pressure of 1 atm
and temperatures varying from 4000-50,000K
Effect of pressure on ionization in argon plasma
Indrani Banerjee, S.V.Bhoraskar and A.K.Das, , , , , Proceedings of theSymposium on Power Beam and Material Processing, 25-28,Sept.2002, BARC, Mumbai, India, pp.313-317
4.16 COMPUTATIONAL FLUID DYNAMICS (CFD) OF
THERMAL PLASMAS
Thermal plasma jets and current carrying arc columns interacting
with materials present extremely complex problems of electro-
fluid dynamics. The presence of sharply varying material
properties, turbulence, arc spot etc need specialized CFD
simulation codes for device design and enhancement of process
performance. L & PT Division has developed 2D (SARCPIS2-
Simulation of ARC Plasma In Steady State in 2 D) and 3D
(SARCPIS3-Simulation of ARC Plasma In Steady State in 3 D)
CFD codes for simulating thermal plasma flows in plasma
chemical reactors.
SARCPIS2 is a 2D finite volume FORTRAN solver of
electromagnetic NS equation for simulation of arc discharge.
The solver can simulate arc discharge with variable material
Temperature distribution inside a reaction chamber of radius3 inch for a 20 kW nontransferred plasma torch.
properties as well as input plasma parameters and geometry.
Once the simulation is completed, graphics files showing
S. Ghorui, A.K. Das and N. Venkatramani, Plasma Sources Scienceand Technology, 9, 1-8, (2000)
A.K. Tiwari, C.S.R. Prasad, V.K. Kansal, S. Ghorui and A.K. Das,Proceedings of the 18th National Symposium on Plasma Science& Technology, Dec, 8-11, 2003, Ranchi, pp.21
A sample output from SARCPIS3 givingassymetric distribution of temperature in the
nozzle region of a plasma torch
4.17 SIMILARITY ANALYSIS FOR PLASMA TORCHES
AND JETS
Dimensional analysis is methodology of representing the overall
effects of complex phenomena of plasma beams in terms of
dynamic similarity in the form of generalised functions,
expressed in terms of generalised arguments in the form of
dimensionless numbers, corresponding coefficients and
constants. The various non-dimensional numbers for a flowing
plasma with electromagnetic fields are derived from the
conservation equations of mass, charge, momentum and
Generalized Current Voltage Characteristic (CVC)relationship of a transferred arc plasma melter torch
with experimental points in literature (6.5-9.3%)
output in simulation of a reaction chamber is given in Fig.
Owing to inherent asymmetry in most of the arc plasma
systems due to attachment of arc root at a particular position
or the typical geometries of the plasma reactors, 2D simulation
becomes inadequate and 3D solver is the only option for
accurate prediction of plasma field quantities. SARCPIS3 is a
FORTRAN code for numerical simulation of arc plasma jet in
3D in laminar as well as turbulent modes. The code uses finite
volume technique, cylindrical coordinate system and SIMPLE
algorithm for solution. A sample simulation result of the nozzle
region of a plasma torch is displayed.
energy. The similarity expressions provide design data for large
systems based on data generated on small systems. Work done
at L & PT Division, BARC has been able to generate generalized
relation for gas stabilized plasma torches. Figures illustrate the
data generated and comparison with data published.
Plot of non-dimensional Current voltage Characteristics(CVC) using Enthalpy number and Voltage numbers
(5.26%). The plot also includes the experimental pointsobserved in literature.
A. M. Paingankar, A. K. Das, V. S. Shirodkar, K. P. Shreekumar,and N. Venkatramani, Plasma Sources, Science and Technology,8,100.
It has long been observed that plasma spray process is beset
with problems associated with partially melted or unmelted
powders that is responsible for severe degradation of process
efficiency. The problem is being looked at both from the point
of view of arc instabilities as well as fluctuations in the power
supply. An attempt was made by us to measure the temperature
fluctuations in plasma jet caused by ripple in DC power supply
through an innovative use of the atomic Boltzmann plot
method. It was found that temperature of the plasma jet
fluctuates widely, from 4400 K to 6500 K during each ripple
period of 6.67 mS. Due to this; the spray powder is non-
uniformly heated, degrading the spray coating. This
Experimental arrangement for measurement of temperaturefluctuation and a sample temperature profile
S. N. Sahasrabudhe, N. V. Kulkarni, S. Ghorui, D. N. Barve, N. K.Joshi and S. V. Bhoraskar, Proceedings of the Symposium on PowerBeam and Material Processing, 25-28, Sept.2002, BARC, Mumbai,India, pp.323
4.19 LASER COOLING OF ATOMS
The field of laser cooling of atoms is one of the prime and
frontier areas of research in contemporary physics today.
BARC is one of the few laboratories in India, which undertook
work in laser cooling and was first to demonstrate laser cooling
of cesium atoms. Notably, this has been achieved using local
expertise and technology. The efforts have realized a
state-of-the-art facility, for laser cooling of atoms, consisting
of Magneto-Optical Traps (MOT), dark trap, sophisticated
diagnostic techniques for temperature and number density of
cold cloud etc. The facility has been routinely used for
generating cold samples of Cs (~100 μK) and Rb (~200 μK)
for a variety of investigations, which include precision
spectroscopy, collision physics and quantum optics. The long-
term vision in this programme includes Bose condensation and
development of advanced devices such as atomic clocks,
gradiometers and futuristic quantum computation.
A view of the Laser Cooling Laboratory at Laser &Plasma Technology Division showing
a magneto-optical trap (MOT) for cold cesium atomsand relevant diagnostic set-up.
measurement was given a pointer to determine the correct
power level for getting low porosity spray coatings. Figure
shows the measurement system and the temperature variation.
N. K. Joshi, D. P. Chakravarthy and M. Madanmohan. Proc. Nat.Laser Symposium- 2003, p.179-180, IIT Kharagpur, Dec. 22-24,2003
4.24 OPERATION OF A HELIUM-FREE TEA CO2
LASER
The major role of the expensive and scarce helium gas that
constitutes majority of the gas mixture in a pulsed Transversely
Excited Atmospheric pressure (TEA) CO2 laser, is to stabilise
Laser head and the pulsar system. Inset shows the laser beam
and facilitate the occurrence of an arc free discharge. We
have, in our laboratory, achieved helium-free operation of a
conventional TEA-CO2 laser under a wide range of operating
conditions. This was rendered possible by integrating the spiker
and sustainer-like actions in to a single pulser network by
making use of a coupling inductance. This, in addition to
delaying the spiker with respect to the preionisation, also
decided the rate of rise of voltage across the electrodes and
therefore, tailored the sustainer pulse such that arc-free
operation at atmospheric pressure in a helium-less gas mixture
became possible. The fact that expensive and scarce helium is
not a constituent of the laser gas mixture makes it an ideal
system for high repetition rate operation as the gas re-
circulatory loop consisting of the heat exchanger and the
catalytic re-converter can be readily dispensed with here.
D. J. Biswas, J. P. Nilaya, and A. Kumar, Opt Commun 248, Opt Commun 248, Opt Commun 248, Opt Commun 248, Opt Commun 248, 521 (2005)
D. J. Biswas, J. P. Nilaya, and A. Kumar, US PUS PUS PUS PUS PAAAAATENT (Granted),TENT (Granted),TENT (Granted),TENT (Granted),TENT (Granted),Publication date: 01-06-2005, Publication No: 2005-0002432A1