PLASMA SHEET”
3.4. EXPERIMENTAL RESEARCHES OF FAST-FLYING BODIES INTERACTION WITH LASER-INDUCED SPARK PLASMA
V.I.Nikolayeva1, A.S.Pashchina2, A.N.Sakhonchik1
1Joint Stock Company “GSKB “Almaz-Antei”, 80-A, Leningradsky prospect, Moscow, Russia, 125178
2Joint Institute for High Temperatures RAS, Moscow, 125412, Russia
Abstract. Experimental results of fast-flying bodies interaction with extended plasma object, created by laser-induced spark, are presented. EPO's formation was carried out by the teamwork of two laser systems including specially developed giant- pulse CO2-lasers and stationary CO2-laser KS-10 created in OKB "Raduga". Giant-pulse CO2-lasers has provided spark ignition on a route site of 45 m. Stationary laser KS-10 in a pulse-periodic mode provided discharge channel development till the demanded geometrical sizes and spark maintenance during the model interaction with EPO's site. Processing of experimental data shows essential deflection of model’s trajectory in laser-induced spark plasma and presence of the model's trajectory fracture on a site where interaction with plasma takes place.
The majority of plasma aerodynamics experimental researches are spent in the conditions of gas flow organizing in the vicinity of immobile explored object. Such experimental simulation is quite full matched flight conditions for the cases when plasma is formed by means of onboard generators. In this case plasma parameters essentially depend on parameters of an incident gas flow. Simulation of body's flight conditions through stationary plasma area created by means of exterior generators can be reached in ballistic experiment.
The aeroballistic method, in essence, is a method of the flight experiment which problems are: extended or local plasma area formation, running flying vehicles models through plasma area, and registration of models parameters before, during and after interaction with plasma area.
Comparison of models parameters in two conditions (with and without plasma area interaction) determines plasma area influence on body’s flight dynamics.
Selection of the extended plasma area formation method
Interaction duration of the flying vehicle model with plasma area is one of parameters that determine degree of plasma area influence on flight dynamics. Necessary interaction duration for appreciable influence on flight dynamics may be provided by various methods. One of them is plasma area formation (for example, by MW radiation focusing) in the vicinity of fast flying body and its accompanying during the necessary time interval. However, for space and time requirements of aeroballistic experiment extended plasma area formation along ballistic route, which dimensions provides necessary for explored object interaction duration, is more suitable.
Laser-induced spark in air can be considered as one of suitable methods of EPO formation for aeroballistic experiment conditions.
This method have been used in researches of fast- flying bodies interaction with extended plasma object (EPO), held by employees of Scientific Research Institute of Radio Device Instruments (the supervisor of studies R.F.Avramenko) in 1992 … 93 on OKB "Raduga" stand, Raduzhny town [1, 2].
CO2 lasers on wavelength λ=10,6 microns, characterized by lower break threshold in air in comparison with smaller wavelength lasers, have been used for EPO formation. In particular, the power density of spark break for CO2 laser wavelength of λ=10,6 microns makes about W≥2·109 W/cm2 in enough transparent atmosphere, and for Nd laser wavelength λ=1,06 microns enlarges to W≥1011 W/сm2. Dust presence in air causes CO2 laser break threshold value lowering for order.
Laser spark ignition in air can be realized by so called giant-pulse lasers, characterized by short radiation pulse (about few tens of nanoseconds). Few tens of Joules radiation energy can provide more than 1 GW of radiation power.
Low beam divergence and great aperture are providing optical break at distances of few hundred of meters. Spark length is approximately equal to half of focus length of optical system. Laser- induced spark is represented by local break centers located along optical caustic. Distance between local sparks is determined by laser power parameters and beam divergence. Laser-induced spark lifetime in giant-pulse mode, however, does not exceed 100 mcs, that is insufficient for plasma interaction with model, taking into account model’s inertance. That’s why for spark evolution and maintenance (increasing of discharge duration, decreasing of distance between local sparks) feeding by radiation on the same wavelength but with essentially greater pulse energy is needed.
Really available length of laser-induced spark is approximately equal to half of focus distance of optical system. In general the focus distance range is determined by radiation pulse
power. So, focus length increase (because of laser beam divergence) causes focal spot diameter increase, that, in turn, results in power density and laser spark length decrease. On the other hand, focus length diminishing at invariable aperture causes optical radiation cone angle increase that leads to waist of space boundaries where the power density exceeds threshold, i.e. to laser spark length diminishing. Taking into account maximal values of interaction duration of plasma and model and power parameters of laser systems, optimal focus length value was selected to be one hundred meters.
Thus real achievable and maximal possible optical spark length can reach 40 m. For characteristic models velocity value 1 km/s interaction duration can reach 40 ms that is enough for model’s flight dynamics influence.
However, laser pulse duration has limitation, caused by plasma front formation, moving towards laser radiation with high speed (v~100 km/s, so called light-detonation wave effect). Plasma front shields up to 80% of power preventing penetration of radiation deep into channel. Specified effect influence on laser spark channel evolution can be essentially weakening by laser functioning in pulse-periodic mode. Thus, laser pulse duration value is selected from condition, when plasma front path length is less than laser spark length L, i.e
τp<0,1L/v,
where τp – laser pulse duration; L – laser spark length, equal to 40 m; v – plasma front velocity, 100 km/s.
Laser pulse duration cannot exceed 40 mcs for specified parameters. That’s why for laser spark maintaining during needed time interval 40 ms it is necessary to realize laser system functioning in pulse periodic mode.
Used in experiments [1, 2] laser KS-10 OKB “Raduga” provided pulse sequence of 30 mcs duration at pulse repetitive frequency of 100 Hz.
During model’s flight throughout EPO area (~40 ms), energy of four pulses will released in the spark region. As in time between pulses practically full recombination of plasma takes place, there is a necessity of giant short pulse for each long pulse of KS-10 laser.
Thus EPO's formation was carried out by the teamwork of two laser systems including specially developed giant-pulse CO2-lasers and pulsing CO2-laser KS-10 created in OKB "Raduga".
Giant-pulse CO2-lasers has provided spark ignition on a route site that approximately equal to half of optical system focal length. Laser KS-10 in a pulse- periodic mode provided discharge channel development till the demanded geometrical sizes and spark maintenance during the model interaction with EPO's site (40 ms for the model velocity of 1 km/s). Algorithm of two laser systems teamwork is presented on fig.1.
Three independent units of giant-pulse CO2-lasers with surface discharge preionization mounted in car box body have been used in experiments. Main parameters of giant-pulse laser are:
• Gas mixtures CO2:N2:He – 1:1:5; 1:1:6, 2:1:7;
1:2:7; 1:2:6;
• Operation pressure – 1 atm;
• Operation volume – 18 liters;
• Input voltage – 70 kV;
• Peak pulsed voltage on plasma electrodes – 70 kV;
• Peak pulsed discharge voltage – 280 kV;
• Pulse radiation energy – up to 300 J;
• Power density – up to 1011 W/cm2
• Radiation wavelength - λ=10,6 microns;
• Output beam diameter – 160 mm;
• Pulse radiation duration- τ=0,05…5 mcs;
• Beam divergence - 5·10-4 rad;
• Operation mode – monopulse.
Fig.1. Algorithm of two laser systems teamwork.
KS-10 stand is ionizing CO2 laser of atmospheric pressure and closed cycle, operating in pulse and pulse-periodic modes. Main parameters of KS-10 stand are:
• Pulse radiation energy – 1,5…2 kJ;
• Pulse radiation duration – 25…40 mcs;
• Pulse repetitive rate – up to 125 Hz,
• Radiation aperture – 20х40 cm2;
• Beam divergence – 0,7…1,5·10-3 rad (depending on operation mode);
• Radiation wavelength - λ=10,6 microns;
• Operation mode – monopulse, pulse-periodic;
• Functioning duration in pulse-periodic mode – up to 1 s.
Teamwork of two laser systems provided EPO formation with length of 45 m and focal spot diameter of 50 mm (fig.2). Distance between local sparks varied from 200 mm to 7000 mm depending on KS-10 laser pulse energy. Thus, most frequently observed distance between sparks was 300…400 mm.
Research technique
The stand represents plurality of the equipment, allowing making aeroballistic experiment on laser spark influence on models flight dynamics in field requirements (fig.3). Stand included next systems:
• EPO’s formation complex, including mobile setup with 3 units of giant-pulse lasers, stationary KS-10 laser, focusing system and synchronization system;
• Powder ballistic unit of 60,5 mm caliber;
• Models catcher;
• Measuring velocity system on the trajectory sites – before and after plasma contact area
(chorographical targets);
• Film recording system, including speed cameras (1000…4000 frames/sec);
• Device of shadow photographing;
• Paper targets;
• Automated management system of experiment.
Disposition of stand main parts were correlated with EPO position and model’s flight trajectory such as to provide maximal interaction duration within model and EPO. Ballistic unit placed on 20 m distance from EPO, and the angle between ballistic unit and EPO axes was about 21´.
Measuring of laser’s KS-10 radiation energy was made by a flow lattice bolometer of 500х700 type terminated to oscilloscope. Laser spark geometrical parameters registration (space position, sizes) was made by single frame speed cam photographing (RFK-5 type).
Flying vehicle model represents cylindrical body with nose needle. Design of one model’s variant is presented on fig.4. Parameters of model variant (МА-3 [2]) are:
Fig.3. Principal stand schematics (distances specified in meters).
Fig.2. Extended plasma area formed by laser-induced spark.
• mass m=0,48 kg;
• relative coordinate of barycenter XT=56%;
• moment of inertia relatively transverse axis, passed through barycenter Jz=0,96·10-3 kg·m2.
Fig.4. Model design.
Model’s aerodynamics parameters in operational Mach numbers range М=2,8…3,5 are constant practically, that simplifies experimental results analyses. For the model integrity control and the fact of model trajectory and laser-induced spark crossing the shadow photographing was spent. The example of a shadow photo of the model flow pattern in the presence of plasma formations is presented on fig.5.
Fig.5. Model’s shadow streamline pattern with EPO presence.
Aeroballistic experiments have been spent in requirements of atmospheric pressure p0=1·105 Pa, temperature T0=265…300 К and model’s average velocity v=1000 m/s. KS-10 laser operation mode – pulse-periodic, pulse repetitive frequency – 100 Hz, duration of operation – 3 s.
Models deflection from trajectory that registered without plasma area influence was the criterion of laser spark influence. For the purpose of reliable definition of the fact of interaction within model and laser spark, transparent for laser radiation polyethylene segments were "sewed“ on paper targets with holes for laser beam. This is
allowed to fix laser beam position in aeroballistic frame. Photo of targets mounted out of (a) and in spark zone (b) are presented on fig.6.
Fig.6. Photos of targets, mounted out of (a) and in (b) the EPO’s interaction area.
Decoding of target holes allows determining models barycenter coordinates, spatial attack angle and precession angle.
Cold laying position by the use of shoot down tube is pointed on each target before every shoot. In requirements of stationary ballistic stand cold laying position coordinates (Y0, Z0) and model’s barycenter coordinates (Yg, Zg) in target plane are determined independently in the stand frame, and after that the deviation values Y=Yg-Y0
and Z=Zg-Z0 are determined. In field requirements the axes 0Yg and 0Zg runs through cold laying position. In obtained frame the barycenter coordinates of model, i.e. deviation values Y, Z from cold laying position, and attack angle α are determined.
Processing observed data permits to determine parameters of technical scattering:
medial hit point coordinates Y1, Z1 and mean- square deviations Sy, Sz:
,
,
where Yi=Yg-Y0; Zi=Zg-Z0 – model’s deflection in i-th shoot from n shoot series.
Research results
Experimental data were processed according to statistical methods which have allowed defining regression model. Statistical parameters of regression permitted to conclude with 95%
probability about linear approximation model reliability that in turn confirms absence of model probable deformations, which are capable to cause additional component of aerodynamics forces modifying trajectory.
Fig.7. Example of model’s barycenter coordinates along ballistic route and piecewise linear approximation (solid straight lines).
Gained in regression analyses mean-square coordinates deviations without EPO influence are:
12…25 mm along Y-axis and 5…70 mm along Z- axis. In condition of EPO influence mean-square coordinates deviation values increase is observed.
For spent experiments series [2] their values were:
7…90 mm along Y-axis and 12…130 mm along Z- axis. This is exceeding technical scattering parameters and may be the proof of scattering rising caused by EPO influence.
For better convergence of experimental values, attempts of piecewise linear approximation were undertaken. Diagrams of calculations on this procedure for one of shoots are given on fig.7.
The straight lines gained at keeping the criterion of the regression residual quadrates sum minimum are shows models’ trajectory fracture on a site where interaction with laser spark takes place.
Value of the model total deviation angle after EPO’s interaction is Δφ=3,2…7,90.
Conclusion
Thus, experimental results permits to conclude about possibility of fast flying body’s trajectory parameters change by plasma formations influence, in viewed case by laser-induced spark.
Trajectory fracture in contact area confirms essential EPO’s influence on models flight dynamics. Obviously, increasing of contact time interval is necessary for EPO’s influence magnification that was limited by laser spark length. That’s why it is interesting organization of experiment, when plasma object accompanies flying body during necessary time interval.
References
1. Experimental researches of flying vehicles models motion in laser-induced spark plasma.
Scientific report on theme 750-1200. № Б- 17/130. Vladimir: OKB «Raduga», 1992, 54 p. (in Russian).
2. Investigations of flying vehicles models interaction with extended plasma formations.
Scientific report. № 98/93. Faustovo:
GosNIPAS, 1993, 39 p. (in Russian).