IEEE TRANSACTlONS ON MAGNETICS, VOL. 27, NO. 1. JANUARY 1991

VISUALIZATION OF PLASMA - FLUID MIXING THROUGH X-RAY SHADOWGRAPHY

Aurel Faibis*, Zvi Kaplan and Shlomo Wald Electromagnetic Propulsion Laboratory

Soreq Nuclear Research Center, Yavne 70600, Israel

*Present address Indigo Systems, Kiryat Weizmann, Rehovot, Israel

Abstract

Changes induced by the impact of a hot plasma- jet on a fluid have been recorded using X-Ray shadowgraphy. The process is of particular importance for the operation of electro-thermal acceleration devices. Sets of frames taken every 40 psec reveal details of the interaction. Fluid erosion is observed to occur only at the fluid-extremity directly hit by the plasma. The erosion-rate has been evaluated for the initial collision stages when the volume accessible to the fluid remains constant; the measured values range between 100 m/sec and 200 m/sec.

m e Electrothermal ProDulsion Method

Electrothermal propulsion is a relatively new concept employed for mass acceleration to hypervelocities (v > 2 km/sec). The energy is stored in a capacitor-bank and delivered via a Pulse Forming Network to a capillary plasma-discharge. The plasma carries material ablated from the capillary-wall through a nozzle into an adjacent chamber filled with working-fluid. Subsequently, part, or all of the fluid, evaporates and gives rise to the high-pressure gas which accelerates the projectile. It is alleged that energy transfer via the working-fluid, rather than directly to the projectile, is to be preferred. The process reduces barrel-wear by significantly decreasing gas-temperature. Concurrently, the gas-density increase ensures the high-pressure required for acceleration.

Compared to conventional acceleration methods, electrothermal propulsion (ETP) has features which facilitate reaching the hypervelocity regime. The most important feature is the ability to choose working- fluids made of low-Z elements, such that sound-velocity is greatest. Secondly, the rate of delivering energy to the plasma can be controlled. All these plus the relative ease to modify a regular barrel into an ETP-accelerator justify the recent interest in such devices.

Comprehension of the mixing-process is decisive for maximizing projectile kinetic-energy. This can be achieved if homogeneous mixing between working fluid and plasma is completed before the projectile starts moving. Still, prior to the results reported here, no direct experimental information on mixing was available. Indirect data have been used instead: the pressure-pulse to estimate the time scale of the mixing-process, recovery of fluid-traces to expose bad optimization of the mixing parameters, a.s.0.

We elected X-Ray shadowgraphy as the simplest means to map the density of the working-fluid during plasma-fluid interaction. An experimental

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set-up was specially designed and constructed. Several experiments were needed to prove the feasibility of the method and provide initial information on mixing. This report contains the description of the experimental set-up and the preliminary experimental results.

The experimental set-up consists of a X-Ray source pointed toward the mixing-cell assembly, the specially designed mixing-cell and the X-Ray detector, placed against the source, on the opposite side of the cell (Fig. la).

The radiation is partially absorbed by the cell-wall and the fluid. The intensity at any point (x,y) on the detection plane (Fig. lb) depends on the total absorption along the line defined by that point and the location of a point-like source. The specific-absorption is a function of the local density and chemical composition of the medium as well as photon energy.

la

Supporling rods

X-Ray Source

Ib

I Mixing Chamber

Working fluid

I

X-Ray Inmge Converler

X-Ray source

linage Converter Fholocalode

lligh Speed Camera

Fig. 1. Schematic drawing of the experimental set-up

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Within some limits, a particular choice of materials may help achieve a situation such that fluid-opacity is much larger than the opacity of the wall.

The fluid specific-absorption is increased by adding water-soluble "sugar of lead" (40 g acetate to 100 g water). Although the supplement of heavy atoms is minor (less than 2%), the opacity is increased by almost five times compared to water alone, for 50 - 60 keV energy photons. The regular metal cartridge is also replaced. The new mixing-cell consists of a polyethylene tube, 9.5 mm inner diameter, shielded by a Kevlar-housing with a 8 mm thick wall. The cell is mounted next to the muzzle in a stainless-steel adapter linking the cell to the plasma-injector, on one side, and the barrel on the other.

The X-Ray source comprises a Scanray DOA-250 tubehead' (focal spot size of about 3 mm) and a IMAX pulsed power-supply made by Hadland Corp. The system (after small modifications) delivers a 100 kV/50 mA X-ray square-pulse about 1 msec wide.

A Thomson-made X-Ray Image Converter' has been used as a "detector". The X-Ray image formed on the 25 mm converter screen is optically relayed onto the photocatode of a Hadland-made IMACON-790 high-speed camera3. The image is recorded on Polaroid-667 paper. Each photograph contains 8 - 10 frames, 40 psec apart, with 8 psec exposure time. The overall system- resolution is about 10 line pairs/".

EXDerimental Results

In all experiments the relative location of the camera and the tubehead was kept unchanged, the energy pulse was 30 kJ, and the working-fluid was a solution of "sugar of lead" (Pb(CH2-COOH) *'3H20) in water.

Each experiment yielded three sets of shadowgrams. The first set is taken prior to acceleration and provides information on fluid location in the mixing tube and on fluid and cell homogeneity. The other two sets are recorded throughout the discharge and after acceleration, respectively. The 10 frames of the second shadowgram reveal the changes taking place in the working-fluid every 40 psec, for the first 400 psec of the discharge. Finally, the post-acceleration picture is used to identify the presence of fluid-traces or eventual damages in the cell wall.

The shadowgram of the cell during the mixing process unveils successive changes in the shape and position of the fluid. The first signs of erosion appear about 200 psec after beginning of discharge. From that moment on, a transition region, about 1 cm in length, of variable opaclty replaces tne previously sharp transition at the fluid-extremity reached by the plasma. Erosion is observed to take place only in this region. The fluid surface recedes while its surface becomes concave. The 5 - 10 mm depth, seen at the boundary, indicates that evaporation starts from the axis and propagates toward the cell-wall. The evaporation leads to a sudden pressure increase inside the mixing-cell cavity. In experiments starting with a gap between fluid and projectile (Fig. 2), the fluid begins moving toward projectile-base under the action of the added pressure. Some 320 psec after beginning of the discharge the pressure is sufficiently high to displace the projectile. The remaining fluid will evaporate inside the barrel, at a different rate, due

to interaction with the surrounding hot-gas. Eventually, fluid-fragments may be found even after the projectile left the barrel.

Fig. 2. Set of shadowgrams showing the erosion and the displacement of the working-fluid during interaction with the plasma. The plasma-jet enters from the upper side of the mixing-chamber; the projectile T-shaped tail can be seen at the opposite end. The frame sequence is:

2 4 6 8 1 0 1 3 5 7 9

The first signs of working fluid erosion may be noticed in the fifth frame; the body of fluid gets visibly shorter from one frame to the next, while it moves away from the plasma-jet. The projectile movement starts sometime between the last two frames.

The features described above are visible in all shadowgrams, including the one showing a hollow-fluid configuration. Contrary to certain expectations, the corresponalng rrames a0 no1 snow any signs of erosion inside the 'in-fluid' cavity.

Apart from providing a qualitative picture of erosion, the shadowgrams can be used to derive quantitative parameters. At this stage, due to the crude resolution of the system, only parameters defining the time scale can be estimated. Two Of these quantities have been mentioned already: the start times for erosion and for the projectile displacement, respectively. A third quantity of interest is the erosion-rate. The fluid-body confined between the high-pressure gas and the projectile-base evaporates and its length decreases. The rate of erosion can be evaluated by determining the location of the fluid-boundary on consecutive frames; the rather diffuse image of the boundary and the finite system-resolution affect the accuracy of the results. A comparison of the different runs shows that the erosion-rate is more or less constant and ranges between 140 - 180 m/s. Variations from one run to another are due to the behaviour of the sealing components and hence the history of gas-pressure inside the cell.

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t

\ Run#l

3 1

b H U I I # ~ ’

I I I I I I t 0.16 0.20 0.24 0.28 032 036 0.40

2 1 - I

Time ( m e )

Fig. 3. Results from three different experiments show that the erosion-rate is practically constant prior to projectile displacement. The length of the remaining (unevaporated) working-fluid (cm) is plotted vs. time (msec) Errors are smaller than symbol-size.

A feasible and relatively simple method for obtaining shadowgraphs of working-fluid/plasma-jet interaction has provided the first direct experimental information on fluid erosion. The data suggest that the erosion process takes place only at the base of the fluid body. Evaluation of the erosion rate was also possible from the experimental data. The simplicity of the system and the results obtained suggest that further work be done to improve the experimental system for more detailed study of the plasma/working-fluid mixing process.

References

X-ray source. 200 kV, 50 mA, Type DOA-250, Scanray A/S, Denmark.

X-ray intensifier (image convertor) - Tpe - Model: THX 1469 GKV, Thomson - CSF. France.

Camera - Imacon 790 Hadland Photonics, England.