IEEE TRANSACTIONS ON MAGNETICS, VOL. 27, NO. 1 , JANUARY 1991 299

EXPERIMENTAL OBSERVATIONS OF PLASMA ARMATURE PROPERTIES AND CHARACTERISTICS

Mary C. Baker Texas Tech University

Dept. of Electrical Engineering Lubbock, TX

Byron D. Barrett and W. C. Nunnally University of Texas at Arlington

Arlington, TX

ABSTRACT

The Railgun Simulator, RGS-11, has operated for over two years with the goals of developing diagnostic techniques for railgun plasmas and using the experiment as a testbed to study the behavior of high current plasma armatures. Over the course of the research, a series of diagnostic techniques were employed, including b-dot and fiber optic probes, laser transmission measurements, interferometry, and spectroscopy. These techniques were used to observe the plasma behavior under varying conditions of chamber pressure, arc current, and arc initiation mechanisms. The experimental techniques and results are described in this paper.

I. INTRODUCTION

The railgun simulator, RGS 11, is a device that was constructed to provide a high current, high velocity plasma without a projectile. It has several unique features. First, the bore is designed such that the insulator side walls may be easily changed, or may be removed from direct contact with the rails by removing the inserts. This places a smooth wall about 1/2 inch from the edge of the rail, leaving an air space between the walls and rails. The power supply is a pulse forming network, made with electrolytic capacitors. The electrolytic capacitors provide a slower risetime, a smoother waveform, a low operating voltage (900 V maximum into the gun) , and a peak current that varies from shot to shot by as much as f 5%. Finally, the simulator, RGS-11, is configured such that current may be fed from both ends of the device in order to obtain some control over the plasma velocity. In practice, this did not prove useful for the objectives that are discussed in this paper. Thus, all of the data presented are for a breech fed free- running arc. The arc parameters range from a 2-20 km/sec velocity and from a 50 to 150 k A current.

11. DIAGNOSTICS

A. VELOCITY PROBES

The b-dot probes used in this experiment were armature b-dot probes, i.e. , they were oriented to measure the magnetic field due to the plasma armature. A fiber optic cable was placed very near each b-dot probe. The cable was taken into a screen room facility with photoconductor circuits to record the optical signals. The fiber optic data and the b-dot

data provided identical velocity profiles for the armature. The signal of the b-dot probe was spread out in time, indicating that the probe was not as spatially resolved as the fiber optic probe. Both of these techniques were used in the course of data-taking, however, the data presented is obtained from the b-dot probe data.

B. LASER TRANSMISSION AND INTERFEROMETRY

The laser transmission data were taken using the experimental arrangement shown in Figure 1. The laser beam was fed into and out of the chamber through 2 quartz windows 1/211 in diameter, mounted flush with the bore wall. The same window arrangement was used for the optical interferometer, shown in Figure 2. The type of interferometer chosen was a Michelson interferometer. Optical isolators were placed in the optical path to prevent feedback of light back into the laser cavity.

7 To SCREEN

t i SCREEN

BEAM SPUTTER

\ PLASMA /

FIGURE 1. Laser Transmission

0018-9464/91/0100-0299$01.00 0 1991 EEE

300

8 -

7 -

6 -

5 - .- 3 -

2 -

q T

Mirrors

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FIGURE 2. Interferometer

C. SPECTROSCOPY

The spectroscopy experiments were performed using a 1/2 meter spectrometer with an optical multichannel analyzer (OMA) . The OMA was triggered from an optical signal at the leading edge of the arc, and then gated to capture a particular time record of the armature as it passed the fiber optic location. The spectroscopy experiments and results are described elsewhere'.

111. RES'JLTS

A . ARMATURE VELOCITY

The armature velocity was a function of several different parameters. Initially, the armature velocity was determined as a function of time and position for various power supply voltages. The data for v(t) and v(x) are shown in Figures 3 and 4 respectively. Figure 5 illustrates the current output for each power supply (PFN) charge voltage. There are two interesting features. First, the velocity of the arc saturates (approaches a constant value) at approximately the same position along the gun. Second, the arc saturation velocity is linearly proportional to the current at the point of saturation as shown in Figure 6, indicating that, in this experiment, the velocity is drag-limited.'

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400 V

300 V

200 v

0.00 ~ , . , . . , . . , , , . , . , . . ~ , ~ , , , , , , , , , ~ , ~ ~ ~ ~ I , , , 0.00 200.00 300.00 400.00

T;;? IN MICROSECONDS FIGURE 3 . Velocity vs. Time

VELOCITY PROF I LE Saturation Effects

9 ,

Displacement in meters

FIGURE 4 . Velocity vs. Displacement

CURRENT VS. T I M E DAT0501, Pressure - a few torr

t20,

Time (seconds)

FIGURE 5. PFN Current vs. Voltage

SATURATION VELOCITY VS. CURRENT Port #3

..3

4 . 1 - .., -

Current in kA

FIGURE 6. Saturation Velocity VS. current

The armature velocity was also determined for both a bore with insulator walls flush with the rails, and a bore with insulator walls 1/2" from the rails, creating an air space. The velocity was observed to be slower for the case with removed rails than for the flush rails, as shown in Figure 7 . This experiment emphasized the importance of a confined current distribution to generate the proper driving pressure at the projectile.

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VELOCITY PROFILE Effects of Bore Confinement

0 . 2 0.4 0.6 0.. 1 1.7 7 . -

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FIGURE 7. Wall Effects

The pressure in the railgun chamber was also varied. With all other factors held equal, the velocity was observed as a function of pressure. Figure 8 illustrates velocity vs. displacement for the pressures indicated. The velocity at each probe location was plotted as a function of pressure, shown in Figure 9. The curve can be fitted to show that

v,,, U p-"3.

A perfect I1snowplow1l type model would indicate

v U p-"2.

Thus, the arc in this experiment, although close to a snowplow effect, is not completely modelled as a snowplow arc. It should be noted that the curve fit is more accurate for pressures above 1 Torr.

The armature velocity measurements in general indicate that a moving arc in a railgun bore is affected by several influences: the ambient gas pressure in the bore, the bore geometry, the intiation mechanism, and the current in the arc. There was no indication that the wall material affected the velocity, indicating that ablation did not play a major role in this experiment.

Finally, experiments were performed using a fuse rather than a spark gap to initiate the plasma. Insufficient data were taken to determine quantitatively the effects of fuses versus spark initiation; however, it was evident that the fuse mass created a slower moving arc. In addition, the copper fuse material at the breech end of the device tended to create additional arcs, particularly for the more massive fuses (> 10 mg).

B. ARMATURE ELECTRON DENSITY

The electron density was measured using a Michelson Interferometer and observing the phase shift as a function of time.3 The experimental arrangement was shown in Figure 2. An example of the results obtained from these experiments is given in Figures 10 and 11. Figure 10 gives the time resolved fringe

data. The fringes appear triangular due to the 1 MHz digitizing rate of the data acquisition system. Figure 11 illustrates the electron density as a function of time, obtained by Itcounting" the fringes. In evaluating the data, it was assumed that an equal number of fringes would appear as the plasma density increased to a maximum, and then decreased with the passage of the arc. Regions of relatively constant density would create a "flat spot11 in the fringe record. Thus if some plasma remains at the probe location, there may not be an equal number of fringes preceeding and following the maximum of the plasma armature electron density profile.

In Figure 10, it was assumed that the flat region from 265 to 275 microseconds was the peak of the current or electron density profile. This time region was checked with the zero crossing of a b-dot probe in close proximity to the interferometer port. The arc passed the probe fro? 255 to 290 microseconds, with a tail region present until 315 microseconds. The time may be correlated with the velocity, to provide an armature electron density versus length profile, as shown in Figure 11. A summary of the interferometry data taken through Sept. 1, 1988 is shown in Figure 1 2 . Much of the data cannot be easily analyzed, or suffered from window coating or alignment probes. Therefore, what is presented in Figure 12 is a collection of the data that could be interpreted. The points were taken at two pressures, 2 and 15 torr, respectively. The pressure is indicated on the plot. At the higher currents, the density appears to increase linearly, with the higher pressure causing a greater electron density value. This is obviously a result of the increase in available ionizable material in the bore. It was assumed that the neutral particles did not contribute to the interferometer fringes at pressures less than 20 torr. In general, at 100 kA peak currents, the electron density began to approach 1 x electrons/cm3.

VELOCITY PROFILE m .

Effects of Chawber Pressure (500 V)

0 : , 0 0 2 0 4 0.6 0 . 8 , 3 . 2 1.4

Displacement in meters 1.5 1 0 4 0 1 I ,mr x'. SO3T

FIGURE 8 . Velocity Profiles

302

VELOCITY VS. PRESSURE A T PORT LOCATIONS

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W H g 13 I z 1 3

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ELECTRON DENSITY vs. CURRENi

L I

FIGURE 12. Electron Density vs. Current

C . ADDITIONAL ARMATURE CHARACTERISTICS

INTERFEROMETRIC FRINGES

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s . , , < , , . , , , , , , ~. 4

TIME (microsecond)

(a) Fringe Pattern

250 153 1‘0 161 170 171 I m i l a , 1.0 29, 300 J O I , I O I t ,

PLASMA LENGTH (cm)

(b) Plasma Electron Density Profile

FIGURE 10. Interferometry Fringes

Additional information became available as the RSG-I1 experiments were conducted. A set of data were taken courtesy of Kodak Spinphysics through the transparent Lexan insulator walls, to observe the plasma in motion. The camera had a resolution time of 8.33 microseconds per frame. The results indicated a plasma armature that was shaped like a comet, with a fairly long tail. One photograph captured a secondary arc, following closely behind the primary arc, with some luminous spots between. These videos have not yet been satisfactorily reproduced for publication.

The plasma length was observed as a function of current for various pressures. The length was proportional to the current , indicating a constant current density profile for current values from about 30 to 100 kA. The relationship between current and arc length remained linear at most pressures, with a change in the slope of the line. For pressures less than a few Torr, or greater than 100 Torr, secondary arc formation was a more frequent occurrence, and made the interpretation of arc length data impossible with armature probes alone.

Iv. Conclusion

The results of the RGS-I1 experiments indicated that for the free arc experiments performed on this device, the arc characteristics could be related to the experimental parameters of pressure and current. These relationships increase the knowledge of how arcs respond to different experimental conditions, and demonstrates one reason why it may be difficult to correlate data from different experimental devices.

FIGURE 11. Electron Density vs. Plasma Length

303

V. REFERENCES

1. Barrett , Byron, et. al. , "Spectroscopic Measurements on the RGS-I1 . . . I f 5th EML Symposium, Sandestin FL, 1990.

2. Parker, J.V., W.M. Parsons, C.E. Cummings, and W.E. Fox, presented to AIAA 18th Fluid Dynamics and Lasers Conference, Cincinnati, Ohio, 1985.

3 . Healdy, Clifford E., 111, ''Interferometric Measurement of Plasma Armature Electron Density Profile in a Railgun Simulator," Master's Thesis, University of Texas at Arlington, Arlington, TX, August, 1988.