IEEE TRANSACTIONS ON MAGNETICS, VOL. 27, NO. 1, JANUARY 1991 165

An Experiment to Measure the Mass Density of a Plasma Armature?

John W. Rogers, Keith A. Thomas, Eugkne J. Clothiaux Department of Physics, Auburn University, AL 36849431 1

and

William C. Condit PO Box 1663, LANL, Los Alamos, NM 87545

Abstract -A diagnostic technique for determining the mass density, and possibly the plasma composition, of the plasma armature in the MIDI-3 free-running arc device is described. The armature consists primarily of the components of polyethylene and copper, and it can reach velocities in excess of 8 kmhec. The approach in this proof-of-principle experiment utilizes a flash x-ray source with x rays having energies from 3 keV to 20 keV, an elliptical x-ray crystal spectrometer, and an appropriate detector array and data acquisition system.

I. INTRODUCTION

Over the past few years there has been considerable research conducted, both theoretical and experimental, on the plasma arc armature electromagnetic launcher. Progress in improving the performance of these devices [1,2] has been slow, and several models [3-61 have been developed for studying the behavior of the plasma arc within the confines of the launcher bore. An important parameter appearing in all models is the density of the armature, which is usually taken to be the density of the plasma arc. The plasma density has never been measured by a direct experiment and is usually inferred from other plasma parameters by assuming that all the armature par- ticles are at least singly-ionized and participate in the arc flow. To improve and extend the current models it is necessary to make a direct determination of the armature density.

Since the plasma created in an electromagnetic launcher is not readily directly accessible, it becomes necessary to make apertures-in the containment structure for pressure, local potential differences, and optical and thermal measurements, all at the risk of compromising the integrity of the containment structure. Besides these passive observations, it may be possible to study the interaction of the plasma arc with electromagnetic and other radiation. Several approaches were considered and included (1) the propagation of AlfvCn waves through the plasma, (2) the scattering of neutron beams, (3) the use of microwave interferometry, and (4) the absorption of x rays by the plasma constituents.

The transmission and detection of AlfvCn waves appears to be too difficult for small bore launchers and for the very noisy plasma characteristics of these devices. The use of neutron beams would likely require the use of a nuclear reactor, and there is always the radiation safety

Fig. 1. (a) A cross-section of the MIDI-3 accelerator showing the locations of the x-ray tube and the PIN diode detector. The G-10 tubes are sealed with Be-foil and backfilled with helium. Slits 1- mm x 10-mm are cut into the polyethylene insulators to allow x rays to reach the bore without absorption. (b) A view showing the locations of the various probes used for diagnostic study of the plasma arc.

problem. Microwave propagation through a plasma with densities greater than 1014 6111-3 is not possible, and the projected density in this plasma arc is estimated to be >lo18 cm-3.

Carbon is suspected to be a principal component of the plasma arc of any launcher using insulators containing hydrocarbons, such as polyethylene or Lexan. If a suffi- ciently intense and soft (-2-3 keV) x-ray source is avail- able, it appears that it may be possible to detect the pre- sence of a low atomic number material such as carbon. Once it is possible to detect carbon, the detection of other dominant bore materials with higher 2 values, such as silicon, aluminum and copper, becomes routine. We describe in this paper the preliminary efforts at a proof- of-principle experiment using a flash x-ray source on the plasma of a free-running arc in the MIDI-3 accelerator.

0018-9464~1/0100-0165$01.~ 0 1991 IEEE

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MIDI-3

' T I Fig. 2. A schematic layout of the elliptically curved

x-ray crystal spectrometer. Bragg reflected monochromatic beams pass through a slit at the second foci and are detected by a scintillator- photomultiplier tube combination.

II. EXPERIMENTAL ARRANGEMENT

The experimental setup is shown in Fig. 1, and it consists of the MIDI-3 free running arc accelerator, a Hewlett-Packard FXR system, a PIN diode, and a spectrograph coupled to the MIDI-3 via quartz fiber optic cables. The rails in the MIDI-3 accelerator are approxi- mately 1.70 meters long with a separation of 1-cm and a rail height of 1-cm. The entire accelerator can be evacuated to a pressure in the tens of milliTorr range using a Welch 1397 mechanical pump. Just before firing it is backfilled with helium gas, and for the experiments reported here the filling pressure was 30 Torr .

The x-ray tube is a molybdenum-beryllium (MoBe) combination for the anode and the window, respectively. The x-ray tube and PIN detector were located 0.86 meter from the breech. A conductivity probe [7] was positioned 0.05 meter closer to the breech than the x-ray port. From this probe it was possible to determine when the plasma arc arrived at that position and to estimate when it would appear before the x-ray port.

Ultimately the x-ray detector system will consist of the elliptically curved analyzing crystal spectrometer shown in Fig. 2. In this arrangement the x rays originate at one foci of the elliptical arc, reflect and disperse from the crystal, focus through the other foci, and are detected by a scintillator-photomultiplier tube combination. A small aperture is located at the second foci to reduce any scattered radiation. A thin long crystal of pentaerythritol PET) cut along the 002-plane gives a 2d spacing of 8.76 8, , which allows the detection of x rays with energies as

low as 1.4 keV. To observe the in-bore radiation in the visible regime of

the spectrum one of the two fiber optic cables used was located 0.39 meter from the breech; the other was placed at the muzzle end looking straight down the bore of the device. The time-integrated spectra were made with Ilford HP5 film with a 1-meter Jarrell-Ash Czemy-Turner spectro raph set to record over the spectral range 2300- 6300 f . The spectrum from a mercury lamp was superimposed for wavelength identification. Argon gas is used as a starter in this lamp and argon lines appear for a short time after the lamp is turned on.

:::::::IF 4.01tl0

3.01tl0

Z.0Itl0

1.0ltl0

10.8 20.6 #.I 48.6 50.0 68.6 "I mrw iw)

Fig. 3. X-ray photon flux versus energy for the Mo/Be x-ray tube.

m. CONSIDERATIONS ON DETECTABILITY OF TRANSMITTED x RAYS

As x rays pass through matter they are absorbed and scattered, and the attenuation of the beam is given by

where Io = the intensity of the incident beam, I = the intensity of the transmitted beam, p = the total mass absorption coefficient (cm2/g), p = the density of the absorbing matter (g/cm3), and x = the thickness of the absorber (cm). When the absorber is a composite system then the above equation becomes

and the sum is over i. The mass absorption coefficient p is found to be independent of the physical state of the element, and to depend on the wavelength of the incident x ray and the atomic number of the element. It can be approximated, within 5% for 3L 2 0.3& by

p = C h n , (3)

with the values of C and IZ given in TABLE 1. [8] The total mass attenuation coefficient, p, consists of

the absorption coefficient, pa, and the scattering coef- ficient, ps, but, in the energy range I 10 keV, p, is small and can be neglected.[9]

A bremsstrahlung curve for our MoBe x-ray tube was obtained by recording ten shots through various filter materials having a range of different thicknesses. The fil- ters used were Mo(2), Cu, Ti, Ta, Zn, Ni, A1 and Sn, plus a calibration shot with no filter. The data from the ten shots are converted to current and given as input to the un- folding routine SHAZAM , which was made available to us by the Los Alamos National Laboratory. The photon flux from the x-ray tube as a function of energy, obtained in this manner, is shown in Fig. 3.

In high energy laboratory plasmas differential absorp- tion of the x rays emitted from the plasma are used to

I - -

TABLE1. Values of C and n for a few of the elements.

Z El

4 B e

6 C

7 N

8 0

13 Al 14 S i

26 Fe

29 Cu

- - - -

- - - -

7.951 - 6.745 - 1.743 - 1.381 -

h<hK C n

0.475 2.83

1.55 2.83

2.4 2.83

3.52 2.83

14.30 2.83

17.80 2.83

91.6 2.83

129.20 2.83

- - - - - - - - 1.10 2.66

1.50 2.66

12.54 2.66

17.22 2.66

determine the electron temperature and density. In a plasma accelerator the plasma arc temperature is estimated to be on the order of =5 eV, not the 1-10 keV temperature associated with energetic fusion plasmas. However, the absorption techniques used with x rays emitted from these hot plasmas can be adapted for a measurement of the mass density of the armature of the accelerator.[ 101 For the EML case, it is the properties of the plasma arc armature (acting as an x-ray filter) which are sought, given that the x-ray emission characteristics of the x-ray source are known.

The intensities of the x-ray beam transmitted through the plasma arc armature at each of four wavelengths are measured. By noting the change in intensity at each wave- length in the presence of the plasma arc compared to the intensity without plasma, it is possible to obtain self-con- sistent mass densities for each component of the plasma arc, as well as inferring the plasma composition,

N. RESULTS AND DISCUSSION

For a series of preliminary shots to check the operation of all the detection equipment and to determine the settings of the various triggers needed, the MIDI-3 was equipped with an array of diagnostic instruments as shown in Fig. l(b). A schematic of the electrical circuit, and of the triggering circuits, is shown in Fig. 4.

For each shot the capacitor bank was charged to 6 kV (=30 kJ for Ctotd= 1650 pF), and commutated into the rails upon the melting of the fuse. An electrothermal plasma was injected into the bore just before the fuse melted. The accelerator peak current for these shots was 40 kA.

The muzzle voltage and conductivity probe signals are given in Fig. 5. It appears that the voltage across the arc remained at - 650 volts. The estimated speed of the arc as it passed the position of the conductivity probe was 6 km/sec, and from the probe signal it appears that conducting media was present for about 50 psecs. This gives a length to the plasma arc on the order of 30-cm, which is longer than the estimated lengths of the arc for an accelerator pushing a projectile. The x-ray signal from the PIN diode obtained before a shot is given in Fig. 6

Fig. 4. Schematic diagram of the electhal circuit layout for the MIDI-3 operation. I -ignitron, C - capacitor bank, L - storage inductor, ETPI - electrothermal plasma initiator.

Fig. 5. Muzzle voltage and conductivity probe signals.

E A

E \ b.0

206.0n. 366.0ns TInl

166.0".

Fig. 6. Pulsed x-ray signal from HF' x-ray tube. EWHM = 100 nsecs.

Timing problems have thus far prevented us from getting a signal with the plasma arc in front of the x-ray port.

Spectrograms were made using quartz fiber optic bundles from the muzzle end of the accelerator and from in-bore at a position near the breech. Microdensitometer scans of spectra from each position are shown in Fig. 7. Unlike the in-bore spectra reported for electromagnetic launchers pushing a projectile [ 111, these spectra are a

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V. CONCLUSIONS The appearance of emission features in the MIDI-3

plasma arc implies the existence of a plasma which is not completely opaque, and may allow the use of optical diag- nostics for probing its characteristics. The presence of ablated material from the polyethylene insulator wall is manifest by the appearance of an ultraviolet cutoff.

With the solution of our timing problems and the com- pletion of the construction of the crystal spectrometer, it will be possible to make preliminary measurements of the mass density of the plasma arc, and determine the viability of extending this diagnostic to larger launchers. In any case, a flash x-ray source will not be suitable for such experiments, since it gives a stop-action view of a single time point in the action. To verify the ablation and viscous drag models will require two or more x-ray units placed along the length of the launcher, with each x-ray unit capable of producing an intense x-ray flux in the wavelength region of interest, for at least 100 psecs.

VI. ACKNOWLEDGEMENTS The authors wish to acknowledge the assistance of

Drs. Richard Blake and Robert Hockaday, Division P-14, Los Alamos National Laboratory for the use of the SHAZAM code; Dr. Y.Chia Thio and Mr. Eugene W. Sherwood, Division P-1, Los Alamos National Laboratory: and Mr. Robert Frierson, SAIC, Shalimar, FL .

Fig. 7. (a) A superposition of the muzzle end spectrum with the in-bore spectrum. The ultraviolet cutoff near 3000 A is evident. (b) Details of in-bore spectrum be- tween 3000 A and 4000 A. (c) Details of muzzle end spectrum for approximately the same range as (b). The designation CS is used for the mercury calibration source.

mix of continuum, absorption and emission l ines. However, an ultraviolet "cutoff' is evident in this case just as has been the case for projectile-pushing devices.

The in-bore spectrum is much fainter than the muzzle end one, as expected. The exposure time for the in-bore plasma arc was not greater than 50 psecs, as indicated by the conductivity probe, whereas the exposure of the muzzle end was for the entire shot. The Ca-I1 lines at 3933 8, and 3968 A appear in the in-bore spectra, but without evidence of being self-reversed, but they are self- reversed in the muzzle-end spectrum. Surprisingly, the A1-I lines at 3944 8, and 3961 A, between the Ca-I1 lines, appear and are strongly absorbed. An aluminum tube is used as the outer electrode for the electrothermal plasma initiator and is the likely source for the aluminum lines. Lines appear at 3882 A and at 3804 8, which maybe Cu-111 lines, which leads us to the conclusion that doubly-ionized copper might be present in the plasma. Lines of tungsten (3856 A) and beryllium (3813 A) are also seen. The appearance of these elements in the spectrum is expected as the conductivity probes are made of tungsten, and the G-10 tubes used to interface the x-ray tube and the crystal spectrometer are sealed with beryllium foils.

REFERENCES [l] J. V. Parker, W. M. Parson, C. E. Cummings and W. E. Fox,

AIAA 18th Fluid Dynamics and Plasmadynamics a n d h s e r s Conference, Report AIAA-85-1575, July 1985.

[2] R. A. Marshall and S. C. Rashleigh, J . Appl. Phys. 49, 2540 (1978).

[3] J. H. Batteh and J. D. Powell, J . Appl. Phys. 52, 2717 (1981)

[4] J. H. Batteh and J. D. Powell, J . Appl. Phys. 54, 2242 (1983)

[5] J. H. Batteh and J. D. Powell, IEEE Trans. Magnetics 20, 336 (984).

[6] J. V. Parker, 14th IEEE Inter. Conf. Plasma Sci., Washington, D. C., June 1987.

[7] Y. C. Thio, W. C. Condit, E. W. Sherwood, I. M. Thio and P. M. Stanley, Proc. 6th EMLaun. Assoc., AFATL, Eglin Air Force Base, Florida; May 2-3, 1989.

[8] G. J. Clark, The Encyclopedia of X-Rays and Gamma Rays , Reinhold Publishing Co., New York (1963). See pp. 10-13.

[9] R. L. Leighton, Principles of Modern Physics, McGraw-Hili Inc., New York (1960). See Chapter 12.

[ 101 R. Huddlestone and Leonard, Plasma Diagnostic Techniques,

I l l ] E. J. Clothiaux and K. K. Cobb, Proceedings of theThird Symposium on Electromagnetic Launcher Technology, 24- 26 April 1986, Austin, TX; p. 13.

Academic Press Inc., New York (1964). See Chapter 7.

.L ' This work supported by the Innovative Science & Technology Division of SDI0 through Contract No. DNA-001-85-C-0183 with the Defense Nuclear Agency.