Detonation Driven Shocks of High Mach NumberBoye Ahlborn and J. P. Huni Citation: Journal of Applied Physics 40, 3402 (1969); doi: 10.1063/1.1658201 View online: http://dx.doi.org/10.1063/1.1658201 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/40/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Note: A high Mach number arc-driven shock tube for turbulence studies Rev. Sci. Instrum. 84, 046102 (2013); 10.1063/1.4799176 Study of high Mach number laser driven blast waves in gases Phys. Plasmas 17, 112104 (2010); 10.1063/1.3491411 Mach number dependence of electron heating in high Mach number quasiperpendicular shocks Phys. Plasmas 17, 042901 (2010); 10.1063/1.3372137 Study of high Mach number laser driven blast waves Phys. Plasmas 11, 4968 (2004); 10.1063/1.1773553 Improving the mach number capabilities of arc driven shock tubes Rev. Sci. Instrum. 51, 1430 (1980); 10.1063/1.1136096

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~his labo.ratory in resistivity studies and superconducting tunnel­mg studies. They appear promising for other studies such as for Toma~h oscilla~ions and ellipsometry. They appear especially attractIve for thiS type of work since so many orientations are available. The fact that the substrate is a polished insulator especially enhances the possibility of their use in electrical measuremen ts.

The authors wish to acknowledge the aid of Dr. A. von Bassewitz and Mr. G. L. Wells in the development of the standard conditions for evaporation. .

* This work was supported by a grant from the U. S. Air Force Office of Scientific Research and a contract from the Advanced Research Projects Agency to the Materials Research Center of the University of North Carolina.

1 D. W. Pashley, Advan. Phys. 5,173 (1956). 'S. lno, D. Watanabe, and S. Ogawa, J. Phys, Soc. Japan 19, 881 (1964).

Sticking Coefficients for Atmospheric Gases Incident on Metallic Substrates*

W. R, OTTt AND W. L. FITE

University of Pittsburgh, Pittsburgh, Pennsylvania 15213

(Received 20 January 1969; in final form 27 March 1969)

The properties of getter materials have been investigated for years because of their applications, first of all, in the electronics industry,!" and more recently, with the development of getter­ion sublimation pumps in high vacuum technology.3 Most of the research has been directed toward the pumping characteristics of getter materials in selected atmospheres. However, there are some applications, for example, in atomic beam experiments, where it is necessary to specify the sticking coefficients, defined as the probability that a given gas molecule incident on a parti­cular surface will be trapped and absorbed by that surface. These sticking coellicients can be estimated from measured pumpdown times,4 but such information is less direct than desired. A dc-beam experiment in which the sticking coefficients were measured directly for beams of O2, N2, CO, He, and H2 incident on gettering surfaces of titanium, barium, or strontium is reported here.

The basic approach was to direct a low-density molecular beam against a liquid-nitrogen-cooled target surface which was being continuously vapor plated with the getter material and to determine the sticking coefficient from mass-spectrometric observations of the neutral gas densities. The experiment was carried out in a three-stage difierentially pumped atomic-beam machlne6 which provided a collimated beam in the main experi­mental chamber through apertures located in the source and buffer chambers. The target area was a sheet of macroscopically smooth copper plate loccated 15 cm downstream from t.he beam­entrance aperture and slightly angled to ensure that all reflected gas molecules remained in the main chamber. The getter evapora­tion source was mounted directly below the molecular beam. The getter materials were bound with tungsten wire and carefully heated inside a stainless-steel cylinder. An aperture was located so that the plating angle was about 45° with respect to the incident beam and so that the deposit covered about 6 cm'. The cooling of the target reduced the rate contamination of the metallic film by surface migration from adjacent uncovered areas' and may also increase the deposition efficiency of the getter materia1.6

With a shutter plated in the main chamber so that it deflected the molecular beam into the chamber before it reached the target area, the signal I, as measured with a electron-impact ionizer and quadrupole mass filter located behind the target area is given by

(1)

where So and So are the pumping speeds of the difiusion pump

TABLE I. Sticking coefficients for various substrates maintained at liquid-nitrogen temperatures.

Titanium Barium Strontium

Ar 0.07

0_ 0.4 0.45 0.2

CO 0.6 0.25 0.17

N_ O.4 0.0 outgas

He 0.0

H. 0.05

and of the getter surface and where nM and nR are the gas densities contributed by the molecular beam and by the residual gas in the system. With the shutter removed, the beam strikes the grtter surface directly and the signal 10 is given by

I_a:nR[So!(So+So) J+nM(1-a) [So!(So+ So) ] (2)

so that the sticking coefficient a is given by

(3)

where 10 is the signal with beam off. The results are listed in Table I. It is known that getter pumping

efficiencies depend on the thickness of the metallic film and on the rate of evaporation.- This effect was observed during all measurements and the table represents only those data obtained at an evaporation rate such that any further increase in evapora­tion rate yielded no improvement in sticking. Irregularities in the target surface and in the deposit itself4 may enable some of the beam molecules to miss the effective interaction areas in which case the results would underestimate the actual sticking coefficients. This effect was not investigated in detail; however, the variation of the coefficients from one run to another was less than 20%.

* Research supported in part by National Aeronautics and Space Ad­ministration under Grant No. NGL 39-011-013.

t Presently an NRC Postdoctoral Fellow at the National Bureau of Standards, Washington, D. C. 20234.

1 L. F. Ehrke and C. M. Slack, J. App!. Phys. 11, 129 (1940). 2 V. L, Stout and M. D. Gibbons, J. App!. Phys. 26, 1488 (1955). 3 L. Holland, J. Sci. lnstrum. 36,105 (1959). • A. L. Hunt. C. C. Damm, and E. C. Popp, J. App!. Phys. 32, 1937

(1961). • L. M. Clendenning. M.S. thesis, University of Pittsburgh, 1966 (un­

published). • S. Wexler. Rev. Mod. Phys. 30, 402 (1958).

Detonation Driven Shocks of High Mach Number

BOYE AHLBORN AND J. P. HUNI

Department of Physics, University of British Columbia.

Vancouver 8, B. C., Canada

(Received 5 February 1969)

In many shock tube experiments it is desirable to produce a plasma which is homogeneous, free of currents, and which can be predictably changed in composition and degree of ionization. Electromagnetic shock tubes are capable of giving high Mach numbers,', but the shock-heated plasma is often unstable, in­homogeneous, and perturbed by electric currents.'" Membrane shock tubes,3 on the other hand, do not reach the high Mach number obtained by the electromagnetic devices and, therefore, fall short of producing a high degree of ionization even when the

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20~-----------+-------------+------------~------------+-------------

FIG. 1. Mach numbers obtained in argon ... and air • with detonation driver (ethane plus oxygen) compared to combustion driven shocks in air 000 (Witliff) and pressure driven shocks in air - - - (Oertel) and in argon --- (Nett).

15~------------+-----------~.~~-----------r-------------+----------~~ .. P, = 3 I rr)/:,"::·

, • 0

,., 0

M

o o

o 0 oO/t 0

... 1 ~ 00

101---------- t "I ----0 0 0 1---f 0

000

-"""

--' -'

I 0 /' 0

" " " " /'

101 102

most effective driver gas is used4 (hydrogen at pressures up to 102 atm). Efforts have been made to increase the sonic speed in the driver gas of the membrane shock tube, thereby extending the working range of this type of tube. Wittliff et al. and others (see Oertel5) used a combustible mixture of hydrogen and oxygen as driver gas and indeed obtained higher Mach numbers than possible with cold hydrogen at the same pressure ratio (Fig. 1). However, homogeneous ignition of the driver gas is a problem.

We have extended the range of Mach numbers in a membrane shock tube significantly by heating and compressing the driver gas in a concentric detonation. The driver gas, a mixture of ethane and oxygen (or hydrogen and oxygen) is ignited electrically, but the ignition spark has long decayed, when the membrane bursts so that the shock is completely free of electrc,­magnetic fields and currents.

The resulting shocks are fast, stable, and reproducible. About 0.1 % of the chemical energy is actually deposited in the shock heated gas. The device is inexpensive to assemble and is presently operated at intervals of about 5 min. It is very likely that even higher Mach numbers may be obtained with a similar device of larger dimensions for reasons given below.

The design of our shock tube (Holiboom I) is indicated in Fig. 2. A detonable mixture of O2 and C2H. is ignited by a spark SP in the ignition side of the detonation chamber, so that a cylindrical detonation wave is formed. This wave turns around the edge of the center plate CP and is subsequently forced to form a concentric implosion wave. At large radii r ",R= 10 em,

o

P, C

HOC/800M I

FIG. 2. Concentric detonation chamber and shock tube.

/' /'

/' /'

/' /'

/' /'

/'

'}A 10 3 10' 105

this imploding front can be considered plane and the pressure in the detonation zone is about PD ",40ps, where po is the filling pressure in the detonation chamber. During the collapse, however, this detonation pressure increases asymptotically and reaches p4=2.5. PD",,100p. at radii smaller than 0.07R (which is the en­trance cross section to the shock tube). This pressure increase was calculated with the Witham" model which was shown to be in good agreement with measurements.7 •S Equally important, the temperature also increases in the over compression phase, al­though it is already above the combustion temperature in a normal, plane detonation. Consequently one obtains temperatures and pressures which by far exceed the values possible in a safe combustion. Further the heating and compression is so rapid that a thin (Mylar) membrane suffices to separate driver and test gas and comfortably stands the initial pressure difference between driver and test section. The membrane bursts practically without jitter, so that the timing of rotating mirror cameras for the observation of the shock front is no problem.

The produced luminous fronts in the 1-in.-diam test section were investigated with a streak camera (j/8) using Polaroid film. Figure 3(a) shows a shot in air, pl=0.7 Torr taken with 10 000 ASA film. The position of the shock front is made visible by an obstacle at 0 (25 tube diameters away from the membrane), which blocked only the center section of the tube (see Fig. 2). The location of the contact surface can be seen on the negative. It is unfortunately not well distinguished on the positive, which only shows the luminous front at a position between shock and contact surface. Figure 3(b) is taken in argon, Pl=2.5 Torr at higher magnification (3000 ASA film). It appears that most of the detonation products enter the observation section only after about 130 p'sec [point A on Fig. 3 (a)]. Probably most of the detonation products are reflected in radial direction, as if the membrane had not given away, and are only eventually blown into the low pressure region. Mach numbers for the leading edge of the luminosity front (which are lower limits for the shock Mach numbers) are given in Fig. 1 for comparison with cold H2 and combustion driven shocks. Addition of He to the driver gas has little effect, but an increase of Ps, the filling pressure in the detonation chamber, helps more to increase the Mach number than a reduction of test gas pressure Pl. A hydrogen-oxygen mixture as driver yields a very similar run of Mach number with pressure ratio p.I Pt. It is interesting to notice that the equilibrium ionization a= 25% in our M = 18 shocks in argon is about two

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A

20 )Jsec

t t

FIG. 3. (a) Incident wave and reflected shock in air. PI =0.7 Torr. (b) Shock in argon at PI =2.5 Torr.

.. X --+-----<1---+-•• X 10em o (b)

3em o (a)

times as high as in the M = 14 shocks obtained by Netta in the membrane shock tube at the same initial pressure Pi = 0.1 Torr.

Pressure and temperature increase in the detonation products, while the detonation front collapses towards the center of the driver section. The Witham model predicts that pn and Tn depend only on the position variable r / R. Therefore by increasing the radius R of the detonation chamber, higher average tem­peratures and higher average pressures across the diameter of the shock tube can be obtained. It seems then quite likely that even higher Mach numbers should be possible with this detonation driven membrane shock tube.

We would like to thank R. Beck for a stimulating question

Abrupt Intensity Transitions in RF Generated Plasmas*

J OH" A. THORNTON

Telic Corporation, .... r.,'anla ..Jlonica. California 9040-1

(Received 29 January 1969; in final form 31 March 1969)

This communication describes some recent experimental ob­servations concerning nonlinear transitions, involving order-of­magnitude increases in the visible luminosity, which have been observed in rf-generated plasmas in several geometries.

The existence of two distinct operating modes for plasmas generated within solenoidal-coil configurations [such as that shown in Fig. 1 (A)], and the transitions between them, was recognized as early as 1929 by MacKinnon.' He concluded that such discharges are generally ignited into a relatively dim (glow) mode by the capacitive field2 and undergo an abrupt transition into an intense bright mode or ring discharge at higher applied potentials due to the inductive field. Subsequent experiments (for example, see Refs. 3 and 4) have clearly substantiated MacKinnon's conclusions. However, published experimental data pertaining to the point of transition are very limited. Thus, although some suggestions have been made,' the details of the causative mechanism are not understood.

Figure 2 summarizes data that were obtained with a S.4-cm i.d. by 20-cm long water-cooled discharge vessel which was placed within an 8.S-cm diamX24-cm long, 14 turn, solenoidal coil. Helium was the test gas and the experiments were conducted at a frequency of 4 Mc. The data were obtained by increasing the coil voltage at a fixed pressure and noting the point of ignition of the glow discharge, line A,A2, and the transition to the bright

which initiated this experiment. The work was supported by a research grant of the Atomic Energy Control Board of Canada.

I W. B. Kunkel, Plasma Physics in Theory and Application (McGraw-Hill Book Co .. New York, 1966), p. 339.

2 H. Munlenbruch, Phys. Fluids Suppl. I-II (1969). 3 H. Nett, Rep. IPP 3/43 Institut fiir Plasma Physik, Garching (1966). 4 The Mach number of a pressure driven shock wave is a function of

P6/Pl (driver to test gas pressure) and of the sonic speed ratio as/ai' 'H. Oertel, Strossrohre (Springer-Verlag, Berlin, 1966). 'G. B. Witham, J. Fluid Mech. 4, 337 (1958). , J. H. Lee and B. H. Lee, Phys. Fluids 8, 2148 (1965). 8 B. Ahlborn and J. P. Huni, AIAA J. 7, No. 7 (1969).

mode, line A3A4. In the glow-discharge region between transition lines A,A2 and A3A4, increases in voltage at a fixed pressure were found to increase the power delivered to the plasma (determined calorimetrically), the electron density (determined from double probe and microwave measurements), and the intensity of the emitted spectra.s Glow-discharge power levels varied from 20 to 200 W, and the electron densities were as high as a few times 10'2 em-a at the point of transition to the bright mode. The bright-mode transition was accompanied by an increase of from 3 to 10 in the power absorbed by the plasma, and an increase of from 10 to 100 in the intensity of prominent lines in the helium emission spectra, the larger increases occurring at the higher pressures.

As with the previous investigations, the bright mode could not be achieved with the shorted coil configuration shown in Fig. 1 (A) or the ring-electrode configuration shown in Fig. 1 (B), even at voltages as high as 10000 V peak-to-peak. When a shield9

consisting of a system of 3-mm-wide strips of electrically con­ducting cement, spaced 3-mm apart and oriented parallel to the discharge-tube axis, was added to the outside of the discharge vessel, the ignition line A,A2 assumed the new position B,B2Ba. For pressures below about 0.1 Torr ignition was directly into the bright mode. At high pressures the plasma behavior was qualita­tively the same as in the case without the surface coating, except that the respective transitions occurred at higher coil voltages, i.e., glow mode along line B2Ba and bright mode along line B2B4. When a more uniform end-to-end shield was formed by spraying a thin layer of the conducting cement on the outside of the dis­charge tube (surface resistivity about 180 fl/square), ignition was primarily a direct transition into the bright mode. However, at a pressure of about 1 Torr and coil voltages of about 5000 V peak-to-peak, weak and localized capacitive discharges were

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