Modified pulsed valve for supersonic jet applicationsA. Auerbach and R. McDiarmid Citation: Review of Scientific Instruments 51, 1273 (1980); doi: 10.1063/1.1136419 View online: http://dx.doi.org/10.1063/1.1136419 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/51/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A pulsed mixing valve for generating reactive complexes in a supersonic expansion Rev. Sci. Instrum. 72, 3375 (2001); 10.1063/1.1386902 Pulsed valve for supersonic nozzle experiments at cryogenic temperatures Rev. Sci. Instrum. 62, 2038 (1991); 10.1063/1.1142362 Electron diffraction investigation of pulsed supersonic jets Rev. Sci. Instrum. 60, 1223 (1989); 10.1063/1.1140294 A hightemperature pulsed solenoid valve for supersonic jet introduction up to 550°C Rev. Sci. Instrum. 60, 499 (1989); 10.1063/1.1140408 A simple pulsed valve for use in supersonic nozzle experiments Rev. Sci. Instrum. 51, 1128 (1980); 10.1063/1.1136387

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60

50

40

In

"" <:

30

~ 20

10

0 .2 10 100 200

Frequency ( MHz)

FIG. 2. Frequency response of the circuit.

Vector MHT-250) are utilized for the output stage. They are connected to the cascode amplifiers through two bipolar transistors. The use of emitter-followers isolates the input and output impedances and the open

loop gain of the front stages is retained. The circuit should be carefully shielded and grounded. The dashed lines shown in Fig. I indicate different compartments of the machined brass housing. All resistors used are metal film type. The measured frequency response of the circuit is shown in Fig. 2. The voltage gain is about 42 dB in the frequency range between 2 and 70 MHz. The maximum input signal level is limited by the output stages to 4 x 10-3 V. The internal noise referred to input of the preamplifier is measured to be 20 x 10-9 V IVHZ. The auxiliary audio frequency input circuit shown in Fig. 1 is for use in regular SQUID operation. By using a cw rf input signal the usual triangular pattern2

can be displayed using a demodulation circuit (not shown in Fig. 1).

We would like to thank Prof. W. H. Parker for the loan of a preamplifier when this work was undertaken.

a) Work performed under auspices of U.S. Department of Energy. I C. M. Falco, Phys. Tech. 9, 148 (1978). 2 R. P. Giffard, R. A. Webb, and J. C. Wheatly, J. Low Temp.

Phys. 6,533 (1972).

Modified pulsed valve for supersonic jet applications A. Auerbach and R. McDiarmid

Laboratory of Chemical Physics. National Institute of Arthritis. Metabolism and Digestive Diseases, National Institutes of Health, Bethesda. Maryland 20205

(Received 15 April 1980; accepted for publication 29 May 1980)

The modification of a commercial, pulsed, piezoelectric valve to enhance its applicability as a moderately rapid (-0.3 ms), pulsed, supersonic molecular beam source is described.

PACS numbers: 47.55.Cy

In the recent past, there has been an increasing use of supersonic molecular jets for spectroscopy and kinet­ics l

- 5 predominantly because the resultant cooling of the molecules greatly simplifies the experimental anal­ysis. The initial experimental systems employed con­tinuous flow nozzles,l which necessitated the use of large pumping systems. However, since most experi­ments involved the use of pulsed lasers, the advisability of using pulsed jets with their higher feasible through­puts, hence larger permissable orifice diameters, more intense beams, and more efficient cooling, became apparent. 3.6 Pulsed valve designs are divisable into two classes: Very fast (~20 fJ-s) and moderate speed valves. The former·6

,7 involve the rapid discharge of several kilovolts and require meticulous rf shielding. The latter2

,5 are usually adapted from commercial devices and are simpler to operate. They differ from the former in not being true "pulsed" valves, but rather valves that can be rapidly opened and closed by the application and

removal of a driving voltage. The pulse duration of such valves equals the sum of the time required to open the valve, the time required to close the valve, plus any additional time the driving voltage is applied. To obtain the minimum pulse duration, it is necessary to insure that the driving voltage is applied for the minimum time required to open the valve. In this note, we describe a design modification of the Veeco pulse valve (PV-IO) to enhance its applicability for the generation of moderately rapid (~0.3 ms) pulsed supersonic molecular beams.

The commercially available PV-IO valve,8 Fig. lea), is unsuitable for the generation of pulsed supersonic molecular beams for two reasons: the orifice throat-to­diameter ratio, ~10, is much too large and the distance from the orifice to the valve exit is too long. To generate good quality molecular beams, the orifice throat-to­diameter ratio should be small (_1).9 The minimum distance from the orifice to the exit obtainable by simply truncating the exit tube is 1-2 cm. This far downstream

1273 Rev. Sci.lnstrum. 51(9), Sept. 1980 0034-6748/80/091273-03$00,60 © 1980 American Institute of Physics 1273

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A

Adjustment

Screw 13)

Crystal

Support Ring

Inlet

Outlet

Piezoelectric

Crystal

Coax Connector

8

FIG. I. (A) Unmodified leak val\c. The relevant part, llf the valve are indicated. (B) Modified parI, of the valve A. (e) All expansion of the modilied valve seal and valve seat.

from the source. the gas density is greatly reduced from its initial magnitude and may be insufficient for some experiments (i.e .. multiphoton spectroscopy).

To adapt the PY-IO valve for supersonic molecular beam applications. the modifications illustrated in Fig. l(b) and in detail in Fig. I(c) were made. The origi­nal valve scat was bored out and a duplicate was ma­chined and silver soldered into a more external location in the valve housing [Fig. I(b)]. The original Yiton seal was detached from the piezolcctric crystal and effec­tively extended with an aluminum rod (0.6 x 1.5 cm in the current valve 10). one end of which was cemented (Eastman 910 adhesive) to the piezoelectric crystal and the other to the Yiton gasket. These modifications re­duced the orifice throat .. to-diameter ratio and moved the orifice exit to the exterior of the valve. as desired. In addition, the new valve seat can be machined to any configuration desired to obtain high quality molecular beams. and the nozzle orifice can be made the desired diameter. For example, the cone-shaped exit port can be machined into a trapezoid to eliminate boundary layer formation by the gas issuing from the orifice. II While longevity studies have not yet been performed on the modified valve, our valve has run for over 100,000 pulses.

The performance of the modified valve is illustrated in Fig. 2 at two different time scales. In each figure, the trace originating at the bottom of the scan is the 100 YDC trigger pulse applied to the valve and the trace originating at the top of the scan shows the pressure surge into the vacuum changer as detected by a nude Bayard-Alpert gauge probe positioned 10 cm downstream from the valve exit and amplified by the Bayard-Alpert gauge control amplifier (Cooke model ICG 20). The amplifier time constant on the 10- 4 torr scale employed is 33 f-tS.

The background pressure in the chamber was 5 x 10 5

torr and the pressure behind the nozzle was 1 atm at room temperature. The valve has been run with both air and butadiene. The estimated flight time from detector to the probe is ~0.2 ms. Each figure represents several

1274 Rev. Sci. rnstrum., Vol. 51, No.9, September 1980

successive valve pubes. The dimensions of the chamber in the current configurations were insufficient to permit a background-free expansion of the beam for the length of time the val ve was open. Consequently. the gas input appears as a rise in pressure in the vacuum chamber. which. although not shown. increases linearly with the length of the time the valve is open. From the rate of this linear increase in pressure in the approximately 100 I volume of the vacuum chamber, the throughput of the modified valve was estimated to be around 5 torr lis, in agreement with the original specifications. Some variability of throughput may be obtained by careful manipulation of the crystal mounting ring adjustment screws. The time delay between applying the trigger pulse and the valve opening is estimated to be 0.6 ms. The time required to fully open the valve is estimated, from the expanded curve, to be 0.15 ms, an order of mag­nitude more rapid than specified by the manufacturer. The time required for the valve to close is similarly esti­mated to be around 0.1 ms. The minimum pulse duration so far obtained is 0.3 m~ FWHM, twice the theoretical minimum, the sum of the opening and closing times. The traces in Fig. 2 were obtained with a trigger repetition rate of 5 Hz. Rates as high as 20 Hz have been used.

(/)

~ o >

0.5 msec/div 0.1 msec/div

FlO. 2. Rate of response of the modified valve. The time scale is 0.5 msfdiv for the left-hand figure and 0.1 msfdiv for the right-hand figure. Seale for the trigger pulse (trace starting at lower left) is 20 V fern. Scale for the signal pulse is 1 V/cm. The delay between the valve opening or closing and the pressure change is a combination of the response time of the valve (-0.15 ms) and the flight time from the valve to the detector (~0.2 ms).

Notes 1274

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In our system, the main limitation to higher rates is the resultant rise in background pressure to the safe operat­ing threshold of our diffusion pump (10-3 torr). For a given backing pressure and pumping rate, higher rates can be obtained with a smaller diameter orifice at the expense of beam intensity.

In conclusion, we have shown how a commercially available leak valve can be modified to produce a mod­erately fast (~0.3 ms FWHM) nozzle source for super­sonic jet formation.

1 R. E. Smalley, L. Wharton, and D. H. Levy, J. Chern. Phys. 63, 4977 (1975); R. E. Smalley, D. H. Levy, and L. Wharton, 1. Chern. Phys. 64,3266 (1976).

Vibrating mirror drive circuital

Alan R. Hawthorne

2 D. Zakheim and P. Johnson, J. Chern. Phys. 68,3644 (1978). 3 M. G. Liverman, S. M. Beck, D. L. Monts, and R. E. Smalley,

J. Chern. Phys. 70, 192 (1979). • P. M. Dehmer and 1. L. Dehmer, J. Chern. Phys. 70,4574 (1979). 5 F. M. Behlen, N. Mikami, and S. A. Rice, Chern. Phys. Lett. 60,

364 (1979). 6 W. R. Gentry and C. R. Giese, Rev. Sci. Instrum. 49,595 (1978);

ibid, J. Chern. Phys. 67,5389 (1977). 1 The valve described in Ref. 6 is now commercially available from

Beam Dynamics, Minneapolis, Minnesota. B Vee co Instruments Inc., Terminal Drive, Plainview, NY 11803. 91. B. Anderson, R. P. Andres, and 1. P. Fenn, Adv. Chern. Phys.

10, 275 (1966). 10 The length of the rod should be approximately equal to the distance

the valve seat is moved. Minor length discrepancies can be accompanied by adjusting the crystal mounting ring adjustment screws.

11 N. Abuaf, 1. B. Anderson, R. P. Andres, 1, B. Fenn, and D. R. Miller, Rarified Gas Dynamics (Academic, New York, 1967), Vol, 2. p, \317.

Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830

(Received 7 Aprii1980; accepted for publication 14 May 1980)

A self-starting, stable, and symmetric driving circuit is described for use with a taut-band optical modulator such as a Bulova L50 or L51. The astable multivibrator consists of a capacitor, two resistors, three adjustment potentiometers, two diodes, and a dual operational amplifier. The circuit is particularly useful in applications requiring a small modulation amplitude that cannot be obtained from the factory-supplied driver.

PACS numbers: 84.30.Ng, 07.50. + f, 07.60. - j

A wide range of applications require the scanning or chopping of uv, visible, or ir light. A low-cost, low­power, reliable method for obtaining a modulated light beam is to use a vibrating mirror or a chopper such as a Bulova L50 or L5I. These scanners or choppers operate with a resonant taut-band providing a fixed-frequency natural oscillation from ~5 Hz to -400 Hz. A drive coil and a sense coil are used with an electronic circuit to amplify the sense coil signal and to provide an adjust­able-amplitude drive signal. The Bulova 4A driver circuit designed for use with these taut band oscillators, how­ever, is limited in the minimum stable amplitude that can be provided. For applications that require low ampli­tude modulation, such as the wavelength modulation required in a derivative spectrometer, 1 an alternative drive circuit must be used. This driver also suffers from lack of symmetry and amplitude stability as reported by Podolski. 2 The circuit described in Ref. 2, which is designed to drive a tuning fork chopper, overcomes the stability and symmetry problems associated with the Bulova driver. This circuit consists of a comparator connected to the pickup coil followed by a four-pole Butterworth filter whose cutoff frequency is set to be the same as the resonant frequency of the tuning fork. The 1800 phase shifted output signal of the filter is used to drive the taut band coil.

The taut-band modulator is a mechanically resonant

device driven by an electronic circuit which amplifies and phase shifts the signal from a magnetic sensing coil to provide the driving signal to a magnetic drive coil. The amplitude of the modulation is adjustable on the 4A driver circuit, but the oscillation frequency is fixed by the mechanical properties ofthe taut band. A slight noise or motion is required in the sense coil to begin the oscillation. This lack of an active starting signal may reduce the reliability of the modulator and at times even necessitates manual initiation of motion by tapping the modulator support.

Figure 1 shows the signal to the drive coil of a 250-Hz

'~~J 0 0 0.0 t-~---+-o-====-"---....... -===-,"----'-

FIG. 1. Bulova 4A driver circuit with 250-Hz L50 vibrating mirror: (a) drive signal, (b) sense signal.

1275 Rev. Sci. Instrum. 51(9), Sept. 1980 0034-6748/80/091275-02$00.60 © 1980 American Institute of PhysIcs 1275

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