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Hot sounds from Orsay
RESEARCH
AND DEVELOPMENT
Hot sounds from Orsay Research into the plasma of an alkalienhanced flame has led to a continuously variable generator of acoustic waves from audible to ultrasonic frequencies. The Rumanian physicist Dmitry D. Sinitean (on leave from the University of TimQoara) has been investigating techniques for producing and detecting acoustic waves from the plasma of a hot flame at the plasma physics laboratory of the FacultC des Sciences &Orsay.1-3 The apparatus consists of a methane flame - not a diffusion flame buming in air but a premixed flame of methane with molecular oxygen into which is introduced a wick soaked in potassium hydroxide and two tungsten electrodes maintained at a potential difference of 250-500 V. To guard against air currents and the formation of standing waves, the flame is walled in by an enclosure lined with glass-fibre wool. An ac signal applied between the electrodes determines the frequency of the sound waves produced by the plasma of the flame while their intensity depends on the voltage drop and on the rate of flow of oxygen. A 500 V potential difference between the electrodes in a potassiumenhanced flame yields a 50 dB sound ressure for oxygen intake at 29 cm ! s-l (Fig.1).
The luminous output varies in step with the acoustic output: the thermodynamic equilibrium (temperature and density) is simply shifted by adjusting the voltage between the electrodes (or alternatively, adjusting the oxygen flow). There is yet a third means of detecting acoustic radiation from the plasma of a seeded flame.2 A fxed electrostatic probe is positioned directly in the flame; fluctuations in the plasma ion current will cause the current in an external circuit containing the probe to vary as well. Because the ion current is closely correlated with the acoustic waves generated in the flame, measurement of the current fluctuation in the external circuit will provide, with suitable calibration, a sensitive indicator of sound-wave intensity. At ultrasonic frequencies, the sensitivity of the probe is superior to that of a standard condenser microphone.
For the present Sinitean claims that his apparatus is continuously tunable in the frequency range 20-20 000 Hz, but another order of magnitude in ultrasonic frequencies should be readily attainable by the same technique. On the issue of commercial application, Sinitean has so far made no written comment.
References Sinitean, D. D. La variation de la lumi&e Bmise par le plasma de la flamme ensemencde, sous l’itiuence des on&s acoustiques dan cette flamme, Comptes Rendus de Z’Acrrdcmic des Sciences de Paris, S&e B 273 (1971) 243 Sinitean, D. D. D&e&ion des ondes acoustiques dans le plasma de la flamme ensemencde 51hide de la modulation de la charact&istique volt-amp&e d’une sonde &ectrostatique, Ibid 273 (1971) 265 Sinitean, D. D. Production d’ondes acoustiques par le plasma d’une flamme act&e, Ibid 275 (1972) 521
Potassium serves as an alkali metal of low ionization potential, and by increasing the ratio of ionized to neutral particles it can enhance the radiative output of the plasma. When the flame is seeded with potassium at a partial pressure of 10m5 atmospheres and a temperature of 1 700 K, the density of charged particles will be 3.1 x lOlo cmm3 - nevertheless, even the seeded flame will be rather weakly ionized, the ratio of neutral atoms to ions being 1 000 to 1. A photomultiplier and filtering amplifier are used to monitor luminous output on an oscilloscope, while a microscope connected to an acoustic spectrometer monitors the intensity of the sound. Using the spectrometer, one can calibrate the oscilloscope in sound-pressure units to facilitate intensity measurements.
ULTRASONICS.
MAY 1973
200
300
400
Fbtentiql difference Fig.1 at: 1 -
Acoustic radiation from at 1 000 22.8 cm3 s-l; 2 - 27.8 cmpiasy s ;3-33.4cm
500
[VI
H~.~C_yrves ara of con;tfyt oxygen flow s ;4-43.3cm s ;5-54.2cm3s-t
99
AND DEVELOPMENT
Hot sounds from Orsay Research into the plasma of an alkalienhanced flame has led to a continuously variable generator of acoustic waves from audible to ultrasonic frequencies. The Rumanian physicist Dmitry D. Sinitean (on leave from the University of TimQoara) has been investigating techniques for producing and detecting acoustic waves from the plasma of a hot flame at the plasma physics laboratory of the FacultC des Sciences &Orsay.1-3 The apparatus consists of a methane flame - not a diffusion flame buming in air but a premixed flame of methane with molecular oxygen into which is introduced a wick soaked in potassium hydroxide and two tungsten electrodes maintained at a potential difference of 250-500 V. To guard against air currents and the formation of standing waves, the flame is walled in by an enclosure lined with glass-fibre wool. An ac signal applied between the electrodes determines the frequency of the sound waves produced by the plasma of the flame while their intensity depends on the voltage drop and on the rate of flow of oxygen. A 500 V potential difference between the electrodes in a potassiumenhanced flame yields a 50 dB sound ressure for oxygen intake at 29 cm ! s-l (Fig.1).
The luminous output varies in step with the acoustic output: the thermodynamic equilibrium (temperature and density) is simply shifted by adjusting the voltage between the electrodes (or alternatively, adjusting the oxygen flow). There is yet a third means of detecting acoustic radiation from the plasma of a seeded flame.2 A fxed electrostatic probe is positioned directly in the flame; fluctuations in the plasma ion current will cause the current in an external circuit containing the probe to vary as well. Because the ion current is closely correlated with the acoustic waves generated in the flame, measurement of the current fluctuation in the external circuit will provide, with suitable calibration, a sensitive indicator of sound-wave intensity. At ultrasonic frequencies, the sensitivity of the probe is superior to that of a standard condenser microphone.
For the present Sinitean claims that his apparatus is continuously tunable in the frequency range 20-20 000 Hz, but another order of magnitude in ultrasonic frequencies should be readily attainable by the same technique. On the issue of commercial application, Sinitean has so far made no written comment.
References Sinitean, D. D. La variation de la lumi&e Bmise par le plasma de la flamme ensemencde, sous l’itiuence des on&s acoustiques dan cette flamme, Comptes Rendus de Z’Acrrdcmic des Sciences de Paris, S&e B 273 (1971) 243 Sinitean, D. D. D&e&ion des ondes acoustiques dans le plasma de la flamme ensemencde 51hide de la modulation de la charact&istique volt-amp&e d’une sonde &ectrostatique, Ibid 273 (1971) 265 Sinitean, D. D. Production d’ondes acoustiques par le plasma d’une flamme act&e, Ibid 275 (1972) 521
Potassium serves as an alkali metal of low ionization potential, and by increasing the ratio of ionized to neutral particles it can enhance the radiative output of the plasma. When the flame is seeded with potassium at a partial pressure of 10m5 atmospheres and a temperature of 1 700 K, the density of charged particles will be 3.1 x lOlo cmm3 - nevertheless, even the seeded flame will be rather weakly ionized, the ratio of neutral atoms to ions being 1 000 to 1. A photomultiplier and filtering amplifier are used to monitor luminous output on an oscilloscope, while a microscope connected to an acoustic spectrometer monitors the intensity of the sound. Using the spectrometer, one can calibrate the oscilloscope in sound-pressure units to facilitate intensity measurements.
ULTRASONICS.
MAY 1973
200
300
400
Fbtentiql difference Fig.1 at: 1 -
Acoustic radiation from at 1 000 22.8 cm3 s-l; 2 - 27.8 cmpiasy s ;3-33.4cm
500
[VI
H~.~C_yrves ara of con;tfyt oxygen flow s ;4-43.3cm s ;5-54.2cm3s-t
99