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Radiometric measurements on a plasma induced by a proton beam
Volume 78A, number 3
RADIOMETRIC
PHYSICS LETTERS
MEASUREMENTS
F. EUVE, M. FITAIRE,
4 August 1980
ON A PLASMA INDUCED BY A PROTON BEAM
J. MARGOT, A.M. POINTU and M. VIALLE
Groupe Plasmad ‘OrigineNuclt!aire,Laboratoire de Physique des PlasmasI, Universitt? Paris-Sud, 9140.5 Orsay, France Received 27 February 1980 Revised manuscript received 19 May 1980
The plasma radiation temperature of a neon plasma created by a dc proton beam has been measured. Variations of this temperature versus gas pressure and beam intensity are presented and related to the theoretical variation of the electronic distribution function.
Introduction. Characterization of the electron velocity distribution function, f(w), is very important for an understanding of direct nuclear pumped (DNP) laser operation. As shown by theoretical calculations [l-3] this distribution is maxwellian in its low energy part, and for energies higher than 1 eV, the maxwellian population becomes negligible as compared to a tail constituted by fast electrons whose number is only one thousandth part of the total population, but nevertheless very efficient in the molecule excitation processes. The plasma radiation temperature, TR, which is highly dependent on this distribution function, is thus interesting to know. Evaluation of the radiation temperature TR has been performed on a proton-beam induced plasma, using a microwave interferometer. Measurements showing the variation of TR versus gas pressure and beam intensity are reported.
where m is the electronic mass, K the Boltzman constant, and v the electron-neutral collision frequency. The plasma is generally not perfectly absorbing, and thus it radiates as a grey body whose absorptivity at angular frequencyo,A, = 1 - exp[-o(o)L] , is determined by the loss coefficient, cy, acting on the whole plasma length, L. When inserted in a wave guide, the plasma radiates, per unit angular frequency, a powerP, =A,KTR. A microwave interferometer (fig. l), allows measuring TR without the knowledge of A,. It uses a reference black-body noise source, with temperature To, which illuminates the plasma; the plasma source is chopped, thus allowing synchronous detection of the microwave power, P,, and Put, respectively reflected and transmitted by the plasma, depending on the position of a rotative switch. It can be shown [5] that
Measurement principle. The microwave radiation of a slightly ionized gas is due to electron-neutral bremsstrahlung. It is well known [4] that the equivalent black-body radiation temperature is: TR = m iv(w)f(w)w4 0
dw K ~v(w)(af/aw)w3
dw,
0 (1)
’ Laboratoire associ6 au Centre National de la Recherche Scientifique.
Klystrm
Fig. 1. Sketch of the interferometric
cl radiometer device.
257
PHYSICS LETTERS
Volume 78A, number 3
I
TRMl
6000
TR
(K)
t
2ccl
300
LOO
so
Fig. 2. Variation of radiation temperature in neon: (a) With beam intensity; O-200 Torr, 2.1 MeV. (b) With gas pressure; 4 2.5 MA, 2 MeV; + 3 PA, 2 MeV.
S=Pwr+Pwt
= Au K(2Tk - To).
(2)
4 August 1980
and beam intensity. We briefly show here how these variations can be understood. The electron-neutral collision frequency in neon is well approximated by the relation v(w) - w~.~. Due to the fact that the product vw4f(w) does not quickly converge with increasing w, it is clear from eq. (1) that TR depends on the whole distribution function and, in particular, on the tail length. This tail length is related to the mean energy of the primary electrons created by the beam. Nevertheless, the dependence of TR on the experimental parameters results from the variation in the shape of the distribution function, mainly due to the variation of its maxwellian-like part. Processes involved in the equilibrium of this part are (i) heating by coulombian collisions with the electrons of the tail, (ii) cooling by electron-neutral collisions and (iii) recombination which makes disappear the slowest electrons. An increase in the beam current increases the number of primary electrons, thus tending to heat the main part of the distribution. Simultaneously, the total density increases so that the recombination effect, involving slow electrons, becomes more efficient. Obviously these two processes lead to an increase in TR . On the other hand, incre,asing the gas pressure mainly results in increasing the electron-neutral collision effects, thus leading to a lower value of TR.
To is adjusted with a calibrated attenuator so as to observe that S = 0, thus allowing one to deduce the TR value.
This work has been supported by the Direction des Recherches, Etudes et Techniques (DRET) and by the Centre National de la Recherche Scientifique.
Results and discussion. The method just described has been applied to a plasma created by a dc proton beam (1-5 PA, 2.1 MeV, 4 cm in diameter) impinging on a gaseous target (neon, 10 to some 100 Torr). The observed plasma part is contained in a cylindrical Pyrex tube (1 cm in diameter), inserted in an X band wave guide. Microwave signals are detected using a mixer followed by an IF amplifier (100 MHz), a crystal and a synchronous detector (fig. 1). Some measurements are reported in fig. 2, showing the variation of TR as a function of gas pressure
References
258
[l] H.A. Hassan and J.E. Deese, Phys. Fluids 20 (1977) 1586. [2] B.D. de Paola et al., Proc. 1st Intern. Symp. on Nuclear induced plasma and nuclear pumped lasers (Les Editions de Physique, Orsay, 1979) p. 221. [ 31 Groupe Plasma d’origine Nudaire, submitted to 3. de Phys. [4] G. Bekefi, Radiation processes in plasmas (Wiley, New York, 1966). [5] J.F. Delpech and J.C. Gauthier, Rev. Sci. Instrum. 42 (1971) 958.
RADIOMETRIC
PHYSICS LETTERS
MEASUREMENTS
F. EUVE, M. FITAIRE,
4 August 1980
ON A PLASMA INDUCED BY A PROTON BEAM
J. MARGOT, A.M. POINTU and M. VIALLE
Groupe Plasmad ‘OrigineNuclt!aire,Laboratoire de Physique des PlasmasI, Universitt? Paris-Sud, 9140.5 Orsay, France Received 27 February 1980 Revised manuscript received 19 May 1980
The plasma radiation temperature of a neon plasma created by a dc proton beam has been measured. Variations of this temperature versus gas pressure and beam intensity are presented and related to the theoretical variation of the electronic distribution function.
Introduction. Characterization of the electron velocity distribution function, f(w), is very important for an understanding of direct nuclear pumped (DNP) laser operation. As shown by theoretical calculations [l-3] this distribution is maxwellian in its low energy part, and for energies higher than 1 eV, the maxwellian population becomes negligible as compared to a tail constituted by fast electrons whose number is only one thousandth part of the total population, but nevertheless very efficient in the molecule excitation processes. The plasma radiation temperature, TR, which is highly dependent on this distribution function, is thus interesting to know. Evaluation of the radiation temperature TR has been performed on a proton-beam induced plasma, using a microwave interferometer. Measurements showing the variation of TR versus gas pressure and beam intensity are reported.
where m is the electronic mass, K the Boltzman constant, and v the electron-neutral collision frequency. The plasma is generally not perfectly absorbing, and thus it radiates as a grey body whose absorptivity at angular frequencyo,A, = 1 - exp[-o(o)L] , is determined by the loss coefficient, cy, acting on the whole plasma length, L. When inserted in a wave guide, the plasma radiates, per unit angular frequency, a powerP, =A,KTR. A microwave interferometer (fig. l), allows measuring TR without the knowledge of A,. It uses a reference black-body noise source, with temperature To, which illuminates the plasma; the plasma source is chopped, thus allowing synchronous detection of the microwave power, P,, and Put, respectively reflected and transmitted by the plasma, depending on the position of a rotative switch. It can be shown [5] that
Measurement principle. The microwave radiation of a slightly ionized gas is due to electron-neutral bremsstrahlung. It is well known [4] that the equivalent black-body radiation temperature is: TR = m iv(w)f(w)w4 0
dw K ~v(w)(af/aw)w3
dw,
0 (1)
’ Laboratoire associ6 au Centre National de la Recherche Scientifique.
Klystrm
Fig. 1. Sketch of the interferometric
cl radiometer device.
257
PHYSICS LETTERS
Volume 78A, number 3
I
TRMl
6000
TR
(K)
t
2ccl
300
LOO
so
Fig. 2. Variation of radiation temperature in neon: (a) With beam intensity; O-200 Torr, 2.1 MeV. (b) With gas pressure; 4 2.5 MA, 2 MeV; + 3 PA, 2 MeV.
S=Pwr+Pwt
= Au K(2Tk - To).
(2)
4 August 1980
and beam intensity. We briefly show here how these variations can be understood. The electron-neutral collision frequency in neon is well approximated by the relation v(w) - w~.~. Due to the fact that the product vw4f(w) does not quickly converge with increasing w, it is clear from eq. (1) that TR depends on the whole distribution function and, in particular, on the tail length. This tail length is related to the mean energy of the primary electrons created by the beam. Nevertheless, the dependence of TR on the experimental parameters results from the variation in the shape of the distribution function, mainly due to the variation of its maxwellian-like part. Processes involved in the equilibrium of this part are (i) heating by coulombian collisions with the electrons of the tail, (ii) cooling by electron-neutral collisions and (iii) recombination which makes disappear the slowest electrons. An increase in the beam current increases the number of primary electrons, thus tending to heat the main part of the distribution. Simultaneously, the total density increases so that the recombination effect, involving slow electrons, becomes more efficient. Obviously these two processes lead to an increase in TR . On the other hand, incre,asing the gas pressure mainly results in increasing the electron-neutral collision effects, thus leading to a lower value of TR.
To is adjusted with a calibrated attenuator so as to observe that S = 0, thus allowing one to deduce the TR value.
This work has been supported by the Direction des Recherches, Etudes et Techniques (DRET) and by the Centre National de la Recherche Scientifique.
Results and discussion. The method just described has been applied to a plasma created by a dc proton beam (1-5 PA, 2.1 MeV, 4 cm in diameter) impinging on a gaseous target (neon, 10 to some 100 Torr). The observed plasma part is contained in a cylindrical Pyrex tube (1 cm in diameter), inserted in an X band wave guide. Microwave signals are detected using a mixer followed by an IF amplifier (100 MHz), a crystal and a synchronous detector (fig. 1). Some measurements are reported in fig. 2, showing the variation of TR as a function of gas pressure
References
258
[l] H.A. Hassan and J.E. Deese, Phys. Fluids 20 (1977) 1586. [2] B.D. de Paola et al., Proc. 1st Intern. Symp. on Nuclear induced plasma and nuclear pumped lasers (Les Editions de Physique, Orsay, 1979) p. 221. [ 31 Groupe Plasma d’origine Nudaire, submitted to 3. de Phys. [4] G. Bekefi, Radiation processes in plasmas (Wiley, New York, 1966). [5] J.F. Delpech and J.C. Gauthier, Rev. Sci. Instrum. 42 (1971) 958.