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Electron energy distribution in the presence of moving striations
Volume 27A. number 8
PHYSICS
therefore, predicts increases in the nucleation field of the correct order of magnitude. This theory also predicts that the critical temperature should be unchanged by the presence of the film. We conclude that, although the presence of a layer of different material, formed on the surface by electropolishing, may account for some values of the ratio Hh3/Hc3 greater than unity reported in the literature, it cannot explain all the experimental results. It is shown that the experimental results presented here can be explained by the theory due to Shmidt if it is assumed that a layer with a high defect density exists on the surface of the sample. It seems very likely that the processes
of swaging on the
outer surface and drilling on the inner should both produce such a layer. The presence of the layer on the outer surface is also indicated by transport current measurements. We note that this work suggests a method of increasing sig-
LETTERS
9 September 1968
nificantly the critical magnetic fields of existing superconducting material. The author wishes to thank Dr. V. V. Shmidt for communicating his results prior to publication. He also wishes to acknowledge the excellent technical assistance of Mr. G. West. The work was financed by a grant from the Science Research Council.
References 1. E.J.ThomasandD.J.Sandiford, Proc.L.T.X., Moscow (1966), Vol. IIA, p.444; and to be published. 2. S. Gygax,. J. L: Olsen and R. H. Kropschot, Whys. Letters 8 (1964) 228. 3. P.R. Doidge, Kwan Sik-Hung and D. R. Tilley, Phil. Mag. 13 (1966) 795. 4. P. C. Wraight, Pbys. Letters 26A (1968) 140. 5. V. V. Shmidt, Soviet Phys. -JETP 26 (1968) 566.
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ELECTRON ENERGY DISTRIBUTION IN THE PRESENCE OF MOVING STRIATIONS M. G. DROUET, M. !&HA and G. G. CLOUTIER Laboratoire de Physique des Plasmas, Universit6 de MontrSal, Mont&al, Canada Received 4 June 1968
Probe measurements obtained in moving striations reveal large variations of the plasma potential and indicate that the Maxwellian approximation is valid for the body of the electron energy distribution.
Several measurements of the electron energy distribution obtained in the presence of moving striations [l-3] have been reported. These measurements revealed an important departure from the Maxwellian distribution and the existence of two groups of electrons. In this paper some measurements obtained in the presence of moving striations of small and large amplitude will be presented. These measurements failed to give evidence for the existence of the two electron groups. They revealed however large variations of the plasma potential in the striations. The probe sampling technique described by Drouet [4] has been used to obtain the oscillographic display of the first derivative of the Langmuir characteristic for different positions in the striations. It is important to note that in the 496
range of voltage for which the measurements are obtained the electron current dependence on probe potential (dZ /dV) is larger than the ion current dependence PcU,/dV). Therefore the latter may be neglected [5] and we have dZ/dV = dZ,/dV. A series of typidal curves obtained for an argon positive column containing self excited moving striations is presented on fig. 1. The brighter points on the light intensity trace numbered 1,2,3, correspond to the curves for the derivative of the characteristic numbered 1,2,3, respectively. The positions of the plasma potential are identified for each curve with the positions of the maxima of dZ/dV and are indicated by arrows on fig. 1. As already noted by Stewart [6] the value of the plasma potential varies in the striations. It is important at this point to reject any
Volume
27A, number
0
PHYSICS
8
I msec.
9 September
LETTERS
ZOOmA
2
Fig. 1. Typical curves corresponding to the light intensity variations in a striation and to the value of the first derivative (dZ/dV) of the Langmuir probe current with respect to the applied probe potential as a function of the applied probe potential for three different positions in the striations (argon 0.5 Torr, 200 mA, discharge tube diameter 6.5 cm). The brighter spots on the light intensity curve indicate the sampled points.
possible suggestions that in the case of the curve numbered 3, for example, the maximum noted by the arrow would not correspond to the plasma potential (i.e. to the inflexion point in the Langmuir characteristic) but to the presence of a fast electron group. Even in the presence of a two group energy distribution there can be only one maximum in the curve of the first derivative of the Langmuir probe characteristic and that maximum corresponds essentially to the position of the plasma potential. In order to test further the nature of the distribution the logarithms of the values of dZ/dV corresponding to the curve numbered 1 in fig. 1 were plotted in fig. 2 as a function of V. Similar measurements obtained in neon in the presence of moving striations of small amplitude are also given. It is noted that the presence of two groups of electrons cannot be observed. However it is evident that if one fails to use a sampling technique and to take into account the variations of the plasma potential, erroneous conclusions may easily be reached. Furthermore it appears
V[volt] I
0
I
I
,
i \
I
1968
I
5
I
I
I
9
Fig. 2. Values of the logarithms of dZ/dV plotted as a function of V in order to determine the character of the distribution.
clearly that logdZ/dV is a linear function of V and therefore [?‘I, for these conditions of discharge, the Maxwell distribution function should be a better approximation, for the body [8] of the distribution, than the Druyvesteyn distribution function.
References 1. F.W.Crawford, A.Garscadden and R.S.PaImer, Proc. 6th Intern. Conf. on Ionization Phenomena of Gases, Paris 1963, eds. Hubert and Cremieu-Alcsn, Vol. 4 (1964) pp.53-56. 2. Yu. M. Kagan, V. M. Milenin and N. K. Mitrofanov Zh.Tekhn.Fis..36.(1966) 2219; Soviet Physics, Tech. Phys. 11 (1967) 1661. and N.D.Twiddy, Nature 216 (1967) 3. S.W.Rayment 674. 4. M.G.Drouet, Csn.J.Phys.46 (1968) 227. 5. Yu.M.Kagan and V.I.Perel, Usp.Fiz.Nauk 81 (1963) 409. 6. A.B.Stewart, J.Appl.Phys.27 (1956) 911. 7. S. Pfau, Beitrage Plasmaphys. 7 (1967) 57. 8. K. W.Gentle, Phys. Fluids 9 (1966) 2203.
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497
PHYSICS
therefore, predicts increases in the nucleation field of the correct order of magnitude. This theory also predicts that the critical temperature should be unchanged by the presence of the film. We conclude that, although the presence of a layer of different material, formed on the surface by electropolishing, may account for some values of the ratio Hh3/Hc3 greater than unity reported in the literature, it cannot explain all the experimental results. It is shown that the experimental results presented here can be explained by the theory due to Shmidt if it is assumed that a layer with a high defect density exists on the surface of the sample. It seems very likely that the processes
of swaging on the
outer surface and drilling on the inner should both produce such a layer. The presence of the layer on the outer surface is also indicated by transport current measurements. We note that this work suggests a method of increasing sig-
LETTERS
9 September 1968
nificantly the critical magnetic fields of existing superconducting material. The author wishes to thank Dr. V. V. Shmidt for communicating his results prior to publication. He also wishes to acknowledge the excellent technical assistance of Mr. G. West. The work was financed by a grant from the Science Research Council.
References 1. E.J.ThomasandD.J.Sandiford, Proc.L.T.X., Moscow (1966), Vol. IIA, p.444; and to be published. 2. S. Gygax,. J. L: Olsen and R. H. Kropschot, Whys. Letters 8 (1964) 228. 3. P.R. Doidge, Kwan Sik-Hung and D. R. Tilley, Phil. Mag. 13 (1966) 795. 4. P. C. Wraight, Pbys. Letters 26A (1968) 140. 5. V. V. Shmidt, Soviet Phys. -JETP 26 (1968) 566.
*****
ELECTRON ENERGY DISTRIBUTION IN THE PRESENCE OF MOVING STRIATIONS M. G. DROUET, M. !&HA and G. G. CLOUTIER Laboratoire de Physique des Plasmas, Universit6 de MontrSal, Mont&al, Canada Received 4 June 1968
Probe measurements obtained in moving striations reveal large variations of the plasma potential and indicate that the Maxwellian approximation is valid for the body of the electron energy distribution.
Several measurements of the electron energy distribution obtained in the presence of moving striations [l-3] have been reported. These measurements revealed an important departure from the Maxwellian distribution and the existence of two groups of electrons. In this paper some measurements obtained in the presence of moving striations of small and large amplitude will be presented. These measurements failed to give evidence for the existence of the two electron groups. They revealed however large variations of the plasma potential in the striations. The probe sampling technique described by Drouet [4] has been used to obtain the oscillographic display of the first derivative of the Langmuir characteristic for different positions in the striations. It is important to note that in the 496
range of voltage for which the measurements are obtained the electron current dependence on probe potential (dZ /dV) is larger than the ion current dependence PcU,/dV). Therefore the latter may be neglected [5] and we have dZ/dV = dZ,/dV. A series of typidal curves obtained for an argon positive column containing self excited moving striations is presented on fig. 1. The brighter points on the light intensity trace numbered 1,2,3, correspond to the curves for the derivative of the characteristic numbered 1,2,3, respectively. The positions of the plasma potential are identified for each curve with the positions of the maxima of dZ/dV and are indicated by arrows on fig. 1. As already noted by Stewart [6] the value of the plasma potential varies in the striations. It is important at this point to reject any
Volume
27A, number
0
PHYSICS
8
I msec.
9 September
LETTERS
ZOOmA
2
Fig. 1. Typical curves corresponding to the light intensity variations in a striation and to the value of the first derivative (dZ/dV) of the Langmuir probe current with respect to the applied probe potential as a function of the applied probe potential for three different positions in the striations (argon 0.5 Torr, 200 mA, discharge tube diameter 6.5 cm). The brighter spots on the light intensity curve indicate the sampled points.
possible suggestions that in the case of the curve numbered 3, for example, the maximum noted by the arrow would not correspond to the plasma potential (i.e. to the inflexion point in the Langmuir characteristic) but to the presence of a fast electron group. Even in the presence of a two group energy distribution there can be only one maximum in the curve of the first derivative of the Langmuir probe characteristic and that maximum corresponds essentially to the position of the plasma potential. In order to test further the nature of the distribution the logarithms of the values of dZ/dV corresponding to the curve numbered 1 in fig. 1 were plotted in fig. 2 as a function of V. Similar measurements obtained in neon in the presence of moving striations of small amplitude are also given. It is noted that the presence of two groups of electrons cannot be observed. However it is evident that if one fails to use a sampling technique and to take into account the variations of the plasma potential, erroneous conclusions may easily be reached. Furthermore it appears
V[volt] I
0
I
I
,
i \
I
1968
I
5
I
I
I
9
Fig. 2. Values of the logarithms of dZ/dV plotted as a function of V in order to determine the character of the distribution.
clearly that logdZ/dV is a linear function of V and therefore [?‘I, for these conditions of discharge, the Maxwell distribution function should be a better approximation, for the body [8] of the distribution, than the Druyvesteyn distribution function.
References 1. F.W.Crawford, A.Garscadden and R.S.PaImer, Proc. 6th Intern. Conf. on Ionization Phenomena of Gases, Paris 1963, eds. Hubert and Cremieu-Alcsn, Vol. 4 (1964) pp.53-56. 2. Yu. M. Kagan, V. M. Milenin and N. K. Mitrofanov Zh.Tekhn.Fis..36.(1966) 2219; Soviet Physics, Tech. Phys. 11 (1967) 1661. and N.D.Twiddy, Nature 216 (1967) 3. S.W.Rayment 674. 4. M.G.Drouet, Csn.J.Phys.46 (1968) 227. 5. Yu.M.Kagan and V.I.Perel, Usp.Fiz.Nauk 81 (1963) 409. 6. A.B.Stewart, J.Appl.Phys.27 (1956) 911. 7. S. Pfau, Beitrage Plasmaphys. 7 (1967) 57. 8. K. W.Gentle, Phys. Fluids 9 (1966) 2203.
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