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Cherenkov ion acoustic wave radiation generated by a pseudowave
Volume 36A, number 2
PHYSICS LETTERS
CHERENKOV
ION ACOUSTIC WAVE RADIATION BY A PSEUDOWAVE*
16 August 1971
GENERATED
K. ESTABROOK and I. A L E X E F F Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA Received 28 June 1971
Ion acoustic waves in a plasma are observed the propagate in a cone of Cherenkov radiation outward from the path of a pseudowave packet (ion burst) that generates them.
We r e p o r t the excitation of ion acoustic waves by pseudowave [1] (ion burst) packets. The ion a c o u s t i c waves propagate in a d i r e c t i o n n e a r l y p e r p e n d i c u l a r to that of the pseudowaves. In our e x p e r i m e n t s , the p s e u d o w a v e - i o n acoustic wave coupling i s a f o r m of Cherenkov r a d i a t i o n since our m a t h e m a t i c a l a n a l y s i s r e q u i r e s we have an exciting packet of i o n s moving f a s t e r than the velocity of ion acoustic waves in the m e d i u m . The e x p e r i m e n t was p e r f o r m e d in a xenon d i s charge p l a s m a [2] of d e n s i t y 107 cm - 3 , e l e c t r o n t e m p e r a t u r e 2 eV and ion t e m p e r a t u r e [3] 1/20 eV. The pseudowaves were g e n e r a t e d by a 0.5 ~tsec pulse of -300 V applied to a fine m e s h grid 5 cm in d i a m e t e r . The negative potential pulse ~ex a c c e l e r a t e d ions toward the grid to a high velocity ( ~ / 2 e ~ ) e x / m i or ~ 10 t i m e s the ion acoustic velocity) and then r e l e a s e d them to f o r m a short c y l i n d r i c a l packet of highly d i r e c tional s t r e a m i n g ions. Following the pseudowave was a r a r e f a c t i o n wave t r a v e l l i n g with the ion acoustic velocity. Fig. 1 shows the e l e c t r o n c u r r e n t to a d e t e c tor probe at four r a d i a l p o s i t i o n s f r o m the axis of a n a r r o w c y l i n d e r of pseudowave b u r s t s . The s m a l l c o m p r e s s i o n (A) i s the ion acoustic wave f r o m the pseudowave moving at a velocity of 1.4 × 105 c m / s e c (theory y i e l d s 1.2 × 105 c m / s e c ) . The l a r g e r a r e f a c t i o n wave (B) i s the ion acoustic wave excited d i r e c t l y by the grid. T h i s peak m o v e s slightly as the probe i s moved, because the d i r e c t d i s t a n c e f r o m probe to grid i s also * Research sponsored by the US Atomic Energy Commission under contract with the Union Carbide Corp. Oak Ridge Graduate Follow from the University of Tennessee under appointment from the Oak Ridge Assoetated Universities.
Fig. 1. Oscillogram of the electron current to the detector probe (a small wire) at radial positions 9..7, 4.1, 5.4 and 6.8 cm from the pseudowave path. The distance between the plane of the probe measurements and the plane of the grid is 7 cm. The time scale is 10 /lsee/dlv. The smA]l bump (A) is the inn acoustic wave excited by the pseudowave and the large dip (B) is the inn acoustic wave excited directly by the grLd. changed. The s m a l l c o m p r e s s i o n (A) was obs e r v e d at s e v e r a l axial d i s t a n c e s f r o m the grid. We have m a t h e m a t i c a l l y d e s c r i b e d the ion wave excitation p r o c e s s as follows: The p s e u d o wave b u r s t p a s s e s through a r e g i o n of p l a s m a in a time short c o m p a r e d to that in which the local p l a s m a density could r e l a x . T h i s i s , in effect, stating that the velocity of the b u r s t i s high in c o m p a r i s o n to the ion acoustic wave velocity (Cherenkov effect). Under this condition, the p l a s m a i o n s p l u s the pseudowave ions produce a local d e n s i t y m a x i m u m , the r e s u l t of which i s a 95
Volume 36A, number 2
PHYSICS
l o c a l p r e s s u r e m a x i m u m that p r o d u c e s an ion a c o u s t i c wave with a d e n s i t y m a x i m u m f r o m the pseudowave edge n e a r e r the p r o b e ( o b s e r v e d ) and a d e n s i t y m i n i m u m f r o m the pseudowave edge f a r t h e r f r o m the p r o b e ( g e n e r a l l y p a r t l y o b s c u r e d l a t e in t i m e by the d i r e c t ion a c o u s t i c wave). We s u s p e c t t h i s pseudowave technique to be v e r y useful in o b s e r v i n g ion a c o u s t i c e x c i t a t i o n u n e n c u m b e r e d by the sheath e f f e c t s i n h e r e n t in the use of t r a n s m i t t i n g e l e c t r o d e s . * *
96
LETTERS
16 August 1971
References [i] I.Alexeff, W.D. Jones and M. Widner, Phys. Fluids 13 (1970) 1519. [2] I.Alexeff, W. D. Jones and D. Montgomery, Phys. Fluids 11 (1968) 167. [3] I.Alexeff, W.D. Jones and D. Montgomery, Phys. Rev. Letters 19 (1967) 422.
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PHYSICS LETTERS
CHERENKOV
ION ACOUSTIC WAVE RADIATION BY A PSEUDOWAVE*
16 August 1971
GENERATED
K. ESTABROOK and I. A L E X E F F Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA Received 28 June 1971
Ion acoustic waves in a plasma are observed the propagate in a cone of Cherenkov radiation outward from the path of a pseudowave packet (ion burst) that generates them.
We r e p o r t the excitation of ion acoustic waves by pseudowave [1] (ion burst) packets. The ion a c o u s t i c waves propagate in a d i r e c t i o n n e a r l y p e r p e n d i c u l a r to that of the pseudowaves. In our e x p e r i m e n t s , the p s e u d o w a v e - i o n acoustic wave coupling i s a f o r m of Cherenkov r a d i a t i o n since our m a t h e m a t i c a l a n a l y s i s r e q u i r e s we have an exciting packet of i o n s moving f a s t e r than the velocity of ion acoustic waves in the m e d i u m . The e x p e r i m e n t was p e r f o r m e d in a xenon d i s charge p l a s m a [2] of d e n s i t y 107 cm - 3 , e l e c t r o n t e m p e r a t u r e 2 eV and ion t e m p e r a t u r e [3] 1/20 eV. The pseudowaves were g e n e r a t e d by a 0.5 ~tsec pulse of -300 V applied to a fine m e s h grid 5 cm in d i a m e t e r . The negative potential pulse ~ex a c c e l e r a t e d ions toward the grid to a high velocity ( ~ / 2 e ~ ) e x / m i or ~ 10 t i m e s the ion acoustic velocity) and then r e l e a s e d them to f o r m a short c y l i n d r i c a l packet of highly d i r e c tional s t r e a m i n g ions. Following the pseudowave was a r a r e f a c t i o n wave t r a v e l l i n g with the ion acoustic velocity. Fig. 1 shows the e l e c t r o n c u r r e n t to a d e t e c tor probe at four r a d i a l p o s i t i o n s f r o m the axis of a n a r r o w c y l i n d e r of pseudowave b u r s t s . The s m a l l c o m p r e s s i o n (A) i s the ion acoustic wave f r o m the pseudowave moving at a velocity of 1.4 × 105 c m / s e c (theory y i e l d s 1.2 × 105 c m / s e c ) . The l a r g e r a r e f a c t i o n wave (B) i s the ion acoustic wave excited d i r e c t l y by the grid. T h i s peak m o v e s slightly as the probe i s moved, because the d i r e c t d i s t a n c e f r o m probe to grid i s also * Research sponsored by the US Atomic Energy Commission under contract with the Union Carbide Corp. Oak Ridge Graduate Follow from the University of Tennessee under appointment from the Oak Ridge Assoetated Universities.
Fig. 1. Oscillogram of the electron current to the detector probe (a small wire) at radial positions 9..7, 4.1, 5.4 and 6.8 cm from the pseudowave path. The distance between the plane of the probe measurements and the plane of the grid is 7 cm. The time scale is 10 /lsee/dlv. The smA]l bump (A) is the inn acoustic wave excited by the pseudowave and the large dip (B) is the inn acoustic wave excited directly by the grLd. changed. The s m a l l c o m p r e s s i o n (A) was obs e r v e d at s e v e r a l axial d i s t a n c e s f r o m the grid. We have m a t h e m a t i c a l l y d e s c r i b e d the ion wave excitation p r o c e s s as follows: The p s e u d o wave b u r s t p a s s e s through a r e g i o n of p l a s m a in a time short c o m p a r e d to that in which the local p l a s m a density could r e l a x . T h i s i s , in effect, stating that the velocity of the b u r s t i s high in c o m p a r i s o n to the ion acoustic wave velocity (Cherenkov effect). Under this condition, the p l a s m a i o n s p l u s the pseudowave ions produce a local d e n s i t y m a x i m u m , the r e s u l t of which i s a 95
Volume 36A, number 2
PHYSICS
l o c a l p r e s s u r e m a x i m u m that p r o d u c e s an ion a c o u s t i c wave with a d e n s i t y m a x i m u m f r o m the pseudowave edge n e a r e r the p r o b e ( o b s e r v e d ) and a d e n s i t y m i n i m u m f r o m the pseudowave edge f a r t h e r f r o m the p r o b e ( g e n e r a l l y p a r t l y o b s c u r e d l a t e in t i m e by the d i r e c t ion a c o u s t i c wave). We s u s p e c t t h i s pseudowave technique to be v e r y useful in o b s e r v i n g ion a c o u s t i c e x c i t a t i o n u n e n c u m b e r e d by the sheath e f f e c t s i n h e r e n t in the use of t r a n s m i t t i n g e l e c t r o d e s . * *
96
LETTERS
16 August 1971
References [i] I.Alexeff, W.D. Jones and M. Widner, Phys. Fluids 13 (1970) 1519. [2] I.Alexeff, W. D. Jones and D. Montgomery, Phys. Fluids 11 (1968) 167. [3] I.Alexeff, W.D. Jones and D. Montgomery, Phys. Rev. Letters 19 (1967) 422.
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