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Direct observation of nonlinear scattering of electrons by laser beam
PHYSICS
Volume 19, number 3
LETTERS
3. S. Berko and F. L.Hereford, Rev. Mod. Phys. 28 (1958) 299.
15 October 1965
4. R.A.Ferrell, Rev.Mocl.Phys.28 (1956) 308. 5. WilsonJobnsonandStump,Phys.Rev. 129(1963)2091. *****
DIRECT
OBSERVATION ELECTRONS
OF NONLINEAR SCATTERING BY LASER BEAM *
OF
H. SCI-IWARZ and H. A. TOURTELLOTTE Rensselaer
Polytechnic East
institute,
Windsor Hill,
Hartford
Graduate
Center,
Connecticut
and W. W. GAERTNER CBS Laboratories,
Stamford,
Connecticut
Received 13 September 1965
We would like to report a few samples of experimental results we have obtained during the past year while studying the interaction of laser and electron beams. This interaction is believed to occur as a Bragg reflection of the electrons in the standing wave pattern set up in the light beam, as was predicted by Eapitza and Dirac in 1933 [l] and again brought to attention by Ter Haar [2] and Hall [ 31. Recently Bartell et al. [4] have also investigated this effect. The direction in which the electron beam is deflected can be expected to depend on the precise angle at whichbthe electron beam enters the standing wave pattern [1, fig. 11. We have found that the deflection angle agrees closely with the prediction of Kapitza and Dirac, but that the magnitude of the deflected beam is greater than one would expect. A fixed laser beam from a 65 joule Neodymium doped laser of X = 1.06 pm enters through a glass window a high vacuum system (see fig. 1). It impinges at right angle on a mirror with a reflectivity in vacuum of higher than 99.99%. The other end of the glass rod also had a 100% reflectivity. A beam of 17.5 keV electrons (0.06 MA) was focused onto a target T in a spot of a diameter which could be varied between 5 pm and 10 pm. In front of this target, very close to it, two parallel wires, WI and W2 were positioned perpendicular to the paper plane. They were connected to the differential inputs, A and B, of an * ExPerimentawere carried out in the Research ratory of Rensselaer Graduate Center.
202
Polytechnic
Institute%
LaboHartford
oscilloscope. Before each experiment the xdeflection (which falls in the paper plane) was adjusted such that the electron beam was positioned between the two wires. The electron beam was then scanned by a y-deflection coil in a plane perpendicular to the paper plane and parallel to the two wires, at fixed frequencies between 400 and 16000 c/s. The geometry of the experimental arrangement allowed the detection of deflections of the order of 10m5 radian in the x-direction. Certain mechanical and thermal disturbances are noticeable in fig. 2b, as well as the fact that the two wires are not ideally straight and parallel. Any stray electrons or ions that may have originated from the laser within the vacuum system would not be detected, since there is no reason to assume (1) that one wire would receive more stray charged particles than the other or (2) that such stray current would follow the modulation frequency (varied between 400 and 16 000 c/s) of the electron beam y-deflection while the laser beam was left unmodulated. A strong argument for a true photon-electron wave interaction is the fact that no effect could be observed even with the “shutter” (see fig. 1) open, if the electron beam did not meet the laser beam under certain geometrical conditions. Fig. 2 shows an example of the over 50 picutre pairs taken of oscilloscope traces during laser action, (a) with the outside shutter open, and (b) with the outside shutter blocking the laser beam. In fig. 2a one can see that one wire receives at least one half of all the electrons during each
PHYSICS
Volume 19, number 3
f
II-
VACUUM
r
/\
‘-I
II
f=r
y
Pumping System
ReflectionI
LASER BEAM
cl
15 October 1965
Scope With Differential Input
-
MIRROR (99.9%
LETTERS
0
x-y
Deflection Coil
0
Focusing Coil
!?%z
. \
Glass Window
Electron Gun
Fig. 1. Diagram of apparatus: WI and W2 straight wires, 6 0.25 mm, 1” long, less than 20 pm apart perpendicular to the paper plane. Electron beam was scanned through laser beam in plane perpendicular to paper plane and parallel to wires, WI and W2, by y deflection coil, and adjusted by x-deflection coil to pass through gap between wires when no laser beam present.
Fig. 2, Electron beam deflection in x-direction. (a) During laser pulse (shutter open, see fig. 1). (b) With laser blocked. Frequency of electron beam deflection 600 c/s. Frequency of resulting x-deflection of electron beam: 1200 c/s. Abscissa time scale: 0.2 m see/ div. Ordinate: Difference between current arriving at WI (signal A} and current arriving at W2 (signal B); scale: IO mV/div. (impedance of circuit 1 megobtn).
electron beam interception with the fixed laser beam. The deflection angle of the electron beam corresponded closely to the Bragg-Kapitza-Dirac value, but the probability of reflection appeared to be a higher than second order function of the photon field intensity. Kapitza and Dirac [l] derived their formula under the assumption of one reflection at the mirror. It reads in practical units: P = 1.9 x 1O-38 ~~6~z/A~~~u (Z - path of electron in light beam in cm, h - wave length of light in Angstrom, D - intensity of light in Watt per cm2, Ah - bandwidth in Angstrom, U - acceleration voltage of electron beam in volt). One finds that to obtain a value of P Z-0.5, our laser would have had to deliver a power of at least 100 megawatts. This would violate the fundamental energy conservation law (appr~i~tely 7000 joule input). It appears to be necessary to extend the Kapitza-Dirac theory.
We do not, however, want to elaborate further on this at this time until we have taken more numerical data under varying conditions. We wish to thank VEECO Instuments Inc., Plainview, L. I., N. Y. for part of the equipment and American Optical Company, Southbridge, Mass., for laser rods. We also thank Dean W. C. Stoker for his constant interest.
References 1. P. L. Kapitza and P. M.A. Dirac, Proc. Cambridge Phil.Soc.29 (1933) 297. 2. D. Ter Haar, Introduction to Vol. I of Kapitza’s Collected Papers (~acmill~, New York, 1964). 3. A.C.Hall, Natire 199 (1963) 683. 4. L.S. Bartell, H. Bradford Thompson and R. R. Roskos, Phys.Rev.Letters 14 (1965) 851.
203
Volume 19, number 3
LETTERS
3. S. Berko and F. L.Hereford, Rev. Mod. Phys. 28 (1958) 299.
15 October 1965
4. R.A.Ferrell, Rev.Mocl.Phys.28 (1956) 308. 5. WilsonJobnsonandStump,Phys.Rev. 129(1963)2091. *****
DIRECT
OBSERVATION ELECTRONS
OF NONLINEAR SCATTERING BY LASER BEAM *
OF
H. SCI-IWARZ and H. A. TOURTELLOTTE Rensselaer
Polytechnic East
institute,
Windsor Hill,
Hartford
Graduate
Center,
Connecticut
and W. W. GAERTNER CBS Laboratories,
Stamford,
Connecticut
Received 13 September 1965
We would like to report a few samples of experimental results we have obtained during the past year while studying the interaction of laser and electron beams. This interaction is believed to occur as a Bragg reflection of the electrons in the standing wave pattern set up in the light beam, as was predicted by Eapitza and Dirac in 1933 [l] and again brought to attention by Ter Haar [2] and Hall [ 31. Recently Bartell et al. [4] have also investigated this effect. The direction in which the electron beam is deflected can be expected to depend on the precise angle at whichbthe electron beam enters the standing wave pattern [1, fig. 11. We have found that the deflection angle agrees closely with the prediction of Kapitza and Dirac, but that the magnitude of the deflected beam is greater than one would expect. A fixed laser beam from a 65 joule Neodymium doped laser of X = 1.06 pm enters through a glass window a high vacuum system (see fig. 1). It impinges at right angle on a mirror with a reflectivity in vacuum of higher than 99.99%. The other end of the glass rod also had a 100% reflectivity. A beam of 17.5 keV electrons (0.06 MA) was focused onto a target T in a spot of a diameter which could be varied between 5 pm and 10 pm. In front of this target, very close to it, two parallel wires, WI and W2 were positioned perpendicular to the paper plane. They were connected to the differential inputs, A and B, of an * ExPerimentawere carried out in the Research ratory of Rensselaer Graduate Center.
202
Polytechnic
Institute%
LaboHartford
oscilloscope. Before each experiment the xdeflection (which falls in the paper plane) was adjusted such that the electron beam was positioned between the two wires. The electron beam was then scanned by a y-deflection coil in a plane perpendicular to the paper plane and parallel to the two wires, at fixed frequencies between 400 and 16000 c/s. The geometry of the experimental arrangement allowed the detection of deflections of the order of 10m5 radian in the x-direction. Certain mechanical and thermal disturbances are noticeable in fig. 2b, as well as the fact that the two wires are not ideally straight and parallel. Any stray electrons or ions that may have originated from the laser within the vacuum system would not be detected, since there is no reason to assume (1) that one wire would receive more stray charged particles than the other or (2) that such stray current would follow the modulation frequency (varied between 400 and 16 000 c/s) of the electron beam y-deflection while the laser beam was left unmodulated. A strong argument for a true photon-electron wave interaction is the fact that no effect could be observed even with the “shutter” (see fig. 1) open, if the electron beam did not meet the laser beam under certain geometrical conditions. Fig. 2 shows an example of the over 50 picutre pairs taken of oscilloscope traces during laser action, (a) with the outside shutter open, and (b) with the outside shutter blocking the laser beam. In fig. 2a one can see that one wire receives at least one half of all the electrons during each
PHYSICS
Volume 19, number 3
f
II-
VACUUM
r
/\
‘-I
II
f=r
y
Pumping System
ReflectionI
LASER BEAM
cl
15 October 1965
Scope With Differential Input
-
MIRROR (99.9%
LETTERS
0
x-y
Deflection Coil
0
Focusing Coil
!?%z
. \
Glass Window
Electron Gun
Fig. 1. Diagram of apparatus: WI and W2 straight wires, 6 0.25 mm, 1” long, less than 20 pm apart perpendicular to the paper plane. Electron beam was scanned through laser beam in plane perpendicular to paper plane and parallel to wires, WI and W2, by y deflection coil, and adjusted by x-deflection coil to pass through gap between wires when no laser beam present.
Fig. 2, Electron beam deflection in x-direction. (a) During laser pulse (shutter open, see fig. 1). (b) With laser blocked. Frequency of electron beam deflection 600 c/s. Frequency of resulting x-deflection of electron beam: 1200 c/s. Abscissa time scale: 0.2 m see/ div. Ordinate: Difference between current arriving at WI (signal A} and current arriving at W2 (signal B); scale: IO mV/div. (impedance of circuit 1 megobtn).
electron beam interception with the fixed laser beam. The deflection angle of the electron beam corresponded closely to the Bragg-Kapitza-Dirac value, but the probability of reflection appeared to be a higher than second order function of the photon field intensity. Kapitza and Dirac [l] derived their formula under the assumption of one reflection at the mirror. It reads in practical units: P = 1.9 x 1O-38 ~~6~z/A~~~u (Z - path of electron in light beam in cm, h - wave length of light in Angstrom, D - intensity of light in Watt per cm2, Ah - bandwidth in Angstrom, U - acceleration voltage of electron beam in volt). One finds that to obtain a value of P Z-0.5, our laser would have had to deliver a power of at least 100 megawatts. This would violate the fundamental energy conservation law (appr~i~tely 7000 joule input). It appears to be necessary to extend the Kapitza-Dirac theory.
We do not, however, want to elaborate further on this at this time until we have taken more numerical data under varying conditions. We wish to thank VEECO Instuments Inc., Plainview, L. I., N. Y. for part of the equipment and American Optical Company, Southbridge, Mass., for laser rods. We also thank Dean W. C. Stoker for his constant interest.
References 1. P. L. Kapitza and P. M.A. Dirac, Proc. Cambridge Phil.Soc.29 (1933) 297. 2. D. Ter Haar, Introduction to Vol. I of Kapitza’s Collected Papers (~acmill~, New York, 1964). 3. A.C.Hall, Natire 199 (1963) 683. 4. L.S. Bartell, H. Bradford Thompson and R. R. Roskos, Phys.Rev.Letters 14 (1965) 851.
203