Millimeter and sub-millimeter radiation source

A radiation source in which a beam of electrons is produced and directed along a path with dielectric material having a constant less than 4 in proximity to the path and an undulator providing oscillatory velocity modulation to the beam along the path for producing millimeter and sub-millimeter wavelength radiation.

The present invention relates to a source for producing radiation, 
particularly radiation in the millimeter and sub-millimeter wavelengths. 
As other parts of the electro-magnetic spectrum are filled, the need has 
arisen to utilize millimeter and sub-millimeter wavelengths for 
communication purposes. In addition, other technical applications exist 
where only such wavelengths can be used. Several devices have been 
proposed and developed for producing radiation at such wavelengths. For 
example, the patent to Walsh, U.S. Pat. No. 4,122,372, describes a method 
and apparatus for producing high power coherent microwave radiation. In 
this device, a beam of electrons is passed through a waveguide tube which 
includes a low dielectric constant material adjacent the path. The 
electro-magnetic radiation is slowed in the material and the beam coupled 
to the radiation so that when the beam is focussed and guided the beam 
energy is converted into high power, high frequency, coherent microwave 
radiation. 
An alternative approach to producing millimeter and sub-millimeter 
wavelength radiation is described in the patent to Phillips, U.S. Pat. No. 
3,259,786. The type of device described in this patent is called a 
Ubitron, and produces a beam of electrons which is passed through a 
rippled or helical magnetic field and Doppler shifted. This rippled field 
can be produced by mounting pieces of iron, for example, rings, separated 
along the beam axis to provide transverse velocity modulation of the beam. 
One drawback with the Ubitron is that, to achieve relatively short 
wavelength radiation, the speed of the electrons passing through the 
device must be relatively high. The frequency of Doppler shifted radiation 
produced by a Ubitron is determined by the following relation: 
##EQU1## 
wherein: W.sub.S is the frequency of radiation produced; 
C is the speed of light; 
.lambda..sub.p is the wavelength of the rippled field which is produced by 
disturbing a uniform axial field with iron pieces; and 
.beta. is the beam speed in percent of light speed. 
From the above relation, it should be obvious that to reduce the wavelength 
of the radiation, the beam speed must be relatively close to 1, i.e., 
close to the speed of light. To produce such high beam speeds requires 
very high voltages applied to the cathode. For example, producing 
wavelengths of the order of one millimeter requires voltages of the order 
of one million volts. Such voltage sources are expensive, difficult to 
handle, and too large in size to be acceptable for many applications. 
The present invention relates to an improved device in which dielectric 
material having a constant less than four is mounted in proximity to the 
beam path. When such dielectric material is provided, the frequency of 
Doppler shifted radiation produced is determined by the following 
relation: 
##EQU2## 
The terms of this relation are the same as those noted above with the 
exception of .beta..sub..phi. the relative speed of the wave in the 
forward direction) which is determined by the frequency of emitted 
radiation and wave number K. The wave number K is in turn a function of 
the dielectric constant of the material, and its volume. Thus, by choosing 
a dielectric material having a low constant, for example, less than four, 
.beta./.beta..sub..phi. can be made relatively large without requiring 
high beam speeds. Thus, millimeter and sub-millimeter wavelength radiation 
can be produced at much lower beam speeds than in the Ubitron, and much 
lower voltages are required to produce those reduced beam speeds. Since as 
.beta. approaches 1, each increment of .beta. requires a much greater 
increment of energy input, the use of dielectric materials to reduce the 
beam speed required to produce a given desired wavelength results in very 
significant reductions in the energy required to produce the beam. 
Many other objects and purposes of the invention will be clear from the 
following detailed description of the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to the FIGURE, a beam of high speed electrons is produced by the 
conventional source 100 and passed along a path lined with dielectric 
material 102. An undulator 104 comprising a plurality of rings of iron 
separated by non-ferromagnetic material, for example, aluminum, is 
disposed along the beam path for providing transverse velocity modulation 
for the beam. Magnetic field coils 106 provide the uniform field which is 
modulated. Any suitable dielectric material can be used, preferably one 
having a dielectric constant less than four. Suitable materials include: 
quartz, polytetrafluoroethylene, polymethyl methacrylate, polyethylene, 
and polystyrene. The guide coils function to focus and guide the beam to 
keep stabilized the guided electro-magnetic modes and to convert the 
electron beam energy into high power coherent radiation. The cathode and 
undulator are preferably disposed within a conventional vacuum vessel 108. 
The electrons leaving the undulator can be collected in any conventional 
way, for example, by a circular waveguide 110. 
Many changes and modifications in the above-described embodiment of the 
invention can be carried out without departing from the scope thereof. For 
example, the rings of iron need not be made as complete rings, but pieces 
of iron separated along the beam path can be utilized. For certain 
applications, it may be desirable to make .beta./.beta..sub..phi. greater 
than one, which, of course, is impossible with a Ubitron as described 
above. Such application would require high voltages but may offer other 
possibilities, for example, possibly higher gains. The illustrated 
embodiment makes use of electron beam modes known as space charge waves. 
Another possibility is to use a bundle of fibers with pieces of iron 
spaced along the bundle axis to operate a number of beams in parallel and 
thus increase total power at short wavelengths. The scope of the present 
invention is intended to be limited accordingly only by the scope of the 
following claims.