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Theory of the Magnetronic Laser

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Theory of the Magnetronic Laser Theory of the magnetronic laser Miroslav Pardy Institute of Plasma Physics ASCR Prague Asterix Laser System, PALS Za Slovankou 3, 182 21 Prague 8, Czech Republic and Department of Physical Electronics Masaryk University Kotl´aˇrsk´a2, 611 37 Brno, Czech Republic e-mail:[email protected] August 13, 2018 Abstract We analyze the motion of an electron in the crossed electromagnetic field of the planar magnetron. Numerically we calculate the parameters of cycloid along which electron moves. We determine the total power of radiation of an electron moving in the magnetron and the power spectrum generated by a single electron and by the system of N electrons moving coherently in the electromagnetic field of planar arXiv:physics/0306024v1 [physics.class-ph] 3 Jun 2003 magnetron. We argue that for large N, and high intensity of electric and magnetic fields, the power of radiation of such magnetronic laser, MAL, can be sufficient for application in the physical, chemical, biological and medicine sciences. In medicine, the magnetronic laser, can be used for the therapy of the localized cancer tumors. The application of the MAL in CERN as an ion source for LHC is not excluded. 1 1 Introduction Any of devices that produces an intensive beam of light not only of a very pure single colour but all colours is a laser. While the gas lasers and masers produce practically a single colour, the so called lasers on the free electrons such as undulators and wigglers produce all spectrum of colours. These spectra are generated in the form of so called synchrotron radiation because they are generated during the motion of electron when moving in the magnetic field. The electrons moving inside the planar magnetron evidently produce also the syn- chrotron radiation because planar magnetron is a system of two metal charged plates im- mersed into homogenous magnetic field which is perpendicular to the homogenous electric field between plates. The planar magnetron producing photons has the same function as the free electron laser such as wiggler or undulator. So, we denote such device by word magnetronic laser, or shortly MAL. The motion of electrons is determined by the equation of motion following from electrodynamic of charges in homogenous electric and magnetic field. The produced energy of electrons is given by the Larmor formula. History teaches us that Van de Graaff (1931), Cockcroft and Walton (1932) used the high voltage principle system for the acceleration of particles and that the next revolution in acceleration physics was caused by Lawrence (Lawrence et al., 1932) who replaced the high voltage acceleration by repeated acceleration of particles moving along the circle. Here, we use the high voltage idea with the difference, to obtain synchrotron radiation only and not to obtain high energy particles. Although the radiation of one electron from one trajectory is very weak, the total radiation of a MAL is not weak because the process of radiation can be realized by many electrons moving along the same trajectory. There is no problem for today technology to construct MAL of high intensity electric and magnetic fields. We hope that the radiation output of such device is sufficient large for application in an physical laboratories. We argue that MAL can be applied for instance in the therapy of cancer and it means that in the specific situations it can be used instead of Leksell gamma knife. On the other hand, the application of MAL in CERN as the ion source for LHC is not excluded. 2 The nonrelativistic motion of an electron in the field configuration of the planar magnetron We shall identify the direction of magnetic field H with z-axis and he direction of electric field E is along the y-axis. Then, the nonrelativistic equation of motion of an electron is as follows: e mv˙ = eE + (v H). (1) c × This equation can be rewritten in the separate coordinates as follows (we do not write the z coordinate): e mx¨ = yH,˙ (2) c 2 e my¨ = eE xH.˙ (3) y − c Multiplying the equation (3) by i and combining with the equation (2), we find d e (x ˙ + iy˙)+ iω (x ˙ + iy˙)= i E , (4) dt 0 m y where ω0 = eH/mc. (5) Functionx ˙ +iy˙ can be considered as the unknown, and as a such is equal to the sum of the integral of the same differential equation without the right-hand term and a particular integral of the equation with the right-hand term. The first integral is ae−iω0t, and the second integral is a constant which can be eliminated from eq. (4) as eEy/mω0 = cEy/H. Thus cE x˙ + iy˙ = ae−iω0t + y . (6) H The constant a is in general complex. So, we can write it in the form a = beiα where b and α are real constants. We see that since a is multiplied by e−iω0t, we can, by a suitable choice of the time origin, give the phase α any arbitrary value. We choose a to be real. Then, breaking upx ˙ + iy˙ into real and imaginary parts, we find cE x˙ = a cos ω t + y , y˙ = a sin ω t. (7) 0 H − 0 At t = 0 the velocity is along the x-axis (for a = cE /H ). 6 − y As we have said, all the formulas of this section assume that the velocity of the particle is small compared with the velocity of light. If we calculate the average value of x and y, we get cE < x>˙ = y , < y˙ >=0, (8) H and, if we we define the drift velocity by relation cE v = y , (9) drift H then, from the nonrelativistic condition v 1, we see that for this to be so, it is drift ≪ necessary in particular that the electric and magnetic intensities satisfy the following condition: E y 1, (10) H ≪ while the absolute magnitudes of E and H can be arbitrary. Integrating equation (7), and choosing the constant of integration so that at t =0, x = y = 0, we obtain a cEy a x = sin ω0t + t, y = (cos ω0t 1). (11) ω0 H ω0 − 3 These equations define in a parametric form a trochoid. Depending on whether a is larger or smaller in absolute value than the quantity cEy/H, the projection of the trajectory on the plane xy have different form. If a = cE /H, then, equation (7) becomes: − y cEy cEy x = (ω0t sin ω0t), y = (1 cos ω0t). (12) ω0H − ω0H − These equations are the parametric equation of cycloid in the plane xy. The cycloid can be formed also mechanically as a curve traced out by a point on the circumference of a circle that rolls without slipping along a straight line. Constant R = cEy/ω0H is equal to the radius of the rolling circle. The distance from the point with parameter t =0 to the point with parameter t = 2π/ω0 is 2πR. The drift velocity is Rω0 and it is the velocity of the center of the circle. On the other hand we see that the average value ofx ˙ is just the drift velocity vdrift in equation (9). The generalization to the relativistic situation can be performed using the approach of Landau et all. (1962). The aim of this article is to investigate the radiation of electron in case that the motion is just along cycloid, i. e. the trajectory of an electron in the planar magnetron. We want to show that such configuration of fields is an experimental device which can be used as the new effective source of synchrotron radiation. For the sake of simplicity we calculate spectrum of radiation of MAL in the system ′ S which moves with regard to the magnetron at the drift velocity vdrift = Rω0. Then, all electron trajectories are circles and the radiation of this system is the synchrotron radiation. Then, we can transform the power spectrum to the system joined with the planar magnetron. However, we shall see that the spectrum is modified slightly, because the drift velocity is not relativistic in our situation. The motion of electrons differs form the motion of electrons in wigglers and undulators where the trajectories of electrons are not cycloids and the motion is highly relativistic. It is also possible to realize the relativistic motion of electrons in MAL. However, it needs special experimental conditions which are very expensive. 3 Physical design of MAL The expression for the instantaneous power radiated by the nonrelativistic charge e undergoing an acceleration a = dv/dt is given by the Larmor formula as follows: 2 e2 dv 2 2 e2 dp 2 P = = . (13) 3 c3 dt ! 3 m2c3 dt ! So we see that only electrons with undergoing big acceleration can produce sufficient power of radiation of photons. If we consider electrons in the system S′ with the drift velocity v = vdrift = cEy/H, then the equations of the trajectory are as follows: cEy cEy x = sin ω0t, y = (1 cos ω0t). (14) −ω0H ω0H − 4 They can be rewritten in the form: cE 2 cE 2 x2 + y y = y , (15) − ω0H ω0H which is evidently an equation of a circle with the radius −1 2 cEy eH cEy mc Ey R = = = 2 . (16) ω0H mc H eH The corresponding tangential velocity is cE v = x˙ 2 +y ˙2 = y = v . (17) t H drift q The corresponding acceleration is eE x¨2 +¨y2 = y , (18) m q which corresponds to the Newton law, mass acceleration = force = eE , which agrees × y with the introducing the electrostatic intensity Ey inside the planar magnetron. Big acceleration gives big radiation and also big tangential velocity for the given intensity of the magnetic field.
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