General Relativity Derivation of Beam Rest-Frame Hamiltonian
Total Page:16
File Type:pdf, Size:1020Kb
Proceedings of the 2001 Particle Accelerator Conference, Chicago GENERAL RELATIVITY DERIVATION OF BEAM REST-FRAME HAMILTONIAN £ J. Wei Ý , Brookhaven National Laboratory, New York, USA Ù = dÜ Abstract where =dØ are the contravariant components of the spatial velocity, the Greek letter indicates the three spatial Analysis of particle interaction in the laboratory frame of = d c components, dØ , is the speed of light, and storage rings is often complicated by the fact that particle ½ µ motion is relativistic, and that reference particle trajectory ´ ¾ ¾ ¾ ½ g Ù Ù 4 is curved. Rest frame of the reference particle is a conve- ¾ Ô = ´ g µ 44 (1) ¾ c g c nient coordinate system to work with, within which particle 44 motion is non-relativistic. We have derived the equations of motion in the beam rest frame from the general relativ- is the generalized Lorentz factor. The contravariant com- i i È = Ñ Ñ Í ¼ ity formalism, and have successfully applied them to the ponents of the four-momentum are ¼ , where analysis of crystalline beams [1]. is the proper mass of the particle. fF g Acted upon by a non-gravitational four-force i , the 1 DERIVATION equations of motion of the particle can be written as The motion of charged particles under Coulomb interac- i DÈ ik i = g F ; F Í ¼; i tion and external electromagnetic (EM) forces can be most k (2) conveniently described in the rotating rest frame of the ref- d ij 4 ¢ 4 fg g fg g erence particle of which the orientation of the axes are con- where the matrix is the inverse of ij , and stantly aligned to the radial, tangential, and vertical direc- the covariant differentiation is defined as tion of the motion. In this frame, particle motion within the i ÐÑ i dÈ @g @g g DÈ @g Ñk ik Ñi Ð k Ð Ð · Í · È ; beam bunch is non-relativistic. = ik ik k i Ñ d ¾ @Ü @Ü @Ü We derive the equations of motion using the general rel- d (3) ativity formalism. First, we express the equations of mo- Among the non-gravitational forces, the external EM force tion in a general tensor formalism. The Lorentz force ex- acting upon the particles is expressed by means of the EM perienced by the particle is constructed as a product of the fF g field tensor ij as EM field tensor and the four-velocity. Starting from the e laboratory frame, the EM field tensor is written by means k F = F Í ; i ik (4) of the components of the EM fields. Then, tensor algebra c is used to transform this field tensor into the rest frame. e fF g where is the electric charge, and ij is anti-symmetric. With a similar transformation, the metric tensor of the rest The Maxwell’s equations are given by frame is also obtained. The equations of motion can thus 8 @F @F be constructed in the rest frame which include centrifugal @F ik kÐ Ði > > · · =¼; < i k force, Coriolis force, time-dependent external EM forces, Ð @Ü @Ü @Ü i Ô (5) @ ½ Í ¼ and electrostatic Coulomb forces. Finally, these equations ik > Ô > ´ jg jF ; µ= : i c are re-scaled in terms of dimensionless quantities for the @Ü jg j convenience of computer simulation and analysis. ij iÐ jÑ F = g g F jg j where ÐÑ , is the absolute value of the de- fg g ¼ 1.1 Tensor formalism terminant of the metric ij , and is the charge density measured in the system of inertia. Adopting the formalism used by M6 oller,[2] we consider the motion of a particle in an arbitrary system of coordi- i Ü µ i = ½; ¾; ¿; 4 nates ´ , where indicate the space-time 1.2 Laboratory frame i fg g components. The metric tensor ij is defined in terms X µ In the Cartesian coordinate system of inertia ´ , the ¾ i j d× d× = g dÜ dÜ of the differential line element as ij , so-called laboratory frame, Eq. 2 can be written in the con- where the summation is performed over the four indices. ventional vector form. In this frame, the only non-zero i i Ü = Ü µ Let ´ be the equation of the time track of the components of the metric tensor are motion, being the proper time of the particle. The con- g = g = g =½; aÒd g ½: travariant components of the four-velocity are = ½½ ¾¾ ¿¿ 44 (6) i dÜ i In terms of the conventionally defined electric and mag- Í =´ Ù ; cµ; d netic fields £ Work performed under the auspices of the US Department of Energy E =´E ;E ;E µ; B =´B ;B ;B µ; Ý ¾ ¿ ½ ¾ ¿ [email protected], on joint appointment at ORNL and BNL ½ (7) 0-7803-7191-7/01/$10.00 ©2001 IEEE. 1678 Proceedings of the 2001 Particle Accelerator Conference, Chicago ¼ fg fg g g the EM field tensor is of the rest frame is related to ij of the laboratory ij ¼ Ð Ñ ¼ ½ g = « « g : frame by the relation ÐÑ Using the transfor- ij i j ¼ B B E ¾ ½ ¿ mation matrix, the contravariant components of a vector B C B ¼ B E ¿ ½ ¾ i B C fa g fF g = : ij (8) in the laboratory frame are transformed into the rest @ A ¼ B B ¼ E ¾ ½ ¿ i i k a = « a frame as : On the other hand, the covariant com- k E E E ¼ ¾ ¿ ½ ponents of a tensor of rank 2, e.g. the EM field tensor, are ¼ Ð Ñ F = « « F It is straightforward with these expressions to verify that transformed as ÐÑ . With these relations, the ij i j Eq. 2 is equivalent to the equations equation of motion in the beam rest frame of fixed orienta- tion can be obtained from Eq. 2 by an explicit evaluation. e d d´ Ùµ ¾ Ñ = eE · Ù ¢ B; Ñ c = eÙ ¡ E; ¼ ¼ (9) c dØ dØ 1.4 Rotating beam rest frame ¡ ½=¾ ¾ ¾ dØ = d = ½ Ù where , and =c is in this case The equations of motion can be greatly simplified in the Lorentz factor. terms of the variables tangential and normal to the direc- tion of the motion of the reference particle. We thus seek 1.3 Beam rest frame of fixed orientation for another transformation into the rotating beam rest frame Consider a reference particle circulating around the ver- of which the orientation of the spatial axes are constantly ¾ Ü Þ Ý ! tical axis X with a uniform angular velocity at a radius aligned to the radial ( ), tangential ( ), and vertical ( ) di- i ff g Ê . Its time track may be described in the laboratory rection of the motion of the reference particle. frame in terms of the proper time Define the rotating beam frame as the rest frame of Þ which the orientations of the spatial axes Ü and rotate i ½ ¿ f ´ µ=´Ê cÓ× ; ¼;Ê×iÒ ; µ ; Ü (10) relative to those of the fixed orientation Ü and ¼ ½ ¼ ½ ¼ ½ ¡ ½=¾ ½ ~ ~ ¾ Ü cÓ× ¼ ×iÒ ¼ Ü = ! = ¬ where is the revolution angle, ½ , ¾ B C B C B C Ý ¼ ½ ¼ ¼ Ü = Ê! and ¬c is the velocity. Introduce a rigid system B C B C B C = : (14) ¿ i @ A @ A @ A ~ ~ Þ Ü ×iÒ ¼ cÓ× ¼ Ü µ of coordinates ´ which follows the reference particle in 4 Ü ¼ ¼ ½ its motion, so that the particle is constantly situated at the ¼ origin of this frame of reference, and that the spatial axes Express the electric and magnetic fields in the laboratory have constant orientations. The transformation connecting Þ i i frame in terms of the tangential component in and the X µ ´Ü µ the variables ´ and are normal component in Ü i i 4 X = f ´ µ·« Ü ; aÒd Ü = ; i (11) E = E cÓ× · E ×iÒ ; B = B cÓ× · B ×iÒ Ü ½ ¿ Ü ½ ¿ = E ; B = B where the check ( ) denotes the inverse operation, the sum- E Ý ¾ Ý ¾ ½; ¾; ¿ = E ×iÒ · E cÓ× ; B = B ×iÒ · B cÓ× mation over is on spatial components . The coef- E Þ ½ ¿ Þ ½ ¿ « ficients ij are obtained by means of successive infinitesi- (15) mal Lorentz transformations without rotation of the spatial The equations of motion can be obtained from Eqs. 2 and axes,[2] 3 by using Eq. 8 and the transformation relations as ¼ ½ 8 ¡ Ñ ~ ¼ « ¼ « ¬ ×iÒ ¾ 4 ¾ ¾ ¾ ½½ ½¿ > Ü ¾ ! Þ_ ! Ü Ñ ! Ê ´½ µ= ¼ > B C > ¼ ½ ¼ ¼ > B C > ¢ £ @Î e f« g = ; > ij ¼ ¼ ¾ ¼ > @ A ~ ; B Ý_ B ´_Þ · !ܵ = e´½ µE · > « ¼ « ¬ cÓ× ¿½ ¿¿ Þ Ý Ü > > c @Ü > Ñ > ¬ ×iÒ ¼ ¬ cÓ× ¼ ¼ > Ý = e´½ µE · < Ý (12) ¢ £ @Î e ~ ~ ~ ¼ ¾ ¼ ¾ « = cÓ× cÓ× · ×iÒ ×iÒ « = ×iÒ cÓ× > ½¿ where ½½ , B ´_Þ · !Ü B ´_Ü !Þ µ ; · > Ü Þ > ~ ~ ~ > c @Ý cÓ× ×iÒ « = cÓ× ×iÒ ×iÒ cÓ× « = > ¿¿ , ¿½ , ¡ > Ñ ¼ > ¾ 4 ¾ ¾ ~ ~ ~ > Þ ·¾ ! Ü_ ! Þ = ×iÒ ×iÒ · cÓ× cÓ× = ! = : , and The fact > > > ~ > ¢ £ > @Î that differs from the revolution angle by the Lorentz e ¾ ¼ ¼ ¼ : ; Ý_ ´_Ü !Þ µ B B · = e´½ µE Ü Ý Þ c @Þ factor is the consequence of the Thomas precession. To obtain the metric tensor of the rest frame, we need (16) to derive the relation between the line elements of the two where coordinate systems. Differentiation of Eq. 11 gives ¼ ¼ E = ´E ¬B µ; B = ´B · ¬E µ Ü Ý Ü Ý Ü Ü ¼ ¼ i E = ´E · ¬B µ; B = ´B ¬E µ Ý Ü Ü Ý (17) Ý Ý « ´ µ f ´ µ i i i j ¼ ¼ dX · Ü =« dÜ · d « dÜ ; i j E = E ; B = B Þ Þ Þ Þ d d (13) i i ½ f« g = f« where g is the transformation matrix be- are the electric and magnetic fields after a Lorentz transfor- j j tween the line elements of the laboratory and the rest frame.