Self-Consistent Computation of Electromagnetic Fields and Phase Space Densities for Particles on Curved Planar Orbits Page: 3 of 5
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Figure 2: Plan for B integration.
second step the variables s and v are interchanged making
s the new independent variable and in the final step z :=
s - 3ru replaces v as a dependent variable and pz := ( -
7,)/7, replaces ps. (11) defines s = s(R) and x = x(R)
so that z = z(R, u) = s(R) - ,/u and we have the identity
R - RT(z(R, u) + ),u) + x(R)n(z(R, u) + ),u). Now z
is small in the bunch and expanding for small z gives R
RrO(rv) + M(j3Tu)r + O(r(z2 + xz) and we obtain ob-
tain the approximate inverse r = MT (,u) (R -RO,u)).
Here M(s) = (t(s), n(s)) and r = (z, x)T.
The equations of motion in (z, x, pz, p; s) have the
fields 8(R, u) evaluated at R = R,(s)) + xn(s) and
u = (s - z)/),. We have the following approximations
8(R,(s) + xn(s), (s - z)/),) 8 S(R,(s + z) + xn(s +
z), s//h) E(R(O,3u) + M( u)r, s). At the first ap-
proximation we use the fact that the fields are slowly vary-
ing in s for fixed r. The second approximation uses the fact
that ), j 1 and we are only interested in the fields in the
bunch for r small. We obtain
z -n(s)x pz Fzi(R, s) +pxFz2(R, s)
X' - pX p' _= (s)pz + Fx(Rs),
where R := Rn(s) + M(s)r and' = d/ds. The self-forces
are given approximately by
Fzi = P Eli * t(s),
Fz2 = -EI n(s)
P c ()
Figure 3: Beam frame coordinates.
The beam frame is defined in terms of the reference or-
bit using the Frenet-Serret coordinates (s, x), where s is the
arc length along the reference orbit and x is the perpendic-
ular distance along n from the orbit at RT (s) as shown in
Fig. 3. Recall the reference orbit for a bunch compressor
in Fig. 1.
The transformation (Z, X) to (s, x) is
R = R, (s) + xn(s), (11)
where R,(s) (Z,(s), X,(s)) and the unit normal vec-
tor n(s) :- (-X'(s), Z'(s)). The corresponding tangent
vector is t(s) R,(s) (Z'(s), X'(s)). In addition,
we define ps and px by P : PT(pst(s) + pyn(s)), where
PT- miyr#c is the momentum of the reference particle.
Finally, we define the curvature K(s) by n'(s) n(s)t(s)
and it follows that t'(s) -n(s)n(s). In terms of Fig. 1
this makes K negative in the first magnet, positive in the
second and so on.
Our lab to beam transformation has three steps:
(Z, X, Pz, Px; u) - (s, x, ps, pX; U) (12)
- (U, x, Ps, P; s) - (z, x, pz,pr; s).
The first step is the transformation just discussed, in the
Fx = q (-EzX'(s) + ExZ'(s) - cBy), (14)
where (Ez, Ex, By) are evaluated at (R, s). We have ex-
panded Fx in order to point out that each of the last two
terms are large whereas their difference is small.
The equations of motion (13), without the self fields,
have been linearized. They can be solved and the solution
written ( 4)(s)(o, where ( (z, x, pz, px)T. Here 4) is
the principal solution matrix which is defined in terms of
the dispersion function, D(s), and R56(s) (see ). The
equations of motion in the interaction picture become
(o 4)-1(s)(0, F, 0, F) T (s, (o).
4)(s) varies more slowly than the self-forces and so we nu-
merically solve these rather than (13). A larger time step,
which is controlled entirely by the self-forces, can be used.
Our field formula is in the lab frame so the lab charge
and current densities must be determined from the beam
frame PSD. The relation between lab and beam PSDs is
4'L(Z,X,PZ,PX;u) =#fB(zxpz,p2; s).
This leads to
pL (R; u) f dpzdp fB =: PB(r; s),
JL(R; u) 0, c[pB(z, x; s)t(s) + T(z, x; s)n(s)], (18)
where r(z, x; s) = f p~fB(z, x, pZ, pX; s)dpzdpz. Us-
ing the fact that fB(z, x, pz, pX; -) is slowly varying
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Ellison, J.A.; Bassi, G.; Heinemann, K.A.; U., /New Mexico; Venturini, M.; /LBL, Berkeley et al. Self-Consistent Computation of Electromagnetic Fields and Phase Space Densities for Particles on Curved Planar Orbits, article, November 2, 2007; [Menlo Park, California]. (digital.library.unt.edu/ark:/67531/metadc888527/m1/3/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.