8. Intro to Nonlinearity#
8.1. Single Sextrupole in a Ring#
Consider a thin sextrupole was added to an otherwise linear ring at \(s_0\). The Hill’s equation will looks like:
(8.1)#\[\begin{align}
&x''+ k_x(s) x = \frac{S}{2} (y^2 - x^2) \delta(s-s_0) \\
&y''+ k_y(s) x = S x y \delta(s-s_0)
\end{align}\]
which can be derived from the Hamiltonian:
(8.2)#\[\begin{equation}
H= \frac{1}{2}x'^2+ \frac{1}{2}y'^2 + \frac{1}{2}k_x(s)x^2+\frac{1}{2}k_y(s)y^2+\delta(s-s_0)\frac{S}{6}(x^3-3xy^2)
\end{equation}\]
Using the action angle variable pairs ((3.26)) for \(x\) and \(y\):
(8.3)#\[\begin{align}
x&=\sqrt{2\beta_x J_x} \cos(\psi_x+ \chi_x(s) -\nu_x \theta) = \sqrt{2\beta_x J_x} \cos(\Phi_x)\\
y&=\sqrt{2\beta_y J_y} \cos(\psi_y+ \chi_y(s) -\nu_y \theta) = \sqrt{2\beta_y J_y} \cos(\Phi_y)\\
\end{align}\]
with
(8.4)#\[\begin{equation}
\chi_x(s) = \int_0^s\frac{1}{\beta_x(s')}ds' \qquad \chi_y(s)=\int_0^s\frac{1}{\beta_y(s')}ds'
\end{equation}\]
Using the trigonometry identity \(\cos^3\alpha = (\cos3x + 3\cos x)/4\), the potential part beomes:
(8.5)#\[\begin{align}
x^3-3xy^2 =& \frac{\sqrt{2}}{12}(\beta_x J_x)^{3/2} \cos3\Phi_x + \frac{\sqrt{2}}{4}(\beta_x J_x)^{1/2}(\beta_x J_x - \beta_y J_y) \cos\Phi_x \nonumber\\
&-\frac{\sqrt{2}}{8}(\beta_x J_x)^{1/2}(\beta_y J_y) \cos(2\Phi_x+\Phi_y) -\frac{\sqrt{2}}{8}(\beta_x J_x)^{1/2}(\beta_y J_y) \cos(2\Phi_x-\Phi_y) \nonumber
\end{align}\]
The Hamiltonian becomes:
(8.6)#\[\begin{align}
H = \nu_x J_x + \nu_y J_y &+ \sum_l G_{3,0,l} J_x^{3/2} \cos(3\psi_x - l\theta + \xi_{3,0,l}) \\
& + \sum_l G_{1,2,l} J_x^{1/2} J_y \cos(\psi_x + 2 \psi_y - l\theta + \xi_{1,2,l}) \\
& + \sum_l G_{1,-2,l} J_x^{1/2} J_y \cos(\psi_x - 2 \psi_y - l\theta + \xi_{1,-2,l}) + \cdots
\end{align}\]
where \(G\) and \(\xi\) are the amplitudes and phases of the Fourier coefficients. For example,
\[
G_{3,0,l}e^{i\xi_{3,0,l}} = \frac{\sqrt{2}}{24\pi}\int_0^{2\pi}S\delta(s-s_0)\beta_x^{3/2} e^{j\left[3\chi_x(s)-(3\nu_x-l)\theta\right]} d\theta
\]
It seems that this single sextrupole can excite \(3\nu_x = l\) and \(\nu_x \pm 2\nu_y = l\) resonances. However, through concatenation, one sextrupole in the ring can excite much more resonances, such as \(4\nu_{x/y}=l\), \(2\nu_x \pm 2\nu_y =l\), etc.
Example: Sextrupole in a linear ring (4-D Henon map).
This examples illustrates different resonances excited by a sextrupole using the following map:
(8.7)#\[\begin{equation}
\left(\begin{array}{c}
x\\
x'\\
y\\
y'
\end{array}\right)_{n+1}=\left(\begin{array}{cc}
\textbf{R}(\mu_x) & 0\\
0 & \textbf{R}(\mu_y)
\end{array}\right)\left(\begin{array}{c}
x\\
x'-x^{2}+y^{2}\\
y\\
y' + 2 x y
\end{array}\right)_{n}
\end{equation}\]
In this case, \(S\) is choosen to be 2. One may notice that for the single sextrupole, the strength does not affect the dynamics, which can be eliminated by the scaling \(\mathbf{z}=(x, x', y, y') \rightarrow \mathbf{z}/S\)
Show code cell source
from map2D import map2D
import numpy as np
import matplotlib.pyplot as plt
#from matplotlib import animation
#from IPython.display import HTML
%matplotlib widget
#resonance 1/3
dvx=0.0003591
tunex=1.0/3.0+dvx
dvy=0.05
tuney=1.0/3.0+dvy
size_scale=0.001
#resonance 1/4
dvx=0.00005
tunex=1.0/4.0+dvx
dvy=0.005
tuney=1.0/4.0 + dvy
size_scale=0.03
#resonance 1/5
dvx=0.0001
tunex=1.0/5.0 + dvx
dvy=0.01
tuney=1.0/5.0 +dvy
size_scale=0.0205
#resonance nux + 2nuy = l
dvy=0.0002
tunex = 0.23
tuney = (1 - tunex) / 2 + dvy
size_scale = 0.001
# resonance nux = 2nuy
dvy=0.000
tunex = 0.23
tuney = tunex / 2 + dvy
# resonance 2nux + nuy =l
dvy=0.000
tunex = 0.102
tuney = 1- tunex * 2 + dvy
pxs=np.concatenate([size_scale*np.arange(10), np.array([])])
pxps=np.zeros_like(pxs)
particles=np.vstack([pxs,pxps])
xpx=map2D(npart=10, twiss=[1,0], twiss_beam=[1,0],tune=tunex, chrom=0.0, espr=0.0,
particles=particles)
pys=np.zeros_like(pxs) + size_scale * 3
pyps=np.zeros_like(pys)
particles=np.vstack([pys,pyps])
ypy=map2D(npart=len(pxs), twiss=[1,0], twiss_beam=[1,0],tune=tuney, chrom=0.0, espr=0.0,
particles=particles)
avex,avep,sizex,sizep,emit=xpx.statistics()
emitlist=[]
sizelist=[]
avelist=[]
N_turn=30000
def evolve_func(turns, kick_turn_start=0, B2=1,
):
for i in range(turns):
if i>=kick_turn_start:
xpx.coor2D[1,:]-=B2*(xpx.coor2D[0,:]*xpx.coor2D[0,:]-ypy.coor2D[0,:]*ypy.coor2D[0,:])/2.0
ypy.coor2D[1,:]+=B2*ypy.coor2D[0,:]*xpx.coor2D[0,:]
xpx.propagate()
ypy.propagate()
#avex,avep,sizex,sizep,emit=xpx.statistics()
#avelist.append(avex)
#sizelist.append(sizex)
#emitlist.append(emit)
yield xpx.coor2D, ypy.coor2D
evolve=evolve_func(N_turn+2)
fig,(ax_x, ax_y)=plt.subplots(1,2, figsize=(6,4))
display_ratio=20
ax_x.set_aspect('equal')
ax_y.set_aspect('equal')
ax_x.set_xlim([-display_ratio*size_scale,display_ratio*size_scale])
ax_x.set_ylim([-display_ratio*size_scale,display_ratio*size_scale])
ax_y.set_xlim([-display_ratio*size_scale,display_ratio*size_scale])
ax_y.set_ylim([-display_ratio*size_scale,display_ratio*size_scale])
ax_x.set_xticks([])
ax_x.set_yticks([])
ax_y.set_xticks([])
ax_y.set_yticks([])
xlist=[]
xplist=[]
ylist=[]
yplist=[]
for i in range(N_turn):
arrx, arry=next(evolve)
#if np.max(arr)>1: break
xlist.append(arrx[0])
xplist.append(arrx[1])
ylist.append(arry[0])
yplist.append(arry[1])
xarr=np.vstack(xlist)
xparr=np.vstack(xplist)
yarr=np.vstack(ylist)
yparr=np.vstack(yplist)
ax_x.plot(xarr,xparr,linestyle='None', marker='.', markersize=2)
ax_y.plot(yarr,yparr,linestyle='None', marker='.', markersize=2, alpha=0.5)
fig.set_label('Linear Tunex='+str(tunex)+', Linear Tuney='+str(tuney))
fig.tight_layout()
Jx=xarr*xarr+xparr*xparr
Show code cell source
par_id =1
every = 1
fig=plt.figure()
ax=plt.axes(projection='3d')
ax.set_xlabel(r"$x$")
ax.set_ylabel(r"$y$")
ax.set_zlabel(r"$x'$")
p=ax.scatter(xarr[::every, par_id], yarr[::every, par_id], xparr[::every, par_id], s=0.1, c=yparr[::every, par_id])
#p=ax.scatter(xcal[-temp::every], ycal[-temp::every], pycal[-temp::every], s=0.1, c=pxcal[-temp::every])
cbar=plt.colorbar(p, location='top', shrink=0.5)
cbar.ax.set_xlabel(r"$y'$");
8.2. Orbit Stability and Resonances#
We have learned that a single sextrupole in an else-where linear ring can excite a large range of resonance and some of then will make motion unstable. A mathematical guidance of finding the orbit stability when an integrable system is nonlinearly perturbed is the KAM theory from Kolmogorov, Arnold and Moser. which states
KAM Theory
A n-dimentional integrable Hamiltonian will confine the motion on a n-D torus. KAM theory state that, such tori will only be distorted not destroyed by a weak nonlinear perturbation und the following conditions:
Non-resonance: the tune is sufficiently irrational, in the Diophantine sense,
\[
\left|x-\frac{q}{q}\right| > \frac{c}{q^2}
\]
Non-degeneracy: the Hamiltonian ‘s Hessian is not singular.
In reality, the working point should avoid any lower order (\(q\)) resoances.
Show code cell source
import numpy as np
def draw_resonance_lines(ax, tunex_range=[0,1], tuney_range=[0,1], synchro_line=False,
synchro_order=[1,], synchro_freq=0.01,
max_order=8, line_width=0.4, coupling=True, working_point=None,
maximum_alpha=0.8, minimum_alpha=0.2):
n_dots = 2
ax.set_aspect('equal')
ax.set_xlabel('Horizontal Tune')
ax.set_ylabel('Vertical Tune')
ax.set_xlim(tunex_range)
ax.set_ylim(tuney_range)
if working_point is not None:
ax.plot(working_point[0], working_point[1], marker='.')
for order in range(1,max_order+1):
xdots=np.linspace(tunex_range[0], tunex_range[1],n_dots)
for l in range(1, order):
ydots=np.zeros_like(xdots)+1.0*l/order
ax.plot(xdots,ydots,'k--', alpha=minimum_alpha+maximum_alpha*(max_order-order)/max_order, linewidth=line_width)
ax.plot(ydots,xdots,'k--', alpha=minimum_alpha+maximum_alpha*(max_order-order)/max_order, linewidth=line_width)
if coupling==False:
return
for xorder in range(1,max_order):
for yorder in range(1,max_order-xorder+1):
order=xorder+yorder
for l in range(-order+1, order):
ydots=np.linspace(tuney_range[0], tuney_range[1],n_dots)
xdots=(l-yorder*ydots)/xorder
ax.plot(xdots,ydots,'k--', alpha=minimum_alpha+maximum_alpha*(max_order-abs(order))/max_order, linewidth=line_width*0.8)
xdots=(l+yorder*ydots)/xorder
ax.plot(xdots,ydots,'k--', alpha=minimum_alpha+maximum_alpha*(max_order-abs(order))/max_order, linewidth=line_width*0.8)
if synchro_line:
for sorder in synchro_order:
for xorder in range(1, max_order-sorder):
for yorder in range(1, max_order - sorder - xorder + 1):
order = xorder + yorder
for l in range(-order + 1, order):
ydots = np.linspace(tuney_range[0], tuney_range[1], n_dots)
xdots = (l - sorder*synchro_freq - yorder * ydots) / xorder
ax.plot(xdots, ydots, 'k--',
alpha=minimum_alpha + maximum_alpha * (max_order - abs(order)) / max_order,
linewidth=line_width * 0.8)
xdots = (l - sorder*synchro_freq + yorder * ydots) / xorder
ax.plot(xdots, ydots, 'k--',
alpha=minimum_alpha + maximum_alpha * (max_order - abs(order)) / max_order,
linewidth=line_width * 0.8)
xdots = (l + sorder * synchro_freq - yorder * ydots) / xorder
ax.plot(xdots, ydots, 'k--',
alpha=minimum_alpha + maximum_alpha * (max_order - abs(order)) / max_order,
linewidth=line_width * 0.8)
xdots = (l + sorder * synchro_freq + yorder * ydots) / xorder
ax.plot(xdots, ydots, 'k--',
alpha=minimum_alpha + maximum_alpha * (max_order - abs(order)) / max_order,
linewidth=line_width * 0.8)
return
import matplotlib.pyplot as plt
%matplotlib widget
fig,ax=plt.subplots()
maxorder=10
draw_resonance_lines(ax, max_order=maxorder)
fig.set_label('Maximal Order='+str(maxorder))