thin multipole h!=k0 off momentum issue

[rdemaria]

p_t and delta dependence in twiss are problematic when h!=k0. Here an example:

ss: sequence,l=1;
m: multipole,knl={0},angle=0.15,at=0;
endsequence;

is a thin multipole with no field and curvature. Effectively, it is a y-rotation.

A twiss on and off momentun should return the same final orbit, becase the particle goes straight in absolute coordinates and the effect of the element is only changing the reference frame.

The calculation

beam,pc=10000;
use,sequence=ss;twiss,betx=1,bety=1,pt=0.0;
xx=table(twiss,ss$end,x);
use,sequence=ss;twiss,betx=1,bety=1,pt=0.001;
xx_pt=table(twiss,ss$end,x);
value,xx_pt-xx, xx_delta-xx;

however, shows a dependence:

value,xx_pt-xx, xx_delta-xx;
xx_pt-xx           =            -0.0001 ;
xx_delta-xx        =    -9.99000999e-05 ;