non linear chromaticity in fast ring

[oscarxblanco]

Dear all,

I am checking the fast ring implementation in AT matlab and pyat, and both are set to provide the linear chromaticity in the non-linear element. See the 'chrom_maxorder' parameter:

at/pyat/at/physics/nonlinear.py

Lines 231 to 246 in a6840dd

	nonlin_elem = Element(
	"NonLinear",
	PassMethod="DeltaQPass",
	Betax=lindata0.beta[0],
	Betay=lindata0.beta[1],
	Alphax=lindata0.alpha[0],
	Alphay=lindata0.alpha[1],
	chromx_arr=np.array([xsi[0]]),
	chromy_arr=np.array([xsi[1]]),
	A1=r1[0][0],
	A2=r1[0][1],
	A3=r1[1][1],
	T1=-orbit,
	T2=orbit,
	chrom_maxorder=1,
	)

at/atmat/lattice/atfastring.m

Lines 82 to 87 in a6840dd

	nonlin_elemrad=atbaselem('NonLinear','DeltaQPass',...
	'Betax',lindata.beta(1),'Betay',lindata.beta(2),...
	'Alphax',lindata.alpha(1),'Alphay',lindata.alpha(2),...
	'chromx_arr',[xsi(1)],'chromy_arr',[xsi(2)],'chrom_maxorder',1,...
	'A1',r1(1),'A2',r1(2),'A3',r1(4),...
	'T1',-orbit6{end},'T2',orbit6{end});

Due to energy spread, the tune shift with energy of a particle bunch also depends on the non-linear chromaticity.
Is there already any work, project or thought to extend the non-linear element to an arbitrary higher order chromaticity ? or at least to second order ?

o

[swhite2401]

I don't know of any work on the topic but this would certainly be most useful.

There are other issues with fastring see #1029 and the interface is not great right now, for example enable_6d()/disable_6() does not work, my feeling is that it would require a complete refurbishment.

@lfarv you mentioned on time you had an improved version, is this correct? Should we start from there?

[lfarv]

I agree with the interest of taking into account higher order chromaticities. I had indeed some time ago an attempt to improve fastring, but it's outdated since long. Better starting from scratch now…