Kickers with non zero length lead to incorrect thick tracking

[chernals]

I understand that this is meant to be used after a MAKETHIN, neverthless:

• there is no complaint from the code when not doing a makethin (ie. doing thick lens tracking)
• this is straightforward physics anyway, so there is no reason not to do it

This goes in multiple steps.

1 - Incorrect drift

Minimal example:

BEAM, PARTICLE=PROTON, BETA=0.5;
SEQ: SEQUENCE, REFER=ENTRY, L=1.0;
    K1: HKICKER, AT=0.0, HKICK=1.0, L=1.0;
ENDSEQUENCE;
USE, SEQUENCE=SEQ;

track, deltap=0.0, onepass=true, onetable=true, quantum=false;
start, x=0.0, px=0.1, y=0.0, py=0.0, t=0.0, pt=0.0;
run, turns=1;
endtrack;

The X coordinate is not modified, indeed the code only applied a kick. The sequence is correctly recognized as having a length of 1m.

2 - Adding a marker

Minimal example:

BEAM, PARTICLE=PROTON, BETA=0.5;
SEQ: SEQUENCE, REFER=ENTRY, L=1.0;
    K1: HKICKER, AT=0.0, HKICK=1.0, L=1.0;
    M1: MARKER, AT=1.0;
ENDSEQUENCE;
USE, SEQUENCE=SEQ;
MAKETHIN, SEQUENCE=SEQ;
USE, SEQUENCE=SEQ;

track, deltap=0.0, onepass=true, onetable=true, quantum=false;
start, x=0.0, px=0.1, y=0.0, py=0.0, t=0.0, pt=0.0;
observe, place=M1;
run, turns=1;
endtrack;

Basically just adding a marker and doing a MAKETHIN.

Now, all the non-modified variables (they should have been but are not), ie. X, Y and PT are nan.

3 - Chromatic effects neglected

Minimal example:

BEAM, PARTICLE=PROTON, BETA=0.5;
SEQ: SEQUENCE, REFER=ENTRY, L=1.0;
    K1: HKICKER, AT=0.0, HKICK=1.0, L=1.0;
ENDSEQUENCE;
USE, SEQUENCE=SEQ;
USE, SEQUENCE=SEQ;

track, deltap=0.0, onepass=true, onetable=true, quantum=false;
start, x=0.0, px=0.1, y=0.0, py=0.0, t=0.0, pt=0.5;
run, turns=1;
endtrack;

Removed the marker, expect the kick to be scaled according to PT, it is not (the scaling should be the usual delta_plus_one).

[rdemaria]

Hello Cedric,
for your point 3, px is the momentum (px=Px/P0) and not the angle (x'=px/(1+delta) = Px/P), therefore it is ok that the change in px does not depend on pt.

[tpersson]

1. I agree. This is now allowed in tracking so we have to follow the implementation there. Thanks for pointing it out!
2. It is not allowed to use Makethin with refer=entry. This is written in the manual but I will have a look and see if we can throw an error when this is being done since this question has come up before.
3. Riccardo already answered and I agree.

[chernals]

@rdemaria

OK I was confused as I looked at the MAD-8 manual where the KICK is the kick angle, while in MAD-X KICK is indeed the momentum change.

@tpersson

1. For the implementation, it seems that there are two ways we could choose: the old code which is commented out but already present, where it is HALF-KICK - DRIFT - HALF-KICK and the MAD8 physics guide which mentions DRIFT-KICK-DRIFT . Not sure which way we should go. Or we could simply make it a kind of BEND to actually treat it as a thick element.

2. OK, I didn't see that but I agree.

3. See above.

[tpersson]

This is fixed in PR #851