The OPAL Discussion Forum

[Christian Baumgarten <christian.baumgarten AT psi.ch>]

Hi, Jianjun.

I already informed Andreas that I suspect a bug
in some coordinate transformation inside OPAL.

I tracked a coasting beam in (an idealized) machine
similar to the PSI ring machine for some turns with 
data output every 10 steps and compared the results 
with a linear TRANSPORT-like code (incl. space charge).

I did this by applying a transformation from the global
cartesic frame (X,PX,Y,PY,Z,PZ) to local (co-moving) coordinates.
I carefully checked my transformation several times, 
but always obtained some very strange results.

I found no problem with the vertical coordinates.
The local horizontal coordinates are the usual (x,x',l,\delta),
where l is the longitudinal coordinate and x is the transverse
coordinate. I transformed all 10^5 particles of the bunch,
computed and analyzed the full covariance matrix.

As usual in beam physics, I take the longitudinal coordinates
with index 5 and 6, and the horizontal to have index 1 and 2, so
that (for instance)

c16=cov(1,6)=cov(x,\delta)
c25=cov(2,5)=cov(px,l)

I attached a plot (loc_cov.pdf) of the results from OPAL-cycl 
and (for comparison) the results I obtained with a 
TRANSPORT-like code (ring_sigma.pdf: one sector only; 
starting with 72 MeV(blue) up to 590 MeV(red)).

As visible in ring_sigma.pdf, the covariance c25/c16 should
be more or less constant at low energy (both about ~0.75).
The OPAL result for c16 is in good agreement with this expectation,
but OPAL shows very different behavior for c25. Not only that
it changes significantly, even the sign changes.

I looked more into the details and plotted also some other
results of interest, for instance sigma_px, which I would expect
to vary, but (at low energy) only a little bit. The OPAL-results
do not only indicate strong variations, but also a very unlikely
behavior at the sector edges. 

My conclusion is, that the results indicate a bug in OPAL and from 
the shape of sigma_px I suspect some bug in a local::global 
transformation (or global::local).

Andreas asked me to inform you about my findings. I hope that
this bug concerns only the data output, but to check this certainly 
requires someone who is more familiar with the code than I am.

Let me know, if you have new about this issue.

Best, Christian Baumgarten

PS.: If you have access to felsim, please find my H5-file here:

/scratch/baumgarten/opal/ideal_ring/noacc/NewSingleBunch.h5

My input file:

~baumgarten/opal/ideal_ring/noacc/NewSingleBunch.in

Attachment: loc_cov.pdf
Description: Adobe PDF document

Attachment: ring_sigma.pdf
Description: Adobe PDF document