Introduction

The Twiss file production is an important step as input for FPTracker which is responsible for the tracking in the ALFA simulation chain. Every time the LHC magnet settings change new Twiss files have to be produced. Here it will be described how to produce Twiss files for the optic version 6.500 as an example and where the element sequence, the magnet settings and so on can be found.

The input

The files which are needed for the Twiss file production which describe the element sequence, the magnet settings and so on as input for MadX are found on lxplus in the optic section:
/afs/cern.ch/eng/lhc/optics
The files alfa_b1_twiss.madx and alfa_b4_twiss.madx are the main input files for MadX and contain a configuration script which is responsible to create the Twiss files in the right format and to use the right files provided in the optic folder. The first step which these configuration scripts do is to create a symbolic link to the optic folder to get excess to the files which describe the optics. This is the reason why one has to run them in default from lxplus or another location where on has access to this optics folder. An easy way to run them stand alone is to download the files from the optic folder for the right optic version. Another important parameter for the Twiss file production is the option for the energy:
column=name,keyword,parent,s,l,hkick,vkick,ANGLE,k1l,k2l,k3l,x,y,px,py,betx,bety,alfx,alfy,mux,muy,dx,dy,dpx,dpy,apertype,aper_1,aper_2,aper_3,aper_4;
This is the format as FPTracker reads it in. The Twiss files itself will be produced in the beam 1 and beam 4 format and are PTC Twiss files. These files are thick lens Twiss files.

The Production steps

The Twiss file production is quite simple. The only thing one has to do is to hand over the configuration scripts to MadX. This can easy be done by using the standard stream parameters of Unix < , >.
./madxp_v6.500 < alfa_b1_twiss.madx
./madxp_v6.500 < alfa_b4_twiss.madx
This will produce the Twiss files needed for FPTracker: alfaTwiss1.txt and alfaTwiss2.txt

The simpler way

A more easy way is to use the the TwissFileProduction.sh which offers the possibility to set the beam energy at the beginning of the production.

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Topic revision: r1 - 2010-03-12 - DanielPelikan
 
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