Elements/MARK

MARK †

MARK elements play special rolls in FFS:

(1) The first element of the beam line must be a MARK element to be used by FFS. In this case the MARK element contains the parameters of the incoming beam (see optical-functions, special-variables EMITX, EMITY, DP).

(2) The calculated optical parameters at a MARK command is saved by SAVE or STOP commands, then it can be used as the incoming condition of other beam lines which have the same MARK element.

Example:

        MARK P1 = (EMITX = .. EMITY = .. DP = ..);
        LINE  A = ( .. P1 ..)
              B = (P1 .. );
        FFS USE = A;
            ...           do mathing on LINE A
            SAVE P1       save the parameters at P1
            USE B;        switch to LINE B
            ...           do matching of LINE B whose entrance is to be
                          matched P1.

(3) If a MARK element has keyword GEO nonzero, this MARK element becomes the origin of the geometric rotation after the last SOL element.

(4) The values of optical-functions of the MARK element at the beginning of the beam line can be specified as matching variables by the FREE command.

OFFSET

OFFSET is a relative position from the current potision. A fraction is allowed to specify a location within an element. If the MARK at the beginning of a beam line has OFFSET nonzero, the optics calculation starts from the shifted location. If the last component of a beam line is a MARK with nonzero OFFSET, the optics calculation stops at the shifted location. The periodic condition is applied between those shifted locations. The geometric origin and the origin of LENG shift to the first MARK. Examples:

(1)  LINE A    = ( ...  QF PQFC  ... );
     QUAD QF   = (L=0.3 K1=0.2);
     MARK PQFC = (OFFSET = -0.5);

Here PQFC represents the center of QF.

(2)  LINE A    = ( ...  PQFC QF  ... );
     QUAD QF   = (L=0.3 K1=0.2);
     MARK PQFC = (OFFSET =  1.5);

Here PQFC represents the center of QF, too (consider why). The value of OFFSET is interpreted taking the direction of the LINE into account, i.e., a MARK in a line A represents the same location in a line -A.
Restrictions:
(1) Function TrackParticles does not take OFFSET into account if the start or stop location is in the midst of a beamline and a Mark with nonzero OFFSET, in the current version. Tracking for entire beam line or MEASURE(MEA) command supports OFFSET.

(2) The outputs by DISPLAY(DISP) outside of the narrowed region by OFFSET are meaningless.

MARK †

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