Projected Emittance

Projected Emittance Computation

Projected Emittance

Projected Emittance In order to compute the Projected Emittance either in x- or in y-direction with a series of measured beam profiles (non-destructive beam data collection) there are several methods well suitable for doing it with two more or less known computer codes.

  • Computing the 86% Projected Emittance with Graphic Transport:

    • From the measured beam profile data collected along the beam line the Sigma-widths have to be computed. The given Fortran Listing shows how this may be done. The 2*Sigma values together with the actual quadrupole settings have to be entered in a special Transport input file in order to perform an Envelope Fit for the actual optics of the beam line.
      In a similar manner the 86% Projected Emittance may be found by measuring one or more beam profiles after a varied set of one or more quadrupoles (with different settings) and by performing a Multiple Envelope Fit with this set of 2*Sigma data.
    • If enough profile data are available, then even information about the momentum spread and the dispersion trajectory may be extracted with the help of an envelope fit procedure. A Simultaneous Fit of two 590 MeV proton beam optics is a typical example for this advanced technique. A necessary condition is that for both sets of quadrupole lens settings the beam has to be transportable with reasonable low losses along the beam line. Furthermore, the 2 settings have to differ enough in values for the quadrupole settings and in the measured profile widths at the corresponding locations.
    • The projected emittances in x and y may be read out with the Ellipse Display routine of the Transport framework, which draws the beam ellipses in (x/x') and (y/y') and shows among several parameters also Epsx and Epsy which represent the respective projected emittances. Beam ellipses and projected emittance in (longitudinal) z-direction are also available if required.
    • In order to learn more about projected emittance, beam ellipses etc. in connection with Transport, the SIN-report Representation of Beam Ellipses for Transport Calculations by Werner Joho is recommended. In this report all aspects concerning beam ellipses are discussed by an expert.

  • Computing the projected emittace by reconstructing the measured beam profiles with a Maximum Entropy Beam Tomography (MENT) algorithm:

    Projected Emittance In order to get this procedure working you need to measure at least 3 profiles either in x- or in y-direction. Their phase advance angles should typically differ from profile to profile by about 60 degrees (3 profiles) or 30 degrees (5 profiles). At one profile location the phase-angle should be zero (waist, most narrow beam width). An 86%-value may also be extracted from this method and compared with the projected emittance value computed with the Transport envelope fit method. More info is given in a 1982 SIN Annual Report about MENT or partially in a 2000 PSI Annual Report. The downloadable MENT-packages contain several input files with profile data taken from several proton beam lines at PSI.
Note: By applying both computation methods to the same profile data for gaining the corresponding 86% projected emittance values, the two values usually agree quite well. But sometimes with imperfect profile data the 2 values may differ by up to a factor of 2. Then the value extracted with Beam Tomography (MENT) has the tendency to be larger than the value extracted with Graphic Transport.

Last updated by Urs Rohrer on 1-Mar-2006