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RE: [Opal] Transit time factor calculation in OPAL

Chronological Thread  
  • From: Daniel Winklehner <winklehn AT>
  • To: Snuverink Jochem <jochem.snuverink AT>, Malek Haj Tahar <malek AT>
  • Cc: "opal AT" <opal AT>
  • Subject: RE: [Opal] Transit time factor calculation in OPAL
  • Date: Wed, 8 Mar 2023 16:34:26 +0000
  • Accept-language: en-US
  • Authentication-results:; iprev=pass ( smtp.remote-ip=; spf=pass; dkim=pass header.s=outgoing header.a=rsa-sha256; dmarc=pass

Hi Malek,


I know you know this, but if anyone else follows the thread: The transit time correction or transit time factor (TTF) is a modifying factor to the energy gain in the idealized cavity that depends on gap length, frequency, and particle velocity like this : TTF =  sin(omega*d/2*v)/( omega*d/2*v). It stems from the voltage changing while the particle traverses the finite-length gap. Energy gain is calculated as q*V*TTF*cos(phi) where phi is the cavity phase.


You might be looking for the following snippet of code in /src/Classic/AbsBeamline/RFCavity.cpp:


    if (gapwidth_m > 0.0) {

        double transit_factor = 0.5 * frequency * gapwidth_m * Units::mm2m / (Physics::c * beta);

        Ufactor = std::sin(transit_factor) / transit_factor;



Also here:





From: opal-request AT <opal-request AT> On Behalf Of Snuverink Jochem
Sent: Wednesday, March 8, 2023 11:09 AM
To: Malek Haj Tahar <malek AT>
Cc: opal AT
Subject: AW: [Opal] Transit time factor calculation in OPAL


Dear Malek


Apologies for the late answer.


The correction is done when a particle step crosses an RF cavity. In this case the step is split in two, one with the old particle energy up to the cavity, and one with a step after the cavity with the new particle energy. I believe this is what is meant with the transit time correction.


To see the code, this is implemented twice (in the same way) at:



Best wishes,


Von: opal-request AT <opal-request AT> im Auftrag von Malek Haj Tahar <malek AT>
Gesendet: Dienstag, 21. Februar 2023 11:35
An: Opal AT <Opal AT>
Betreff: [Opal] Transit time factor calculation in OPAL


Dear OPAL developers,


Looking at the RFcavity model implementation in OPAL, one could read in the manual that


"The RF cavities are treated as straight lines with infinitely narrow gaps and the electric field is a δ function plus a transit time correction."


Could you please let me know where to find more info regarding how OPAL computes the transit time correction? Is this an analytical model available somewhere?


Many thanks for your time and support!


Best regards,





Dr. Malek Haj Tahar

Computational physicist

Transmutex SA

Mobile: +41 784 202 022

Chemin de Blandonnet 8

1214 Genève (Vernier)


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