The OPAL Discussion Forum

["Power, John" <JP AT anl.gov>]

Thanks to everyone for your answers.   I really appreciate the work you guys 
are doing here since the community is need of a modern, well-documented, 
well-vetted code. Please keep up the good work.

I should have written that I was looking for something similar to GPT (other 
than the price) which uses a 5th order embedded Runge-Kutta integrator with 
adaptive stepsize control.  Further, as a code user it is really the adaptive 
timestep that I find attractive; not the integration algorithm.  Setting the 
initial time step is not a problem.

By the way, I would have thought that an adaptive timestep would be helpful 
in more cases than just those having to do with space charge. For instance, 
an adaptive step would insure that one does not "step" over a short element.

Cheers,
John



>>-----Original Message-----
>>From: Matthias Toggweiler [mailto:matthias.toggweiler AT psi.ch]
>>Sent: Sunday, May 06, 2012 8:05 AM
>>To: Power, John
>>Cc: opal AT lists.psi.ch
>>Subject: Re: [Opal] Runge-Kutta
>>
>>Hi John
>>
>>My colleagues already answered most points, but to summarize and to give a
>>little bit more detail:
>>
>>- Runge-Kutta integrator per se does not use adaptive time steps, but is 
>>just
>>an integrator of higher order available in OPAL-CYCL in addition to the
>>standard Boris-Buneman integrator.
>>
>>- OPAL-T has only the Boris-Buneman integrator (second order leapfrog-like
>>scheme). There is no plan to add a Runge-Kutta integrator here; memory
>>requirements of RK are higher.
>>
>>- The adaptivity in our current work deals with varying the time step
>>according to how strong space charge forces are. Simulations where space
>>charge effects are important (RF photoinjector is a perfect candidate) can
>>profit from this adaptivity a lot.
>>
>>- Simulations where space charge is less important do not profit noticeably
>>from the variable time stepping. There, another approach makes more
>>sense. For example, a multiple-time-stepping integrator in OPAL-CYCL
>>evaluates space charge effects less often than external field effects.
>>
>>- Not everything will be automatic: for example, you will still have to set 
>>an
>>initial time step. Nevertheless, less manual work will be required to 
>>achieve
>>better quality solutions.
>>
>>- In my masterthesis, good results were obtained with a first prototype (see
>>scenario EGun-CTF3 in http://e-
>>collection.library.ethz.ch/eserv/eth:5175/eth-5175-01.pdf). The problem
>>there was that the time step used for space charge effects dictates the time
>>step used for external field effects. We are working on an improved
>>prototype where the integration of these two effects is less tightly 
>>coupled.
>>Our understanding how to the time step should be chosen has also improved
>>since then.
>>
>>To conclude, I am happy that you are interested in this work and we are
>>working hard to get this going. However, as already noted, it is not magic 
>>that
>>will do everything automatically, but one step towards more intelligent
>>integration.
>>
>>Best,
>>Matthias
>>
>>Am 04.05.2012 um 17:55 schrieb Power, John:
>>
>>> Dear All,
>>>
>>> I am looking for an alternative to PARMELA so I am lurking on your list. J
>>>
>>> I see in the user's manual that the Runge-Kutta time integrator is only
>>available in OPAL-CYCL.  Since I work mostly with RF photoinjectors, and I
>>would rather not have to set the time step manually as is done in PARMELA, I
>>wonder if there are any near term plans to add this to OPAL-T since.
>>>
>>> cheers,
>>> John