The OPAL Discussion Forum

[Norman Huang <norman-huang AT hotmail.com>]

Thanks for the clarification, Steve.

I assume that SigmaPx and SigmaPy suffer the same mislabeling, and are also in units of eV. But as eV describes energy, I find this to be a very puzzling way to describe beam distribution.

I have the magnitude of momentum deviations along X and Y direction, how can I convert these values into SigmaPx and SigmaPy in the input file?

Regards,
Norman

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Date: Thu, 13 Oct 2011 17:41:46 -0600
From: srussell AT lanl.gov
To: norman-huang AT hotmail.com
CC: andreas.adelmann AT psi.ch; krab AT triumf.ca; raoyn AT triumf.ca
Subject: Re: [Opal] Single quadrupole test

Norman,

The units of PT are in eV. This is not an approximation but is more a poor choice of variable names.

I realize this is confusing and some poor choices were made out of expediency in some cases in order to get things working quickly. The Distribution class was probably the most hard hit as a lot of experimentation went on to get specialized distributions for certain problems and as our thinking evolved on the proper way to generate and emit particles.

We realize this is all an issue, of course, and anticipate a major rewrite of how particles are generated in OPAL to make that part of the code more transparent. In the mean time though, we are stuck with what we have.

The inconsistent use of units is also something we are working to fix.

Steve


The distribution class is probably the most confusing part of the code as it has probably had

On 10/13/2011 03:15 AM, Norman Huang wrote:

Hi Steve,

Thanks for the tip, I've ran the test again following your suggestion and I'm seeing the correct energy. But now I'm even more confused.

It now appears to me that the average energy (at least that printed out by OPAL on the cosole) is taken directly from the value of PT. This doesn't make sense to me... Is it doing an approximation? Is there an implicit assumption that the resting mass of the particle is much smaller than the kinetic energy? In fact, I've noticed many places in the code where momentum and energy are used almost interchangeably.

I would much appreciate it if anyone could help me out with this.

Thanks,
Norman

Steve Russell <srussell AT lanl.gov> wrote:

Hi Norman,

When I run this file I get a distribution with a kinetic energy over 1000 MeV. Try changing the distribution command from

Dist1:DISTRIBUTION, DISTRIBUTION=gauss,
                    sigmax=  0.005, sigmapx=0.0, corrx=0.0,
                    sigmay=  0.005, sigmapy=0.0, corry=0.0,
                    t = 0.0, sigmat= 0.00040, pt=P0*1e9, sigmapt=0.0,corrt=0.0;

to

Dist1:DISTRIBUTION, DISTRIBUTION=gauss,
                    sigmax=  0.005, sigmapx=0.0, corrx=0.0,
                    sigmay=  0.005, sigmapy=0.0, corry=0.0,
                    t = 0.0, sigmat= 0.00040, pt=Edes*1e9, sigmapt=0.0,corrt=0.0;

I used "Edes" instead of "P0" to set the value of pt.

This at least gives the right average energy.

Regards

Steve


On 10/12/2011 04:19 PM, Norman Huang wrote:

Hi Andreas,

Following up on my request for a sample test file, I've attached the test case that we wrote to simulate a pencil beam through a single focusing quadrupole. Maybe I'm misinterpreting the input format, as the result does not match with expectation; in any case, could you take a look?

---

Here are the specs for our configuration:

Track: Drift1---->Quarupole---->Drift2

Drift1 = 10 cm
Drift2 = 122.7647 cm

Beam: Proton

Kinetic energy = 460 MeV

Magnetic rigidity = 3.45816 Tm

Initial condition:

Sigma X = Sigma Y = 0.5 cm

Sigma Px = Sigma Py = 0

r12 = r34 = 0

Quadrupole:

Pole tip field = 0.5 T

Apperture radius = 3.55 cm

Length = 20 cm

---

We still don't know what SigmaT and SigmaPt refer to, but OPAL wouldn't run if we set them to 0. 

The expected output is an upright X-Px distribution at the end of Drift2, which we're not getting.

Thanks,

Norman

-- 
Steve Russell
Technical Staff Member
MS H851
P.O. Box 1663
Los Alamos National Laboratory
Los Alamos, NM 87545
USA
Phone: (505) 665-3712
Fax: (505) 667-8207

-- 
Steve Russell
Technical Staff Member
MS H851
P.O. Box 1663
Los Alamos National Laboratory
Los Alamos, NM 87545
USA
Phone: (505) 665-3712
Fax: (505) 667-8207