Hi,
Thanks for the earlier reply. I'm trying to run a simple simulation
for the resonant frequencies of an empty metal cavity, I know these
can easily be calculated analytically but I would like to confirm I'm
operating meep correctly before I move onto a more complicated model.
The ctl file for my model is attached to the end of this email and I'm
wondering if I have specified the geometry correctly. I have placed a
cylinder of air inside a cylinder of metal. I assume the thickness of
the perfect metal shield does not matter as meep sets the E field to
zero at the boundary interface? I would like to place a point source
at the centre of the cavity in order to excite the TM modes, my base
unit of distance is 1 cm so my Gaussian source has a centre frequency
of roughly 5GHz and a bandwidth of 3GHz. When i run the simulation I
can't make much sense of the png output for the dielectric function,
does meep output the rz plane when in cylindrical co-ordinates and is
it possible to show the r theta plane? When working in cylindrical co-
ordinates are centre values specified as r,theta,z and if so does
0,0,0 represent the centre of the base of the cylinder or the
geometric centre of the object.
I apologise for so many questions in one email but any help is greatly
appreciated.
Best Regards, Simon.
-- ctl file
; Calculating 3d cylindrical resonator modes using cylindircal co-
ordinates
(set! dimensions CYLINDRICAL)
(define-param ra 2.74) ; radius of air cylinder
(define-param rm 2.75) ; radius of the metal shield
(define-param hm 5.49) ; height of metal shield
(define-param ha 5.48) ; height of inner air region
(set-param! m 3)
(define sr (+ rm)) ; radial size (cell is from 0 to sr)
(set! geometry-lattice (make lattice (size sr no-size (+ hm))))
(set! geometry (list
(make cylinder (center 0 0 0) (height hm)
(radius rm) (material metal))
(make cylinder (center 0 0 0) (height ha)
(radius ra) (material air))))
(set-param! resolution 10)
(define-param fcen 0.17) ; pulse center frequency
(define-param df 0.1) ; pulse width (in frequency)
(set! sources (list
(make source
(src (make gaussian-src (frequency fcen) (fwidth df)))
(component Ez) (center 0 0 0))))
(run-sources+ 200 (at-beginning output-epsilon)(after-sources (harminv
Ez (vector3 0 0 0) fcen df)))
On 2 Jan 2008, at 03:53, Steven G. Johnson wrote:
> On Tue, 1 Jan 2008, Simon J. Bale wrote:
>> I'm interested in using meep to model the resonant frequencies and
>> field
>> patterns inside a high Q cylindrical cavity resonator in 3
>> dimensions.
>> I'm totally new to meep and I've had a read through the tutorial and
>> documentation and I'm concerned that I may not be able to accurately
>> model the metal end wall and side wall losses. Is it possible to
>> specify
>> a lossy metal over a wide bandwidth in meep and if so are there any
>> examples?
>
> You can specify, in principle, just about any loss spectrum by
> fitting to
> a series of Lorentzian functions in the form described in the Meep
> manual.
>
> Of course, if you have a resonant cavity, then only the absorption
> losses
> at the resonant frequencies matter, so I'm not sure why you're
> concerned
> about losses over a wide bandwidth.
>
> A more efficient alternative, if the losses are small (which should
> be the
> case if you have a resonant cavity with a reasonable Q), is to model
> the
> cavity using perfect metals, get the field pattern of the resonant
> mode,
> and then compute the absorption losses a posteriori using perturbation
> theory. (I seem to recall that Jackson includes an example of how to
> compute absorption losses in metallic cavities/waveguides using
> perturbation theory.) This should be essentially exact if the
> absorption
> losses are small (Q > 100 or so), and has several advantages -- you
> can
> model different metals with no additional calculation, and you may not
> need as high a resolution (since you do not need to numerically
> model the
> finite skin depth of the field in the metal).
>
> [In general, I highly recommend perturbative methods as a supplement
> to
> brute-force numerics whenever you are trying to compute small effects
> (such as absorption loss in a low-loss cavity), as for such problems
> they
> can be far more powerful and revealing than numerics. However, they
> require a bit more care, so it is important to do a simple test case
> or
> two to be sure you are applying them correctly.]
>
>> I'm also interested in modelling the quality factor of some very
>> high
>> Q distributed Bragg structures but again metal loss is important.
>> Will
>> meep be a suitable fit for this type of problem?
>
> Yes, probably. See above. (Although if by "Bragg structure" you
> mean a
> multilayer film, then transfer-matrix methods are far more efficient
> for
> that specific geometry.)
>
> Regards,
> Steven G. Johnson
>
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