| Topic: |
Science > Physics |
| User: |
"DAH" |
| Date: |
04 Feb 2005 02:26:06 PM |
| Object: |
dispersion question |
Pulsed-wire is a standard technique used in magnetic survey, along
with Hall probe mapping, vibrating-wire, moving wire loop and
rotating coil. When conducting a pulsed-wire survey of a periodic
magnetic device like an undulator, an electrical impulse is applied
to a fine wire which is tensioned along the axis of the undulator.
The current in the wire passing through the undulator orthogonal to
the field creates a force on the wire. If the period of the
undulator is large, then a 250 micron diameter copper-beryllium wire,
tensioned to 80 percent of its breaking force, will respond with low
acoustic distortion, and the micron amplitude wire vibration signal
defined by the periodic undulator field and not by the position of
the wire anchor points, which can be detected with a focused optical
interrupter, is an accurate depiction of what the trajectory of an
electron-beam passing along the magnetic axis of the undulator would
be, because the detected wire vibration is the second integral of
field vs distance. The technique is particularly useful for precise
determination of the magnetic axis, for measuring trajectory steering
errors, and for detection of periodic phase errors, all of which are
important issues if the Insertion Device is to be used for generating
coherent synchrotron light.
We intend to push the technique to the limit to measure
superconducting, short period, narrow-gap undulators installed in a
cryostat. The severe technicalities of the design impose a
limitation on the thickness of the wire to at least 250 micron
diameter—unfortunately we can’t easily use thinner wire, which has
been shown to correct the distortion issue we are observing. We have
varied the wire tension and the pulse current—the force on the
wire--but that did not have an appreciable effect upon the observed
acoustic distortion, which suggests that the cause is not a simple
matter of inertial resistance, but rather an issue of dispersion in
the wire, which brings me to my question.
I need to find a rigorous treatment from first principles of acoustic
dispersion in wires—a rough approximation will not do. I suspect
that density, elastic modulous, tension, diameter, temperature and
undulator period will come into play, but just exactly how? If I can
model the dispersion accurately, perhaps I can post-process the
detected signal in LabVIEW and rectify the distortion. Does someone
out there have knowledge of acoustic dispersion in wires?
DAH 2/4/05
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| User: "Franz Heymann" |
|
| Title: Re: dispersion question |
04 Feb 2005 04:16:59 PM |
|
|
"DAH" <dharder@bnl-dot-gov.no-spam.invalid> wrote in message
news:4203da5e$1_1@127.0.0.1...
Pulsed-wire is a standard technique used in magnetic survey, along
with Hall probe mapping, vibrating-wire, moving wire loop and
rotating coil.
It is an excellent analogue technique for exploring magnetic fields.
We used it in the 1950's and 60's to determine the electron-optical
characteristics of magnetic lenses and focussing bending magnets, and
to determine pion trajectories in a beam transport system.
When conducting a pulsed-wire survey of a periodic
magnetic device like an undulator, an electrical impulse is applied
to a fine wire which is tensioned along the axis of the undulator.
The current in the wire passing through the undulator orthogonal to
the field creates a force on the wire. If the period of the
undulator is large, then a 250 micron diameter copper-beryllium
wire,
tensioned to 80 percent of its breaking force, will respond with low
acoustic distortion, and the micron amplitude wire vibration signal
defined by the periodic undulator field and not by the position of
the wire anchor points, which can be detected with a focused optical
interrupter, is an accurate depiction of what the trajectory of an
electron-beam passing along the magnetic axis of the undulator would
be, because the detected wire vibration is the second integral of
field vs distance. The technique is particularly useful for precise
determination of the magnetic axis, for measuring trajectory
steering
errors, and for detection of periodic phase errors, all of which are
important issues if the Insertion Device is to be used for
generating
coherent synchrotron light.
We intend to push the technique to the limit to measure
superconducting, short period, narrow-gap undulators installed in a
cryostat. The severe technicalities of the design impose a
limitation on the thickness of the wire to at least 250 micron
diameter-unfortunately we can't easily use thinner wire, which has
been shown to correct the distortion issue we are observing. We
have
varied the wire tension and the pulse current-the force on the
wire--but that did not have an appreciable effect upon the observed
acoustic distortion, which suggests that the cause is not a simple
matter of inertial resistance, but rather an issue of dispersion in
the wire, which brings me to my question.
We used to use beryllium copper wires more like 46 SWG, which is
around 60 micron diameter. Under those circumstances, our wires
behaved like flexible stretched strings to a good enough
approximation, and we were not unduly troubled by the elastic
properties of the material of the wire itself.
I need to find a rigorous treatment from first principles of
acoustic
dispersion in wires-a rough approximation will not do. I suspect
that density, elastic modulous, tension, diameter, temperature and
undulator period will come into play, but just exactly how? If I
can
model the dispersion accurately, perhaps I can post-process the
detected signal in LabVIEW and rectify the distortion. Does someone
out there have knowledge of acoustic dispersion in wires?
Franz
.
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| User: "DAH" |
|
| Title: re:dispersion question |
07 Feb 2005 08:30:32 AM |
|
|
Yes, typically we use 125 micron diameter be-cu wire, and maybe I
could design the optical pickup to provide enough gain to detect the
vibration on 60 micron diameter wire, but the technician is adamant
about not using wire of diameter less than 250 microns, and it has
the associated problems. It is a formidable technical challenge to
apply the technique to a magnet submerged in liquid helium, with a
vacuum pipe passing through it containing the wire, feeding out of
the top of the cryostat, over pullies, and loaded with a weight. If
the wire breaks, it would take a lot of hours to replace it, and of
course the assembly would have to be brought to room temp. 125
micron be-cu wire kinks and breaks easily, so I imagine one would
have to be extremely careful with 60 micron wire.
If we can characterize the elastic/acoustic properties of the 250
micron wire accurately, maybe we can remove the distortion. It's a
tall order.
DAH 2/7/05
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