From: terry@asl.dl.nec.com
Newsgroups: sci.physics.fusion
Subject: Re: Responses to Dale Bass
Date: 7 Jan 93 08:02:52 GMT
Organization: (Speaking only for myself)
Originator: terry@aslss01

Hi folks,

In article <1993Jan5.180115.17549@murdoch.acc.Virginia.EDU>
crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:

> [Terry Bollinger writes:]
> 
> | Can anyone out there show one way or the other whether this wedge-out
> | idea is either bogus or has merit?
> ...
>
> 1) in stable cavitation in acoustical fields, the sonoluminescence occurs
> shortly after the bubble reaches maximum radius (i.e. just as the bubble
> starts its inward cycle.  Keep in mind that in stable sonoluminescence,
> the bubble itself does not completely collapse.).  This implies that the
> sonoluminescence is mediated by a strong shock in the vapor itself (See
> Barber and Putterman PRL: 69:3839 (1992) among others), 

In the UC draft I did assume vapor or gas in most bubbles, but (by choice)
emphasized keeping the vapor level low.  I dropped the use of "shock
wave" from my first outline for that same reason.  I was interested mainly
in _very_ thin shock wave media, not the conventional thick stuff.  (In
retrospect I wish I had left the "shock" terminology in there.)

I find the fact that the surface is able to accelerate to Mach 1 that
quickly to be most interesting.  So there _really is_ an implosion, hmm?


I would love to get this (and other) references, but in my current location
I no longer have easy access to my old resource (the Library of Congress).

I thus will beg a bit:  If anyone out there has a copy of this and any other
related cavitation papers that they might be willing to fax to me (without
violating any university rules, please), I would be extremely appreciative.

Could any kind souls out there help me out a bit on this?


> 2) it is not clear what physical/thermodynamic meaning 100,000 K has for 
> this process (a discussion of 'huge' fugacity should be kept in mind here).

Sorry, you need to talk to Seth Putterman about that one.  You know more
about the details of his peak extrapolation than I do anyway.


> However, all of the component species should be fairly well ionized at that
> point if it has any physical meaning...

You mean at the void center when your shock wave hits?  OK by me.  You are,
after all, dumping a lot of energy into a small region over a short time.


> Putting aside the thought that it probably has limited physical meaning,
> at that point there are very good electrostatic reasons for the character 
> of the interaction to change somewhat drastically.

Sorry, I don't get it.  If the plasma is created through thermal methods and
is not being subjected to intense currents or magnetic fields, my assumption
was (and is) that it will be subject to the same general dynamic behavior as
any very hot gas.  Are you implying that you think a strong field or current
will be present?  If so, why?  It would seem like an unnecessary assumption
without a lot more evidence for the presence of such phenomena in the center.


> I would also point out that the concept of ordinary fluid dynamic surface
> tension breaks down well before that point (to put it mildly).

From conversations with you offline, I gather that you were originally under
the impression that I was trying to whack the fluid surface all the way into
the center of the void.

I most decidely proposed no such thing in UC, and I explicitly described the
"wedge-out" as a proposed _gaseous_ effect.


Moreover, in UC I described the wedge-out in the context of a very fast,
inwardly moving gaseous front that has a very sharp transtion from high
pressure to (hopefully) low void pressure on its leading edge...

You'll pardon my lack of using standard terminology at times, but I believe
that such a thing is called a "shock wave," is it not?


> 3) If you seriously want to present a model, the burden of proof is on you
> to describe such things as
>
> a) why your process does not violate the second law, 

Hmm?  Since when does the second law prevent _local_ increases in temperature
and pressure as long as they don't reduce entropy for the system as a whole?

Whatever sonoluminscence is, it's clearly expending a lot of energy in the
whole bubble to produce an increase in energy in a _much smaller_ central
region.  So what?  Multi-stage rockets behave quite similarly when they
accelerate a payload, but they certainly don't violate the second law.


Speaking of such things, David Cyganski sent me a very nice (and easy-to-do)
example of how mechanical bouncing around (literally!) can result in the
energy of one object being transferred largely or entirely into another one:

Take two elastic balls, one of which is about 1/3 the weight as the other.
Carefully drop them so that the light ball rest exactly on top of the heavy
ball as they fall.

If the weights are about right, then when they both hit the ground the heavier
ball will transfer _all_ of its kinetic energy to the lighter ball, which will
then rebound much higher than the height from which it was dropped.

Does the ball violate the second law?  No, of course not.  Nor does the idea
that the collapse of a highly symmetrical void could produce a brief, highly
energetic central region.  A close cavitation system with a self-contained
energy source increases in entropy quite nicely over time, no matter how
hot certain individual events within that system may or may not become.


> b) why surface tension itself does not seem to operate in the 'wedgeout
> process', 

Because if it exists, it's a gaseous process...  :)


> c) why diffusion and damping do not significantly operate,

The stupid answer is "because it's spherical and it's collapsing inward."
Things are getting damped, alright, but most of what is getting damped is
also getting "left behind" as the wave front moves inward.

I guess I just don't believe that the models for all this are necessarily
all that complete or insightful.  You get out what you put in.


> d) how one applies a fluid continuum approach to a putatively ionized gas,

Again, I'm ignorant.  Is neutal plasma "sticky," or what?  Can you elaborate?


> e) how one can get a further 11 order of magnitude concentration of energy
> beyond the concentration probably caused by an ionizing shock, especially
> considering that at indicated spectral temperature, electromagnetic
> interactions will dominate long before fusion energies are reached,

Hey, I said measureable _low level_ fusion.  I don't recall the curves
that used to be discussed in this group, but it seems to me that a real
kinetic energy of 100,000 K might _already_ be high enough for some very
low level increases in, say, T-T that could be detectable.  (Anyone game?)

On the other hand, I honestly _don't_ feel that the concentration curve has
been anywhere near exhausted yet, because I don't think there is a full
understanding of what is going on for some ranges of the effect -- such as
for exceptionally low void pressures.

I say that if you thin the medium through which the shock wave travels, you
will get an intensification.  I also note that the slanting of SL spectra
towards the UV when the water is cooled are nicely compatible with that
prediction.  Have any of the other seven or so models predicted that?


> f) why it seems to be somewhat at variance with current indications of the
> mechanism of stable sonoluminescence.

I did not take shock waves composed of void media into account, yes.  I only
mentioned elastic compression and the idea that more void vapor should reduce
the overall intensity of the event.

But I don't think we're talking about a major mismatch here.  Indeed, there
shoud be a continuum of effects between whacking a thick void gas and sending
off a shock that way, versus cavitations in which the void surface itself is
vaporizes and begins "filling in" a sparsely filled void with a particularly
vicious (and dense) shock wave.

It is the latter that I am betting on as being an interesting effect.  That
the former void-shock gives sonoluminscence simply makes me that much more
interested in the thin-vapor scenio.

> Among other side issues, quantification also seems to be a fairly
> significant burden.

Sounds about right.


> | Also, is there any chance someone out there can simulate this issue?
>
> Sure, for $50,000 + overhead, I'd be glad to.  Sonoluminescence is not a
> trivial problem, but there are certain obvious ways of approach at this
> juncture.

Heh.  Thanks, but no thanks.


> ... However, the mechanism [for increasing the interior energy] is likely
> somewhat different than you propose, in that it may be the shedding of 
> a strong shock into the vapor bubble...

See above.


> If so, it appears to be of limited value in fusion studies unless you can
> explain why a partially ionized gas can further concentrate energy.

How about the other way around:  Why can it _not_ for if the plasma is
electrically neutral and no large fields are present?


> Our good friends in the hot fusion business can probably explain better
> than I how difficult that is.

Sounds good.  Could some kind soul explain the added instabilities that
make neutral (?) plasmas behave differently from high-temp gases?


> | Thus liquid helium would be a _really_ poor UC choice by these criteria,
> | and liquid mercury would be a quite good one.  Mixtures of liquids will
> | general do better by this criterion than single-composition liquids
>
> So something that is doubly ionized is harder to get close together than
> something that is 80x ionized?  Keep in mind, at hot fusion energies your
> original fluid is gone.  All that remains is that nucleus and all its
> charges.

Again, you seem to be assuming that the plasma as a whole has lost the
neutralizing effect of its electrons.  I have no knowledge of any physical
basis for that kind of assumption.  Also, most plasmas do _not_ totally
ionize all elecrons, especially from heavy metals.  I don't understand
your point in suggesting such extreme ionization, which almost certainly
could not occur even in a tokamok or other comparable plasma devices, let
alone in whatever (if anything) is going on in cavitation.

........

P.S. -- I will be re-issuing a discussion of the wedge-out with a new emphasis
        on development of flow-cells (formerly called wedges) at the margin
        of a converging "shock front" (or whatever you wish to call it).

        A new version will also take Dale Bass' excellent shock wave points
        into account -- but not until _after_ I can get hold of a few of the
        various references I need to get UC "synched up" with standard ideas
        and terminology.  For all I know I'm exactly duplicating one of the
        seven mechanisms of the caviation book (I actually doubt that), and if
        so I need to know it and to properly reference it.
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