Rock Excavation by Pulsed Electron Beams Page: 4 of 5
6 pagesView a full description of this report.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
4
THEORETICAL UNDERSTANDING
The mechanism of fracture on this short time scale has been pur-
sued theoretically and our results seem now to have a satisfac-
tory description. If there are many incipient flaws in the ma-
terial from which cracks can propagate, only a short time is
needed before cracks coalesce and fracture occurs; alternatively,
if there are few such sites more time is needed for the cracks
to grow to coalescence. Thus the stress duration, T, is import-
ant: where T - x/c Beam voltage/c. The other important in-
gredient is the statistical distribution of crack sizes normally
described in terms of a Weibull distribution. The three Weibull
parameters for each of the five rock types have been determined
by Vardar (2) by means of 3-point bending tests, and fracture
criteria developed which agree well with the experimental results
for all rock types. In particular, the dependence of failure on
, and hence voltage, is well explained.
EXAMPLE PULSED ELECTRON TUNNEL EXCAVATOR
The experiments described were carried out at available accel-
erators under limited conditions. In particular, the beam pro-
files tended to be sharply peaked - a more uniform distribution
could result in reducing the specific energy by a factor of as
much as three. Also if a rapidly-pulsed scanning beam were used
enhanced spalling would be expected because of heating and inci-
pient cracking produced by preceding pulses. Thus we expect
under real operating conditions a lower specific energy than the
a 1 kJ/cc observed in the tests; we choose as a reasonable de-
sign value 250 J/cc. We specify the example excavator to be
capable of removing 104 cu. m (136 cu. yds.) of rock per hour,
i.e., to advance a 6.4 m (21 ft.) diameter tunnel at a rate of
3.2 m (10.6 ft.) per hour. This is an order-of-magnitude greater
advance rate than by present-day drill/blast techniques.
The average beam power required is 9 MW and at present we envis-
age using a linear induction accelerator producing 25 kJ pulses
at a 360 Hz rate. This can be achieved with a 5 kA pulse 1 usec
long, with an electron beam voltage of 5 MV. The induction
accelerator is composed of many separate ferro-magnetic cores
through which the electron beam passes. By pulsing a primary
winding an increment of energy is supplied by each core through
transformer action to the beam which acts as the secondary cir-
cuit. This type of accelerator seems to have several advantages
over other types considered (coaxial pulseline, transformer or
Marx generator types) for the following reasons:
- Its modular construction allows continued operation near
full output should one or a few modules fail.
- Only modest voltages (< 100 kV) to ground exist at any
point.
- The stored energy that would be released by an arc-over
in a module is not large and would cause little damage.
- The total beam voltage can be very well-regulated as
needed for scanning.
- The overall electrical efficiency (mains to beam) can be
greater than 50%.
Upcoming Pages
Here’s what’s next.
Search Inside
This report can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Report.
Avery, Robert T.; Keefe, Denis; Brekke, Tor L. & Finnie, Iain. Rock Excavation by Pulsed Electron Beams, report, March 1, 1976; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1443035/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.