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Laser Power Supply
Second Semester Report
Spring 2007
By
Josh Cook
Prepared to fulfill the requirements for
EE 402
Department of Electrical and Computer Engineering
Colorado State University
Fort Collins, Colorado 80523
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ABSTRACT
In an academic environment where we design and build our own lasers to meet
the needs of specific applications, it is often impossible to find a high voltage,
commercially available, power supply which can produce the exact electrical
requirements necessary for its operation. It is then necessary to design and build a
specialized power supply to for these unique requirements. This project is to build, test
and implement two pulsing units for the 46.9 nm capillary discharge high repetition rate
soft x-ray lasers employed here at CSU, extrapolating from and improving on a previous
design.
The bulk of the design in this project is not in the circuit itself but in the
implementation of the connections between the components and the support structures
within the pulsing unit. The electrical connections with in the unit have been altered
from the original unit to be more uniform, smaller, easier to fabricate and easier to
connect and disconnect. The support structure within the pulsing unit which consists of
half inch Plexiglas panels has been altered to be less complicated, more stable, and to
facilitate cooling oil flow to components which may overheat. All of this has been done
bearing in mind that all components and connections will be carrying high voltages and
currents.
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TABLE OF CONTENTS
Title
Abstract
Table of Contents
1
2
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I Introduction
II Review of Previous Work
III Physical Design
A.
Electrical Connections
B.
Internal Support Structure
IV Costs
V Results and Future Work
Acknowledgements
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6
7
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LIST OF FIGURES
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Barrel connector
Lengthwise Dividers
8Ω Resistor Supports
Pspice Circuit
Pspice Output
Unit 1 Output
Unit 2 Output
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8
9
11
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LIST OF TABLES
Table 1
Cost
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CHAPTER I - INTRODUCTION
For this project we will need to take in a DC voltage of approximately 50 kV and
provide a pulse of 80-90 kV for a very short time of < 1 micro second. We will also need
to provide a pre-ionization voltage to the lasers cathode prior to the main voltage pulse.
We have available to us a commercial high voltage DC power supply that can produce
50kV, we will use this to supply the pulsing unit and spark gap switches to pulse it. The
step input is then sent through an RLC resonance circuit that will provide overshoot
which we will use to charge a second capacitor bank, which is external to the pulsing
unit, to approximately 80kV. The second capacitor bank will then be switched by a
second spark gap switch and discharge directly to the laser.
The unit is housed in a Steal box that has internal dimensions of 20 and 5/8 inch
by 15 and 1/8 inch by 7 and ½ inches deep. The box is water tight and has water tight
fittings on the back for wire and tubing penetrations. The lid is ½ inch thick aluminum
with an o-ring groove and a connection for the oil out. Inside the steal box is a Plexiglas
box which is used to electrically isolate the entire circuit from the grounded outer steal
box. The only penetrations through the Plexiglas box are in the back for wire and tubing
which must go to external equipment, and in the bottom for the grounding bar stands,
which are welded to the steal box. Inside the Plexiglas box the circuit is divided into
three sections by ½ inch Plexiglas dividers. The left section is for the pre-ionization
circuitry, the center holds the 60nF capacitor bank, the bus bars and the spark gap switch,
and the right sections will hold the trigger circuitry on the bottom level and the inductor
and output resistors on the top level.
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The main issues when physically assembling this circuit are controlling the
extremely high voltage, and providing cooling to the components that may overheat.
Both problems are solved by immersing the circuit in high impedance oil. Air will
conduct at about 30kV per inch, so using voltages as high as 90kV can cause problems
with arcing. Arcing will not only interfere with the operation of the circuit but may also
cause damage to it. By submerging the circuit in transformer oil, which has a dielectric
strength of 100 kV/cm, all of the wires and connections will be sufficiently isolated from
ground and from lower voltage components such that no arcing will occur. Still it is
important to keep components of different voltages a distance of at least 1cm apart to
maintain a sufficient margin of error (at least 1 inch from ground was also maintained).
The dielectric oil is also used to cool the circuit by being constantly circulated
through the circuit and an oil chiller. The two components which are most in danger of
overheating are the 1 MΩ pre-ionization resistor and the two 8 Ω output resistors. In
order to achieve this, the oil inlet tubing has been routed within the pulsing unit directly
to these components and the Plexiglas dividers designed to guide the flow of the oil to
these vital cooling loads. This was achieved simply by not allowing other paths for the
oil to flow and opening paths of low resistance for the oil to flow past the vital cooling
loads. In the second unit the 1 M Ω resistor came from the template unit and already had
an oil tube around it so the oil is directly routed in to the tube.
The support structures and the internal components are not attached to the
insulating Plexiglas box or the external steal box except at the grounding points for the
grounding bar. This power supply is not meant for use in situations where it may be
jarred or moved while in use so this will not put additional strain on components or
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connections. This is done to make it easier to disassemble for quick access to internal
parts in order to facilitate maintenance, repairs and trouble shooting. This being said, the
components inside do not move freely because the internal support structure is designed
to be a tight fit.
CHAPTER II – REVIEW OF PREVIOUS WORK
Prior to the current two units that are being produced now there have been several
units built by different people. The only ones that have been available for reference on
specific design improvements during this project the one that I was given as a template
and the one built by Scott Heimberg who is the graduate student I am working with on
this project. This is due to the fact that they are all currently in use. I was also provided
with a binder containing the dimensions and specifications of many of the parts I would
need to fabricate, which was made by David Braley.
The base circuit design has not changed since the original unit, but there have
been various improvements along the way. For example in the original unit the circuit
and Plexiglas box was attached to the lid of the steal box, effectively making it upsidedown in the box and very difficult to take apart for repairs. The units now rest inside the
outer box such that the circuit can be accessed easily without draining the oil, just by
taking off the lid. Also the means by which the high voltage wires that enter and exit the
box have been change form an L type connector pipe that came out of the lid to a straight
water proof fitting that comes out of the back of the box. This makes it easier to remove
any damaged wires and replace them relatively quickly as well as more space efficient in
the rack where the unit is usually stored.
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CHAPTER III – PHYSICAL DESIGN
PART A – ELECTRICAL CONNECTIONS
In the unit that was being used as a template there are a large number of different
types of connections. Some were simple barrel connectors and some were complicated,
using stands and brass pieces that were welded together and some had specially
fabricated parts. In this version all connections for wires are made using either standard
ring connectors or barrel connectors as shown in figure 1. The barrel connectors are
small easy to make and provide for a large surface for a strong
electrical connection. These barrel connectors are made from ¼ inch
brass rod stock and drilled down the center for the wire to be inserted
and soldered. A ¼ inch hole is then drilled into the large brass discs
and a set screw hole is tapped perpendicular into the side of the hole.
A set screw is then used to hold the barrel connector in place. Ring
connectors were used in places where barrel connectors would not fit
or would be inconvenient. By using only ring and barrel connectors the construction of
this unit is significantly less complicated and more easily taken apart for maintenance or
repairs. It should be noted that in the second unit some of the specially fabricated
connection pieces from the template pulsing unit were reused along with much of the
template units components in order to save time and money.
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B – INTERNAL SUPPORT STRUCTURE
The internal support structure is used both for mechanical support and to
electrically isolate lower voltage components from higher voltage components. There are
two large dividers that divide the circuit
as describe in the introduction, these
dividers are 19 and 5/8 inches by 6 and
½ inches and have sections cut out to
allow wires to pass through and to
facilitate oil flow. There is a hole in the
right divider for an oil inlet line to be
run to in order to cool the output
resistors and inductor directly.
There are several other smaller pieces in the unit which support individual pieces
of equipment. The spark gap has two Plexiglas pieces which are 9 and 1/8 inches long by
6 and ½ inches tall that support it completely via holes in the center which the brass input
and output connectors for the spark gap fit into. On both of the outside divisions there is
an upper and lower level. On the left side the lower level contains the 1MΩ preionization resistor which is supported with two Plexiglas pieces, one on each end; these
pieces also support the platform for the upper level. The piece that supports the 1MΩ
resistor near the back of the unit has one 5/8 inch hole in it for an oil line to supply
cooling oil directly to the resistor and the one on the front has two large holes (one above
the resistor one below) to allow the oil to flow freely. On the right side the trigger circuit
is on the bottom and the capacitors for the trigger circuit are suspended by the Plexiglas
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dividers which supports the top level. The right side top
level contains the two 8Ω output resistors which are
supported together by two Plexiglas pieces which each
contain four additional large holes to allow the cooling oil
to flow freely to the oil return line and ensure adequate
cooling.
CHAPTER IV – COSTS
Due to the nature of high voltage equipment and the fact that most of the items
were bought one at a time rather than in bulk, many of the pieces of equipment that have
been purchased for this project are quite expensive. Things like screws, ring connectors
and 100kV cable are not included because many of the components were found already in
the lab. I was able to save money by using the scrap materials found in the machining
room next to room B-310 at the ERC for the raw materials necessary to fabricate many of
the components. Also the steal box and the associated fittings were already completed
before I started this project, though the bill for having the lid machined and the matching
hole pattern put into the box and lid is on the list under CNC Steal Work. I do not expect
that there will be many more large purchases as we have all materials necessary to
complete the second unit and finish the project next semester. Table 1 is a list of all of
the major expenses that have occurred while I was working on the project. If you include
the cost of the capacitors, the spark gap, and the other resistors the cost of a single unit is
about $5,500. This can very by several hundred dollars depending on whether the
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resistors are ordered in bulk or not. Ordering in bulk can reduce the cost to around
$100.00 apiece, and ordering them one at a time will raise the cost to about $425.00 per
resistor.
Item
Tube Fittings
HV Cable
CNC Steal Work
Reducing Union
Rubber Tubing
500Ω Oil
Impregnated
resistors x2
Oil System
Components
Male Elbow,
Reducing Union
Ball Valve
Bought From
Denver Valve and
Fitting
McMaster-Car
Acra-Tech
Denver Valve and
Fitting
McMaster-Car
U.S. Resistor
Denver Valve and
Fitting
Denver Valve and
Fitting
Denver Valve and
Fitting
TOTAL
Date
Cost
07/07/06
$52.00
07/07/06
07/18/06
$101.00
$400.00
07/21/06
$56.00
8/14/06
$36.75
08/15/06
$532.36
08/18/06
$256.20
09/08/06
$105.30
09/22/06
$160.90
$1700.51
Table 1
CHAPTER V – RESULTS AND FUTURE WORK
Testing was performed on the first unit successfully producing the expected
results. Testing did take longer than expected because we did not have the pre-ionization
circuit properly grounded causing it to act as a very large capacitance and giving the
circuit an extremely over damped response. This had nothing to do with the unit itself, it
was a problem with the way we were testing it. The results of testing for the first unit are
shown in Figure 6, the expected results from the Pspice model are shown in Figure 5 and
the circuit is shown in Figure 4. Testing for the second unit went much smoother and
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only took about 45 minuets once the equipment was set up. The results for the second
unit are shown in Figure 7.
Figure 4
Figure 5
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Figure 6
Figure 7
With both units now completed and tested the support structures for the laser
itself are currently being constructed and the discharge head being assembled so that the
units can be tested, actually firing a laser at full power.
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ACKNOWLEDGEMENTS
This project has been funded and supported by Colorado State University
Extreme Ultraviolet Engineering Research center and Jorge Rocca. Guidance on the
process and techniques used came from Scott Heinbuch as well as Dale Martz and
Michael Grisham.
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