Coherent light generators – Particular beam control device – Optical output stabilization
Reexamination Certificate
2002-12-27
2004-02-17
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Optical output stabilization
C372S029020, C372S029016, C372S033000
Reexamination Certificate
active
06693931
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the use and operation of particle accelerator systems. More specifically, the invention relates to techniques for synchronizing signals within a particle accelerator system such that a stable, reliable and predictable output is obtained from the particle accelerator system.
2. Description of the Related Art
Conventional particle accelerator systems serve to manipulate and control atomic and sub-atomic particles through the use of electromagnetic fields. For example, some particle accelerators are used to cause the collision of atomic particles (with one another or with a separate object), so that sub-atomic particles can thereby be obtained. As another example, particle accelerators can be used as part of systems designed to produce high-energy, coherent light sources (e.g., the Free Electron Laser (FEL), which uses an electron beam as its lasing medium).
In accelerator systems operating to create an electron beam having desired properties, electrons can be obtained from a radio-frequency (RF) photocathode electron gun. Such a photocathode electron gun typically requires a pulse of (laser) light to be provided to the cathode at a time that is very precisely determined with respect to the phase of the electromagnetic field inside the resonant cavity of the electron gun. This phase of the field within the cavity is conventionally approximated by using the phase of the RF drive applied to the gun, as will be discussed in more detail below.
Providing the laser pulse to the photocathode at the appropriate time is typically accomplished by driving the source laser with an RF source (i.e., frequency generator) carefully phase-locked to a higher-frequency RF drive being applied to the electron gun (as well as to the accelerator itself). This technique, using common lasers for the purpose just described, provides what is known as a “lock-to-clock” capability to thereby provide a conventional level of timing accuracy.
FIG. 1
illustrates a known accelerator system
100
. In
FIG. 1
, master RF oscillator
105
generates and supplies a low-frequency signal to seed laser
110
. Seed laser
110
may include, for example, a mode-locked Ti
3+
doped Sapphire (Ti:S) oscillator operating at 81.6 MHz, thereby providing a train of pulses with a pulse width of around 150 fs (1.5×10
−13
sec). The output of seed laser
110
is fed into amplifier chain
120
. Together, the seed laser
110
and amplifier chain
120
form a laser system
125
.
The pulses output by laser system
125
are directed, for example by mirror
130
, through entrance window
135
of electron gun
140
. A photocathode (not shown) within electron gun
140
is thereby stimulated to produce electrons, which are then supplied to Linear Accelerator (LINAC)
145
.
Master RF oscillator
105
also generates a high-frequency signal output to high power RF amplifier
150
for use by electron gun
140
and LINAC
145
. The electron gun
140
and LINAC
145
may operate in the S-band of the microwave spectrum, generally defined as within a range of 2800-3000 MHz. The high-frequency signal sent to high power RF amplifier
150
may be locked to a multiple of the low-frequency laser signal sent to seed laser
110
. In the system of
FIG. 1
, the high frequency signal may be, for example, at a frequency of 2856 MHz, the 35th harmonic of the low-frequency signal to laser system
125
. High power RF amplifier
150
and variable power splitter
155
can be used to manipulate the high-frequency output of master oscillator
105
in terms of power and direction, respectively.
Various difficulties are associated with the operation of conventional accelerator systems such as system
100
shown in FIG.
1
. For example, the laser pulse should be provided to the electron gun
140
within a window of approximately 1° of phase with respect to the RF field within the electron gun
140
. This correspond to about 1 ps of timing jitter and drift between the laser system
125
and the master RF oscillator
105
. Because the RF field has a frequency of about 35 times the frequency of the laser system, a lock stability of about 1/35° exists on the laser system
125
, which is extremely difficult to maintain.
As another example of the shortcomings of system
100
, amplifier chain
120
often contributes timing drift to the system
100
. Because the desired 1 ps timing window corresponds to as little as 1 part per million of the total time the pulse is propagated between the laser system
125
and the electron gun
140
, even a tiny drip in the delay of the pulse through the laser system
125
can significantly degrade the timing of a pulse. This degradation can occur even if the signal applied to seed laser
110
by master RF oscillator
105
starts out perfectly timed with the phase of the electric field within a resonant cavity (not shown) of electron gun
140
. For example, a 1% change in atmospheric density can easily provide this much timing drift.
A final example of problems associated with system
100
results from changes in the phase of the RF field inside the resonant cavity of the electron gun
140
relative to the phase of the RF drive supplied to the electron gun
140
by master RF oscillator
105
. Such phase changes can be induced by changes in the operating temperature of the electron gun
140
, for example, or changes in the transit time properties of a waveguide (not shown) to the electron gun
140
(such as could be induced by changes in the dielectric gas pressure). Thus, some variable phase may exist between the frequency of the signal supplied by master RF oscillator
105
and the frequency of the electromagnetic field present within the resonant cavity (not shown) of the electron gun
140
.
An operator might monitor the output of LINAC
145
, then adjust the output of master RF oscillator
105
in an attempt to obtain a desirable output from LINAC
145
. However, such observations and adjustments are difficult to make, particularly in real time. Moreover, the quality of output will vary according to the skill of the operator of the system. Thus, such a method and system is not capable of providing desirable outputs on a consistent basis.
Therefore, a need exists for a method and system for easily achieving stable and predictable timing between a laser signal and an electromagnetic field(s) within an accelerator system, whereby a satisfactory output of the accelerator system itself can be easily, inexpensively and reliably obtained.
SUMMARY OF THE INVENTION
An improved method and system for synchronizing signals in a particle accelerator system is disclosed, overcoming at least the aforementioned disadvantages.
In one embodiment, a method and system is disclosed whereby phase of laser pulses are monitored, and a high-frequency signal is adjusted as necessary to be substantially in-phase with the laser pulses. In another embodiment a method and system is disclosed whereby a phase of an electromagnetic field in an electron gun is monitored, and a high-frequency signal is adjusted as necessary to be substantially in-phase with the electromagnetic field.
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patent: 6473255 (2002-10-01), Hatanaka et al.
patent: 2001/0024458 (2001-09-01), Fermann
patent: 2002/0168161 (2002-11-01), Price et al.
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Mendenhall Marcus H.
Shearer Gary R.
Cooley & Godward LLP
Jr. Leon Scott
Vanderbilt University
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