Commissioning a Gas Filled Energy Monitor for the AMO Hutch at the LCLS Ankur Dhar

20 Pages·2012·4.62 MB·English

Commissioning a Gas Filled Energy Monitor for the AMO
Hutch at the LCLS
Ankur Dhar
Office of Science, Science Undergraduate Laboratory Internship (SULI)
University of California Berkeley
SLAC National Accelerator Laboratory
Menlo Park, CA
August 15, 2012
Prepared in partial fulfillment of the requirements of the Office of Science, Department of
Energy’s Science Undergraduate Laboratory Internship under the direction of Christoph
Bostedt at the Linac Coherent Light Source, SLAC National Accelerator Laboratory.
Participant:
Signature
Research Advisor:
Signature
Table of Contents
Abstract ii
Introduction: 1
Materials and Methods: 2
Results 8
Discussion and Conclusion 9
Acknowledgments 11
References 11
i
Abstract
Commissioning a Gas Filled Energy Monitor for the AMO Hutch at the LCLS. ANKUR
DHAR (University of California Berkeley, Berkeley, CA, 94720) CHRISTOPH BOSTEDT
(Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA
94025)
Measuring X-ray pulse energies with minimal attenuation is extremely important for
FEL facilities like the LCLS. This measurement can be done by using nitrogen gas at
pressures between 20mTorr and 50mTorr and measuring the decay channels proportional
to the incoming number of photons. A gas detector was designed and built by LLNL
to use this principle to measure pulse energies at the LCLS. Preparations were made to
commission one of these detectors for the AMO hutch to measure soft X-ray pulse energies.
A new compact differential pumping system was designed and the vacuum controller was
successfully tested. Further plans were made for first calibrations in October during the
next AMO beam time.
ii
Introduction:
The Linac Coherent Light Source (LCLS) is the most recent of the experimental innova-
tions at the SLAC National Accelerator Laboratory. The electrons accelerated by the linac
are used to create a coherent pulse of X-ray light in an undulator. The photons are created
by coherent radiation from the electron bunches in the undulator. Unlike synchrotron
sources, the photons are bunched in extremely short pulses, on the femtosecond time scale,
and have a bandwidth less than 1%. As a result the LCLS is the world’s first X-ray free
electron laser (FEL) capable of producing photons with energies from 500 eV to 10 keV
and pulse energies up to 4mJ. The coherence of the pulse also allows the LCLS to image
molecules and materials with unprecedented luminosity [1].
An important part of studies with intense X-ray pulses is precise calibration of the
pulse energies. This measurement can be done with calorimeters that absorb the entire
beam energy, but only work at the end of the beam and do not give shot by shot metrics.
Furthermore, beam users want the maximum luminosity for their experiments, so the
diagnostics need to attenuate the beam as little as possible while still maintaining accuracy.
This requirement resulted in Lawrence Livermore National Lab (LLNL) designing and
constructing gas detectors for the LCLS.
These gas detectors were designed to measure UV florescence produced by nitrogen gas.
The X-ray photon is absorbed through K-shell photoionization, releasing an electron. This
hole is filled by an outer shell electron, which releases enough energy to excite a second
electron. This decay process is called Auger relaxation, which leads to the production
of secondary electrons in addition to the primary photoelectron. These electrons further
interact with the gas through impact ionization and excitation. These processes eventually
resultinUVemission, withanintensityproportionaltothenumberofelectronsreleased[2].
Once calibrated against other absolute energy measurements, these detectors can
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be used to give shot by shot energy readings for soft X-rays (photon energy between
500eV and 2keV) [3]. Three detectors were built by LLNL for the LCLS, two for the
Front End Enclosure (FEE) and one for the Atomic Molecular Optical hutch (AMO).
The gas detector belonging to AMO has been offline until now, so planning is needed
to incorporate this useful diagnostic device into the current AMO hutch. The detector
will need to be remotely controlled, using a system known as the Experimental Physics
and Industrial Control System (EPICS). This control system is used throughout the
LCLS for controlling every remote device. The gas detector, incorporating the improve-
mentsofthiscurrentwork, willbeintegratedintothenewAMObeamlinediagnosticssuite.
Materials and Methods:
System Overview:
Figure1 shows thegas detector andall themajorelements. Oneither endof thecylindrical
chamber is a 1cm aperture to limit gas flow but not cut the beam. The UV photons from
the Auger electrons are collected by two photomultiplier tubes (PMT) mounted orthogonal
to each other. They also have optical bandpass filters to stop any other photons that
might distort the UV signal. The pressure sensor used on this gas detector is the Baratron
627D, a capacitance manometer with a range from 10−4 Torr to 1Torr. The MKS Type
248A control valve on the bottom controls the amount of nitrogen that flows into the
chamber by opening a certain amount proportional to an applied voltage between 0 and
15V. Therefore, by adjusting the flow of nitrogen into the gas detector the pressure is
precisely controlled.
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Pressure Control System:
Since the energy measurement requires direct interaction with the beam, precise control of
the number of the particles in the chamber is required. Too many particles will attenuate
the beam significantly, and too few will lead to an inaccurate measurement. By the ideal
gas law, at a set volume and temperature, the pressure inside the chamber is proportional
to the number of molecules inside [3]. Therefore, by controlling the pressure precisely the
detector can be tuned for optimal performance.
Controlling the pressure through a feedback loop requires three components: a sensor, a
controlled element, and a proportional–integral–derivative (PID) controller. The controller
works by receiving a measurement from the sensor and comparing it to the set value.
After also considering previous measurements and predicted future values, a new signal
is sent to the controlled element to bring the next measurement closer to the set point.
This process loops until the measurement converges on the set point. The PID controller
is integrated into the MKS Type 146C Vacuum controller (as seen in Figure 2) as a
controller card. It is capable of storing four different recipes for different pressures and
PID parameters. This whole setup can be seen in Figure 3.
Operation:
Control through EPICS can be accomplished through the PID controller’s serial port and
the appropriate serial commands to control the pressure and valve. The commands most
relevant for EPICS integration from the MKS Type 146C manual are listed in Table 1.
Once the pressure within the gas detector is set, the PMTs can be read out using a 1GHz
sampling flash digitizer. A baseline is determined from reading a running average of about
175 samples over 100 pulses. This baseline is subtracted from the signal region, which
is also averaged [4]. This signal only provides a relative reading; an absolute reading
would require calibration against another energy monitor such as a calorimeter. Since
the timing for the samples comes from the LCLS operators, once calibrated the detector
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could provide shot by shot feedback without additional operational controls. The only
parameter that needs to be adjusted on a regular basis is the pressure set point. The
excitation cross section drops as photon energy increases, so the pressure in the chamber
need to increase along with the photon energy [5].
Differential Pumping System:
The gas detector will need to operate between about 0.1Torr down to 0.01Torr. The
normal operating range in the gas detector for soft X-rays is 10 to 50mTorr, but higher
pressure overall may be needed for lower pulse energies[5]. Experiments in the hutch
normally operate somewhere between 10−8Torr and 10−10Torr, so a differential pumping
system will be needed to prevent leakage from the gas detector into the experimental
region.
Flow Regimes:
Gas flow can be broken up into three main regimes: viscous flow, transitional flow, and
molecularflow. TheseregimesarecharacterizedbytheKnudsennumber, thedimensionless
ratio of the mean free path of the gas molecules to the characteristic length scale of the
system. For our purposes this length scale can be the diameter of the aperture or tube we
use, thus
πc¯η
K = (1)
n
4Pd
Here, ¯c is the mean velocity of the gas particles (510 m/s for nitrogen), η is the viscosity
(1.3×10−7 Torr-s), and P is the pressure of the system in Torr [6]. Therefore for nitrogen
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at 20◦C with an aperture diameter of 1 cm, the regimes are broken down as follows
K > 0.5 P < 0.01Torr Molecular Flow (2)
n
0.5 > K > 0.01 0.5 > P > 0.01Torr Transitional Flow (3)
n
K < 0.01 P > 0.5Torr Viscous Flow (4)
n
At the peak of the operational range is in the transitional regime, but the gas detector
will usually be operating below that. The first stage will be assumed to operate in the
transitional regime, but the rest will be in the molecular regime. Calibration will be
needed to precisely determine the operational range.
Variables:
Torr L
Q : Throughput [ ] (5)
s
L
S : Pumping Speed [ ] (6)
s
L
C : Conductance [ ] (7)
s
P : Pressure [Torr] (8)
Throughput is defined by gas volume flowing through a system at a certain pressure. So
for a vacuum chamber being pumped with a pump speed S at pressure P, the throughput
is simply Q = SP. Conductance is defined as Q for a component between two chambers
∆P
of different pressures. The throughput of a system has to be constant since all the gas is
flowing together, so given a component connecting two chambers at pressures P and P
0 1
and a pump in the second chamber
Q SP
1
C = = (9)
01
P −P P −P
0 1 0 1
P C
∴ P = 0 01 (10)
1
S +C
01
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This equation can be used to predict the pressure in next chamber of a differential pumping
system given the effective pumping speed of the chamber and the conductance of the
connecting component.
Pumps have an ideal pumping speed as part of their specification, but is characteristically
too high. This is due to the conductance of the tube connecting the pump to the chamber
limits the pumping speed slightly. Thus, the effective pumping speed S is the series
eff
sum of the ideal pumping speed and the conductance of the connecting component.
Conductance for components in series adds as capacitors do, since the drop in pressure
from one component to another is additive. Thus, the effective pumping speed is as follows
1 1 1
= + (11)
S S C
eff
S
∴ S = (12)
eff 1+ S
C
Therefore,tomaketheeffectivepumpingspeedasclosetoidealaspossible,theconductance
of the connecting component should be as large as possible [6]. In the more general sense,
conductance adds in series as follows
1 1 1
= + (13)
C C C
tot 1 2
Molecular Flow:
For the molecular regime, gas particles rarely interact with each other, so they can be
imagined as billiard balls bouncing around randomly inside the vacuum chamber. In this
regime, the gas flow through an aperture or tube is proportional to the input gas flow
times the probability of a molecule exiting the component. For an aperture the probability
of transmission is 1 since once a particle enters the aperture, it will exit. However, the
probability for a tube is more complex. While no precise solution exists for a tube of
arbitrary length, an analytical solution exists from minimizing the relative error[6]. These
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solutions results in the following formulas for conductance
c¯
C = A (14)
a
4
π 14+4l
C = C P = c¯d2 d (15)
t a t 16 14+18l +3l2
d d2
π d3
≈ c¯ when d << l (16)
12 l
Based on these equations tubes should be more effective than apertures at limiting
conductance since they are proportional to d3. However, tubes can create a beaming effect
that will increase the probability that the gas molecules will enter the next tube along
the beam line, reducing the efficiency of the system [7].
Transitional Flow:
Transitionalflow is modeled by simply combiningthe modelsfor viscous andmolecular flow
in different proportions. For an aperture, there are no precise equations, but simulations
show that between 1Torr and 0.01Torr, the conductance of an aperture ranges from
1.53C to 1.05C . However, the conductance of a tube in this regime can be found
m m
analytically
C ≈ C +C (17)
m v
π 3 p¯d d3
∴ C = ( +Z) c¯ (18)
12 32ηc¯ l
1+ 1.28
Where Z = Kn (19)
1+ 1.58
Kn
Notice in the case of a tube, the average pressure from either side of the component p¯
matters, so finding the pressure in the second chamber becomes slightly more difficult [6].
However, if the difference in pressure is assumed to be several orders of magnitude, then
the average pressure is roughly P0.
2
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