\begin{spacing}{0}
\chapter{Optical Diagnostics for Investigation of Mercury Jet Flow in a Magnetic Field}
\label{chap:exp_method}
\end{spacing}
\
\par

The optical method is considered to investigate MHD processes.
Optical methods have considerable advantages over other measurement
techniques: they do not introduce any perturbations into the
medium being investigated, they possess high sensitivity and
accuracy, their response is practically instantaneous, which enables
them to be used to investigate turbulent flows and transition
states, since they provide the possibility of visually following
the phenomenon being investigated, and they
enable one to obtain the physical characteristics for the whole
space being investigated at the same instant of time. Unlike other
probeless methods, optical methods possess high spatial resolution.
All these features enable optical methods to be widely employed in
MHD experiments and underlie the need to search for new ways of
using optical methods which have not yet been employed.

Direct visualization techniques for hydrodynamic examination have often been employed to investigate the dynamics
of MHD flows. In this method, one can measure the time taken for the particles to traverse a given path.
Because no quantitative results can be deduced from direct visualization methods and difficulties often
arise when investigating things like thin boundary layers in liquids, attention
has turned to the use of optical techniques for the investigations of fluid dynamics \cite{Fedin}.

\section{Optical Diagnostics as A Principal Diagnostics for Proton Target Experiment}

\subsection{Working principle of shadowgraph}

The major advantage of optical measurements is the absence of an instrument
probe that could influence the flow field. The light beam can also be considered as essentially inertialess,
so that very rapid transient effects can be studied.

Shadowgraph is often employed in studying shock and
flame phenomena, in which very large density gradients are present.
It integrates the quantity measured over the length of the light beam.
For this reason they are well suited to measurements in two
dimensional fields, where there is no index of refraction or density
variation in the field along the light beam.

Figure~\ref{fig:light_shadow} shows the displacement of a light for shadowgraph.
The index of refraction of a homogeneous transparent medium obtained by Lorentz-Lorenz
relation reduces to the Gladstone-Dale equation when $n \simeq 1$:

\begin{equation}
 \label{e:eq31a}
\frac{n-1}{\rho} = C  \ ,
\end{equation}

where C is the Gladstone-Dale constant and $n$ is the index of refraction of a homogeneous
transparent medium. The first or second derivative is given as follows:

\begin{equation}
 \label{e:eq31b}
\frac{\partial \rho}{\partial y} = \frac{1}{C} \frac{\partial n}{\partial y}  \ \  , \ \ \ \
\frac{\partial^{2} \rho}{\partial y^{2}} = \frac{1}{C} \frac{\partial^{2} n}{\partial y^{2}}  \ .
\end{equation}

Employing the ideal gas law and assuming that the pressure can be constant, the effect of temperature
on the index of refraction and its derivatives can be shown as follows:

\begin{equation}
 \label{e:eq31c}
\frac{\partial n}{\partial y} = - \frac{C P}{R T^{2}} \frac{\partial T}{\partial y}
\end{equation}

and

\begin{equation}
 \label{e:eq31d}
\frac{\partial^{2} n}{\partial y^{2}} = C [-\frac{\rho}{T} \frac{\partial^{2} T}{\partial y^{2}} + \frac{2 \rho}{T^{2}} (\frac{\partial T}{\partial y})^{2} ] \ .
\end{equation}

If the illumination is uniform entering the test section, it should still be fairly uniform at
the exit of the test section because of the short distance the light has traveled.
The light, however, is deflected by an angle $ \theta $, which is a function of y.
The illumination within the region defined by $ \Delta y $ at the position is within the region
defined by $ \Delta y_{sc} $ at the screen. The intensity at screen is given as follow:

\begin{equation}
 \label{e:eq31}
I_{o} = \frac{\Delta y}{\Delta y_{sc}} I_{T} \ ,
\end{equation}

where $I_{T}$ is the initial intensity of light. The contrast is given as follow:

\begin{equation}
 \label{e:eq32}
\frac{\Delta I}{I_{T}} = \frac{I_{o} - I_{T}}{I_{T}} = \frac{\Delta y}{\Delta y_{sc}} - 1 \simeq -z_{sc} \frac{\partial \theta}{\partial y} \ ,
\end{equation}

where $z_{sc}$ is the distance to the screen. Combining Eqn.~(\ref{e:eq32}) with the angular
deflection of ray $\theta = \frac{1}{n_{a}} \int \frac{\partial n}{\partial y} dz$,
the contrast is given as follow:

\begin{equation}
 \label{e:eq33}
\frac{\Delta I}{I_{T}} = - \frac{z_{sc}}{n_{a}} \int \frac{\partial^{2} n}{\partial y^{2}} dz  \ ,
\end{equation}

where $n_{a} \simeq 1$ for the ambient air. For gas, Eqn.~(\ref{e:eq31b}) and Eqn.~(\ref{e:eq31d})
can be substituted into Eqn.~(\ref{e:eq33}):

\begin{equation}
 \label{e:eq33a}
\frac{\Delta I}{I_{T}} = - \frac{z_{sc}}{n_{a}} \int  C \frac{\partial^{2} \rho}{\partial y^{2}} dz =  - \frac{z_{sc}}{n_{a}} \int  C [-\frac{\rho}{T} \frac{\partial^{2} T}{\partial y^{2}} + \frac{2 \rho}{T^{2}} (\frac{\partial T}{\partial y})^{2} ]   dz  \ .
\end{equation}

Shadowgraph system are rarely used for quantitative density or temperature measurements because the
contrast would have to be measured accurately. However, if large gradients of density are present,
as in a shock wave, shadowgraph images can be very useful \cite{Goldstein}.
Thus, shadowgraph is used principally for qualitative descriptions of a density field because
it yields information on the first and second derivatives of density.
Coupled to a state-of-the art high-speed camera and the long interaction path length of a light beam
with a field of view adjustable to arbitrary dimensions, the optical shadow photography enables one
to obtain the physical characteristics for the entire subject being investigated in a short period
of time.

\subsection{Development of optical diagnostic system}

An optical diagnostic system is designed and constructed for imaging
a free Hg jet interacting with a high intensity proton beam in a
pulsed high-field solenoid magnet. The optical imaging system
employs a back-illuminated, laser shadow photography technique.
Object illumination and image capture are transmitted through
radiation-hard multi-mode optical fibers and flexible coherent
imaging fibers. A retro-reflected illumination design allows the
entire passive imaging system to fit inside the bore of the solenoid
magnet. A sequence of synchronized short laser light pulses are
used to freeze the transient events and the images are recorded by
several high speed charge coupled devices.

\subsubsection{\textit{optical imaging system and Viewports design}}

Laser back-illuminated shadow photography technique is employed in experiment to
capture the dynamics of the interaction of the proton beam with a
moving free Hg jet. The design of the optical imaging system is
based on a few essential criteria which are described below. The entire optical imaging
head has to fit inside a small portion of a 1 meter long, 150~mm
diameter bore magnet. Figure~\ref{fig:optics_install} shows the conceptual
back illuminated optics design, the installation of 4~Viewports on the primary
containment vessel, and the schematic layout of optical components, respectively.

Note that all optics placed inside the interaction beam
tunnel are required to be radiation-hard because of high radiation levels
in the beam tunnel and the activation of the Hg after proton beam interactions.
\\
A Viewport is located at the beam interaction center and two additional Viewports are located at $\pm$ 152.4~mm up/down stream locations. Viewport 4 is positioned at +457.2~mm and is designed to capture the residual dynamics of the proton interaction. Because of limited space inside the magnet bore, object illumination and image capture are transmitted
through multi-mode optical fibers and coherent imaging fibers, respectively, all positioned on one
side exterior to the primary containment vessel.
Figure~\ref{fig:optics_assembly} shows the fabricated and
assembled optical head containing the integration of ball lens,
imaging lens, illumination fiber, and imaging fiber, where a red laser diode is used to illuminate the optical head.
\\
The arrangement resembles a compact endoscope design but with a
different illumination scheme. Illumination light pulses are coupled
into a 15 meter long multi-mode fiber (ThorLabs BFL22-200). It has a
numerical aperture of 0.22, 25$^{\circ}$ cone angle, with a core
diameter of 200~$\mu$m that matches that of the fiber-coupled lasers.
To provide a $\sim$ 55~mm illumination area at the center of the
primary containment vessel over a limited short working distance of $<$ 100~mm, the
illumination cone angle has to be opened up to a 43$^{\circ}$ full
cone angle. This is achieved by placing a tiny  $\sim$ 0.5~mm
diameter sapphire ball lens (Edmund Optics M46-117) at the tip of
the illumination fiber and secured by a thin stainless steel plate.
At the heart of the illumination arrangement is a 76~mm diameter
Au-coated concave spherical retro-reflector that has a short radius
of curvature of 124~mm (Rainbow Research Optics). When the much
diverged illumination fiber is placed at the radius of curvature and
shined onto the optical axis of the reflector, a retro-reflected beam
returns back to the illumination fiber providing the
back-illumination scheme. Again, because of the tight environment
inside the primary, a Au-coated 90$^{\circ}$ prism mirror turns the
optical path from longitudinal to transverse onto the center of the
primary. Two anti-reflection coated sapphire windows (Swiss Jewel
Company) are mounted on the primary with airtight seals tested up to 1.4~bar pressure.
The diameter and the thickness of the window is 100~mm
and 6~mm respectively, sufficiently large enough for the observation
of a 1~cm diameter jet and mechanically strong enough to withstand
the momentum of a direct impact from Hg jet with a mean velocity of 20~m/s \cite{Simos}.
\\
Based on this optical arrangement, a Hg jet in front of the
reflector naturally makes a shadow on the retro-reflected beam.
The shadow is collected by a 1~mm diameter AR-coated
cylindrical grin objective lens (GrinTech, GT-IFRL-100-inf-50-CC)
which has an optical path length of 2.43~mm. The grin lens is
coupled onto a coherent image fiber. This flexible coherent
imaging fiber is the key optical element of the imaging system. It
is a 10~meter long Sumitomo IGN-08/30 fiber with 30,000 picture
elements (pixels). Each individual fiber has a core diameter of $\sim$ 4~$\mu$m
with a total fiber diameter of merely 0.96~mm including coating. It
has a bending radius of 40~mm, sufficiently small to allow curving
and arching inside the primary containment vessel. All imaging
fiber ends are hand polished in-house to optical finished quality to
allow high quality images with maximum light intensity transmission. Figure~\ref{fig:fiber_surface} shows the final finished end of an imaging fiber after polishing with 0.3~$\mu$m lapping
film (ThorLabs, LFG03P). The surface quality and the flatness of the imaging fibers are
inspected under a microscope. The imaging fibers are jacketed
in-house with reinforced furcation tubing (ThorLab FT030-BK). One
end of the imaging fiber is finished with an SMA 905 fiber-optics
connector to facilitate coupling to a CCD camera. The other ends of
the illumination and imaging fibers are positioned next to each
other with $\sim$ 2~mm separation inserted inside a specially fabricated plastic
ferrule. The integrated all-in-one ferrule (ball lens, illumination fiber,
objective lens, and imaging fiber bundle) is placed at the radius of
curvature as well as on the optical axis of the reflector so that it allows both the
illumination and the imaging collection to work on one side of the
primary.
The liquid Hg target is enclosed in a stainless steel primary containment vessel which is placed in the primary beam tunnel (TT2A). A total of four optical imaging heads for each Viewport are mounted on the exterior of the primary, designated as channels 1 to 4. All fibers are routed through a $\sim$ 150~mm diameter,
2~meter long concrete passage to an adjacent beam tunnel (TT2), where radiation is much reduced. All electronics control for the optical diagnostic as well as all other electronics control for
the solenoid magnet operation and hydraulic power unit used to generate the Hg jet are also placed in the adjacent tunnel. The exit end of each imaging fiber is coupled to an SMA fiber
adaptor (ThorLabs SM1SMA) mounted on an x-y translator (ThorLab
LM1XY). Four 40~$\times$ infinitely corrected microscope objective (Newport
M-40x) relay the  $\sim$ 0.96~mm image outputs of each imaging
fiber onto each corresponding CCD with appropriate lens tubes to
fully expand the images onto a typical 10 $\times$ 10~mm CCD array. A
non-rotating adjustable lens tube zoom housing (ThorLabs SM1ZM)
provides fine and accurate adjustment of image focus on CCD.

\subsubsection{\textit{consideration for alignment of optics}}

A retro-reflective mirror captures the output beam of the laser
diode and focuses it through the field of view at the target onto
the lens of the telescope. The CCD camera views the target through
the telescope.
Tilting alignment by using fine adjustments on the side of the
retro-reflecting mirror can be made and the field of view can be adjusted by
moving the imaging lens forwards or backwards. The system is
designed to make 6 possible alignments for adjustment.  After the
retro-reflecting mirror moves forward or backward, the field of
view can also be adjusted. The maximum field of view that could be obtained
is $\sim$ 5.0~cm diagonally.

For target to be in focus, one must obey the lens formula:

\begin{equation}
 \label{e:eq35}
\frac{1}{f} = \frac{1}{c} + \frac{1}{d} \ ,
\end{equation}

where $c$ is the distance from the target to the objective lens, $d$ is the distance from the
objective lens to the camera, and $f$ is focal length.

\subsubsection{\textit{high speed cameras and light sources}}

Table~\ref{tab:spec_camera} gives the specifications of high
speed cameras in terms of some selected attributes. Two FastVision cameras with CCD size of 15.4 $\times$
12.3~mm run with a full 1280 $\times$ 1000 pixel resolution at a 0.5~kHz frame rate.
One Olympus Encore PCI 8000S camera with 1/3 inch
CCD size runs with a 480 $\times$ 420~pixel resolution at a 4~kHz
recording rate. A high speed ``Silica Mountain Devices (SMD)" 64KIM
camera with a CCD size of 13.4 $\times$ 13.4~mm runs with a reduced
single frame size of (960 $\times$ 960)/4~pixel resolution at up to 1~MHz frame rate. For the three slower cameras, images collected by each individual imaging fiber overfill the CCD pixels by a factor of $\sim$ 6 and $\sim$ 3,
respectively, i.e. one fiber projected onto 6 $\times$ 6 and 3 $\times$ 3 CCD pixel area,
respectively. However, for the SMD camera, each imaging fiber
slightly underfills the CCD pixels by a factor of 0.83, i.e. one
fiber projected onto nearly a single CCD pixel area. Due to the
nature of spatial superposition, an array of imaging fibers imaged
by an array of CCD pixels, some images might compose of a honeycomb
pattern caused by this pixelation artifact. However, the artifact
can be minimized by slightly defocusing the image on the CCD. The FastVision and Olympus CCDs are capable of recording at a frame rate higher than 500~Hz, the architecture for binning at reduced resolution requires a change of the
zoom ratio on the image head doom. The SMD camera has a different but fixed binning architecture so that the full field of view is taken at a high speed frame rate with reduced resolution. Except for
the SMD camera where images are frozen by the short 150~ns
illumination laser pulses, all other images are arrested by the
short adjustable electronic exposure time of 10 $\sim$ 50~$\mu$s set on the
CCDs.
\\
Synchronized short laser light pulses are used to illuminate the
target and freeze the motion of the jet after the impact of the proton beam.
For SMD camera, the mask reduces the photosensitive area to 0.03 of the nominal pixel area.
The quantum efficiency of the photo-resistive area is 0.18 at
800~nm, and the pixel fill is 200000 electrons. Therefore, a full exposure
of a frame of the CCD therefore requires (960)$^{2} \times$ 200000/0.03/0.18 $\approx$ 3.4 $\times$
10$^{13} $ photons or 10 Watts for 800~nm photons. For FastVision camera,
the sensor is 1280 $\times$ 1024 pixel (1.03 megapixel) of CCD of total area 15.36
$\times$ 12.29~mm$^{2} $ in 8 bits at 500 frames per second (10 bits at 400 frames per second).
Maximum frame rate is 500,000 at 1 $\times$ 1280.
The mask reduces the photosensitive area to 0.4 of the nominal pixel area.
Based on the estimation of required photons, a full exposure
of a frame of the CCD therefore requires 1280 $\times$ 1024 $\times$ 200000/0.4/0.18
$\approx$ 3 $\times$ 10$^{12}$ photons or 1 Watts for 800~nm photons.
\\
Optical light pulses are sent through 15 meters of multi-mode
illumination fibers. The light sources used in the experiment are
all Class 4 lasers, emitting at wavelengths of 808 to 850~nm.
Three lasers are capable of emitting a peak optical power of 1 Watt (JDS
Uniphase SDL-2300-L2) driven by three independent current drivers
(ThorLabs LDC220C). These 1 Watt lasers can be operated from CW to a
minimum programmable pulse width of 1~$\mu$s limited by the trigger logic pulse.
The 4$^{th}$ laser emits at a peak optical power
of 25 Watt (Bright Solution BDL20-808-F6) limited by the pulsed
current driver (Avtech AXOZ-A1A-B). It provides a current pulse of
150~ns and is capable of running at the maximum 1~MHz repetition
rate, i.e. a frame rate of 1~$\mu$s/frame.

The complete transmission of the imaging system is $\sim$ 0.2 per Viewport channel, including 0.85 for the
15~meter long illumination fiber, 0.86 for the sapphire
ball lens, 0.86 for each pass of the sapphire Viewport, 0.91 for the
retro-reflector, 0.67 for the 10 meter long imaging fiber, and 0.86
for the grin lens and the relay lens. For the SMD camera, the
imaging circle filled $\pi$/4 of the CCD array. A measured output
energy of 3.5~$\mu$J/pulse is obtained from the Bright Solution
(BDL20-808-F6) laser illumination light source for Viewport 2.
Therefore the calculated number of photons impinging on
the SMD camera reaches 4.2 $\times$ 10$^6 $ photons/pixel. After
taking into account the 18$\%$ quantum efficiency of the CCD,
7.5 $\times$ 10$^5$ photoelectrons are generated at the full
illumination intensity. Since the SMD camera has full well capacity
of 2.2 $\times$ 10$^5 e^-$ , there is a factor of $\sim$ 3 on the
optical power budget reserved for unanticipated optical power loss
and for overcoming the possible attenuation due to ionization
radiation. Similar calculations for Viewport channels 1 and
3 give a factor of $\sim$ 10 on the optical power budget. This
larger factor is mostly due to the long, 10~$\mu$s, exposure time
set on the FastVision cameras. Overall, the imaging system is
designed to have sufficient optical power budget for the
illumination of each Viewport throughout the entire experiment.

\subsubsection{\textit{radiation-hardness}}

Because of the high radiation level in the beam tunnel and the
activation of the Hg after the proton beam interactions, all optics
placed inside the interaction beam tunnel are required to be
radiation-hard. One complete set of optics was selected for radiation
resistance test done at CERN. This complete set of optics included an Au-coated reflector, sapphire window, illumination fiber, imaging fiber, and Grin objective lens. The experiment has anticipated a total of 200 proton pulses at 14 and 24~GeV with a total of $\sim$ 3 $\times$ 10$^{15}$ protons. The calculated total radiation reaches $\sim$ 1~Mrad equivalent radiation dose. Therefore, all optics except the grin objective lens were irradiated at CERN to a lower energy 1.4~GeV proton beam but up to an equivalent radiation dose of 5 $\times$ 10$^{15}$ protons.
Because we missed an opportunity to deliver the grin lens to the CERN irradiation facility, the grin objective lens was instead irradiated at BNL using a Co-60 source up to a total dose of $\sim$ 3~Mrad.
\\
The reflectance of the Au-coated reflector and the transmittance of all other optics are measured at the wavelength of 830~nm before and after irradiation.
Table~\ref{tab:irrad_optics} shows the effects of irradiation up to an equivalent radiation dose of 1~Mrad on the reflectance and transmittance of the components of the optical diagnostic system. No noticeable change in the reflectance was observed
on the Au-coated reflector even though the substrate of the reflector
has turned nearly opaque. The sapphire, 5 meter long of illumination
fiber, and 0.3 meter long of imaging fiber do not show any
additional insertion loss. They are all radiation hard up to a 1~Mrad
dose. However, the small grin objective lens did suffer radiation
damage resulting in a 0.73 transmission. This tiny grin objective
lens is made of silver-ion exchanged index modification internal to
a glass substrate. Therefore it was not anticipated to have a high
radiation resistance.
However, it is well known that although
glass (and silica fibers) lose its transmission in the visible
wavelengths, near infrared (NIR) light can still has adequate light
throughput for some applications \cite{Kakuta}. This is one of the reason we
select NIR rather than visible laser light for back-illumination of
the Hg jet. Since the back-illuminated NIR light passes the
grin objective only once, the 0.27 transmission
loss over the entire experiment is tolerable and can be
recovered with the present designed laser capability. We should note
that the integrity of the imaging properties of the grin lens was
unchanged, i.e. no image distortion was observed after the 1~Mrad
radiation resistance test.

\subsubsection{\textit{scintillating fiber channel}}

A jacketed 2~meter long 1~mm diameter blue emitting scintillating
fiber is attached along with the imaging head to
register gamma emission during the proton beam and Hg jet interaction. A 12~meter long 1~mm diameter fiber patch-cord (ThorLabs BFH37-1000) carries the blue scintillated light signal and is fiber-coupled to an Avalanche photodiode (ThorLabs APD210),
designated as channel 0. The overall transmission at
the center wavelength of 480~nm of the fiber patch-cord is measured
to be 0.77. The scintillating signal trace is displayed on an
oscilloscope and data can be retrieved remotely from the control
room. This scintillating signal serves to confirm the arrival of the
proton beam and has the potential to extract the proton intensity
from the scintillating signal pulse level.

\subsection{Schematic of electronic trigger and high speed camera control}

Because we are using several high speed cameras from different
vendors, we must use separate camera control software for each camera. The limitation on their exposure time also requires two different set of illumination laser pulse
trains. A master trigger pulse, synchronized to the arrival of the proton
bunch, is delivered to trigger the Hg loop system, the solenoid
magnet system, and the optical diagnostic system together.  The
Hg jet reaches its steady state for 1~second when the
solenoid magnet reaches the highest magnetic induction field of 0~T $\sim$ 15~T.
However, there is a significantly long time lag of $\sim$ 10~seconds
for the solenoid system to power up to its full capacity. Therefore,
the master trigger signal is first sent to a digital delay generator
(Stanford Research DG535) to provide a sufficient long delay to synchronize
with all other electronic components. These relative and absolute
delays are measured by an oscilloscope. By adjusting each
independent delay channel, complete synchronization of all cameras
with the pulsing of the laser light sources can be achieved by comparing the bright/dark image intensities
of each frame for each camera.

Figure~\ref{fig:trigger_signal} shows the two sets of pulse sequences used to simultaneously trigger
all cameras. The 25W infrared laser consisted of a 17 pulse sequence with a pulse
width of 150~ns. This determines the exposure time of the SMD camera on the Viewport 2.
The laser pulse period is set to match the frame rate of the images. The SMD camera collects 16 frames of image. Figure~\ref{fig:scope_beam} shows the traced signals on an oscilloscope when the beam and
the beam triggering signal are delivered.
After the master trigger from the synchrotron is delivered at time $t=0$, the
proton beam comes in $\sim$ 3~$\mu$s. The photodiode response from scintillating fiber has a 20~ns
rise time and the level indicates the beam intensity and beam position.
The scintillating fiber signal gives the beam arrival time. Therefore, it is possible to set
the trigger timing for the cameras and laser driver inputs, which is $\sim$ 2~$\mu$s after the
master triggering from the proton synchrotron.

Three 1 Watt lasers pulsed to a 0.5~second duration are used to independently
illuminate Viewport 1, Viewport 3, and Viewport 4, respectively.  Typically
the FastVision and Olympus cameras continuously collect 220 frames of images. The
exposure times on the cameras are set at 10 $\sim$ 50~$\mu$s respectively to
give a sharp image quality.  Although the sharpness of images
increases with reduced exposure time, much more light is required for
illumination. Therefore, a trade off between exposure time and
laser intensity is made. On the contrary, the exposure time for SMD
camera is determined by the laser pulse width. As the pulse width of
the laser decreases, the laser intensity also decreases. In order to
utilize the maximum allowable intensity of the 25~W laser, the
maximum pulse width of 0.15~$\mu$s is used. This pulse width
should not seriously jeopardize the image quality even running at
its highest frame rate of 1~$\mu$s/frame.
A schematic diagram linking all cameras, triggering electronics, and
controlling computers are shown in Fig.~\ref{fig:trigger}. 2 desktops reside in the control room to master the optical diagnostics system. All other electronics and desktops are placed in the TT2 tunnel adjacent to the interaction beam tunnel TT2A. In our setup, all cameras, lasers, and all other associated electronics are placed in an adjacent beam tunnel controlled locally by several desktop computers. Remote control of the entire system is achieved through designated control desktops located in the control room via MS Window XP remote desktop connections from the
ethernet network. (see Fig.~\ref{fig:trigger}).

\section{Windows Consideration as Viewports}

The Hg jet target is observed through four windows.
These windows must contain any possible spray of Hg due
to intense beam energy deposition, and remain transparent after a
radiation dose from the interaction of beam and Hg.
Four Viewports are installed within the solenoid bore, which are intended for observation of the Hg jet
within primary containment vessel. Viewport 2 is positioned at the center
of the solenoid and is the location where the center of the proton beam interacts with the Hg jet.

\subsection{Fiducial mark on windows}

We put fiducial mark on each sapphire window to set the magnitude of the referenced length. The size of fiducial on the back and front windows is varying in image according to the changed field of view. i.e, the back fiducial looks smaller than the front fiducial. Figure~\ref{fig:window_fiducial} shows the artificially marked fiducial on the sapphire window.
It gives referencing length scale when we measure the size of jet, velocity, rotation of windows, and the location of magnetic axis on image.

\subsection{Impact resistance test}

We used sapphire windows to obtain enough strength and did surface coating on
both sides for anti-reflection at 800~nm wavelength.
In order to check the survival from Hg droplet impact, we tested sapphire window
using a paint ball gun.
A paint ball is a 2.7~gram sphere of radius 8.6~mm containing a colored gel that
readily ``splats" on impact. The velocity of a paint ball was 95~m/s. The
ratio of the force from a paint ball to that due to the dispersal of the
entire Hg jet by the proton beam is

\begin{equation}
 \label{e:eq37}
\frac{F_{paintball}}{F_{mercury}} = \frac{m_{paintball} \textit{v}^{2}_{paintball} r_{mercury}}{m_{mercury} \textit{v}^{2}_{mercury} r_{paintball}} \ .
\end{equation}

The momentum of the paint ball is the same as that of a 7~mm diameter Hg
drop at 95~m/s. The sapphire window survived in the test \cite{McDonald2006}.

\subsection{Pressure leaking test of sapphire windows}

The primary containment is mostly welded and the window ports are sealed
with rubber gaskets (BUNA-N). Each window is sealed with two sheets of rubber
gaskets per port. 21~psi is loaded inside the primary containment to check the sealing of the primary containment.
To locate leaks, a Metheson 8850 flammable gas sniffer, which has a 5~ppm sensitivity,
and Ar/Methane (90 $ \%$ / 10 $\%$ ) was used. All of 8 windows survived the 21 psi pressure for over 17 hours.

\section{Water Jet Observation for Nozzle Performance Test}

Prior to Hg injection in the primary at Oak Ridge National
Laboratory(ORNL), extensive optical diagnostics were carried out by
pulsing water jets in the system using 4 different types of nozzle
configurations. One nozzle showed the most stable shape of jet motion with fairly uniform velocity,
$\sim$ 10~mm diameter and 20~m/s respectively.
\\
Due to the spray and wetting of water on the interior of windows,
only ambiguous shadow of the water jet was observed. A clear surface
motion was required in order to obtain accurate velocity measurement.
Therefore, only very qualitative diagnostics was made on the water jet.
The traveled distance and diameter of the water jet flow was measured by overlaying a grid of
referenced field of view onto the images. The time lapse of each frame
was read from the camera frame rates. So, the velocity of jet flow was able to be estimated.
These measurements of the water jet tests were done at ORNL. The observations led us to select the design of the final nozzle for the subsequent jet runs.

\setlength{\baselineskip}{12pt}

\newpage
\clearpage
\begin{landscape}
\begin{table}
\centering
\tablecaption{\label{tab:spec_camera} Specifications of high speed
cameras. }
\begin{supertabular}{|l|l|l|l|}
\hline
\hline
Attributes & SMD 64KIM & FastVision & Olympus Encore PCI 8000S \\
\hline
CCD chip size & 13.4~mm $\times$ 13.4~mm & 15.4~mm $\times$ 12.3~mm & 1/3~inch \\
Pixels & 960 $\times$ 960 & 1280 $\times$ 1024 & 480 $\times$ 420 \\
Pixel size & 14~$\mu$m & 12~$\mu$m & 13~$\mu$m \\
Single frame & 240 $\times$ 240  & 1280 $\times$ 1000 & 480 $\times$ 420 \\
Maximum frame rate & 1~MHz\footnote{16 frames} & 0.5~kHz\footnote{at full resolution} & 4~kHz\footnote{$12.5~\mu$s electronic shutter, with reduced frame size} \\
Full well Capacity & 220,000 $e^-$ & $\sim$ 1000 LSB/lux-sec & - \\
ADC & 12 bit  & 8 bit & 8 bit \\
\hline
\hline
\end{supertabular} \\

\ \\
\ \\
\begin{flushleft}
%\parbox[b]{3.5in}{
\shortstack[l]{\vrule width 3.5in height 0.4pt\\
        \ \ \ $^{1}$ 16 frames. \\
       \ \\
       \ \\
        \ \ \ $^{2}$ at full resolution. \\
       \ \\
       \ \\
       \ \ \ $^{3}$ 12.5~$\mu$s electronic shutter, with reduced frame size. }

%\ \ \ \ \ \ \ \ \ $^{1}$ 16 frames. \\
%\ \ \ \ \ \ \ \ \ $^{2}$ at full resolution. \\
%\ \ \ \ \ \ \ \ \ $^{3}$ 12.5 $\mu$s electronic shutter, with reduced frame size.
\end{flushleft}
\end{table}
\end{landscape}
\clearpage

\setlength{\baselineskip}{12pt}

\newpage
\clearpage
\begin{landscape}
\begin{table}
\centering
\tablecaption{\label{tab:irrad_optics} Effects of irradiation up to an equivalent radiation dose of 1~Mrad on the reflectance and transmittance of the components of the optical diagnostic system. Reflectance is inferred on the Au-coated mirror and transmittance is inferred on all other components.}
\begin{supertabular}{|l|l|l|l|}
\hline
\hline
Optical component & Before radiation&  After radiation & $\%$ difference \\
\hline
Large Au-coated mirror & 0.91 & 0.92 & no change \\
Sapphire window(1~mm) & 0.86 & 0.87 & no change \ \ \ \ \ \ \ \ \ \ \ \ \ \ \  \\
Illumination fiber(5~m) & 1 & 1.02 &  no change \\
Imaging fiber (30~cm) & 0.67 & 0.71 & no change \\
Grin lens & 0.90 & 0.66 & 73 $\%$ \\
\hline
\hline
\end{supertabular}
\end{table}
\end{landscape}
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\newpage
%\input{epsf}
\clearpage
\begin{landscape}
\begin{figure}
\centering
\leavevmode \epsfxsize=12 cm \epsfysize=8 cm
\epsfbox{light_shadow.eps} \caption{Displacement of light beam for shadowgraph.}
\label{fig:light_shadow}
\end{figure}
\end{landscape}
\clearpage
%\input{epsf}

\newpage
\clearpage
\begin{landscape}
\begin{figure}
\centering
\includegraphics{optics_install}
\caption{Design of optical layout and installation of 4 Viewports of
primary containment vessel. a.) Conceptual integration of optics to primary containment vessel. b.) Photograph of installation of optics to primary
containment vessel. c.) Schematic layout of optical components. } \label{fig:optics_install}
\end{figure}
\end{landscape}
\clearpage

\newpage
\clearpage
\begin{landscape}
\begin{figure}
\centering
\includegraphics{optics_assembly}
\caption{Photograph of optical head assembly and its illumination of laser. a.) Front view of optical head assembly. b.) Side view of optical head assembly. c.) Illumination of fiber-optics head assembly. } \label{fig:optics_assembly}
\end{figure}
\end{landscape}
\clearpage

\newpage
\clearpage
\begin{landscape}
\begin{figure}
\centering
\includegraphics{fiber_surface}% Here is how to import EPS art
\caption{\label{fig:fiber_surface} Polished fiber end with 50~X and 800~X
magnifications, respectively.}
\end{figure}
\end{landscape}
\clearpage

\newpage
\clearpage
\begin{landscape}
\begin{figure}
\centering
\includegraphics{trigger_signal}% Here is how to import EPS art
\caption{\label{fig:trigger_signal} Schematic of synchronized signal
of high speed camera and laser pulse.}
\end{figure}
\end{landscape}
\clearpage

\newpage
\clearpage
\begin{landscape}
\begin{figure}
\centering
\includegraphics[width=8in]{scope_beam3}
\caption{The triggering time for high speed camera upon beam arrival.} \label{fig:scope_beam}
\end{figure}
\end{landscape}
\clearpage

\newpage
\clearpage
\begin{landscape}
\begin{figure}
\centering
\includegraphics[width=8in]{trigger2}% Here is how to import EPS art
\caption{\label{fig:trigger}Schematic of electrical triggering and
high speed camera control in tunnel for experiment.}
\end{figure}
\end{landscape}
\clearpage

\newpage
\clearpage
\begin{landscape}
\begin{figure}
\centering
\includegraphics{window_fiducial}% Here is how to import EPS art
\caption{\label{fig:window_fiducial}Top fiducial on the front window and bottom fiducial on the rear window. a.) Photo of fiducial on the sapphire window assembled in Viewport. b.) Image of fiducial captured by camera.}
\end{figure}
\end{landscape}
\clearpage

