Review of

ITER Toroidal Interferometer/Polarimeter Design

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height="5.094444444444444in"}

October 21, 2005

**C.W. Domier, K.C. Lee, N.C. Luhmann, Jr.**

UNIVERSITY OF CALIFORNIA, DAVIS

Department of Applied Science

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height="1.2708333333333333in"}

PRELIMINARY REPORT ON THE ITER MULTICHANNEL TOROIDAL
INTERFEROMETER/POLARIMETER DESIGN

> C.W. Domier, K.C. Lee, N.C. Luhmann, Jr.
>
> University of California at Davis\
> Dept. of Applied Science\
> Davis, CA 95616

# ABSTRACT

Designs for a multi-channel toroidal interferometer/polarimeter (TIP)
system to measure the electron density profiles on ITER are reviewed. It
is concluded that a two wavelength system design for vibration
compensation is still required; however, the choice of wavelengths is
not yet fixed. Newly reported studies into environmental effects on
retroreflectors and in-vessel mirrors reveal serious problems with the
short wavelength 10.6 µm/5.3 µm choice for the TIP system. These
problems are alleviated somewhat by going to a slightly longer
wavelength choice of 10.6 µm/9.27 µm or 12.1 µm/9.0 µm. Consideration of
a longer 57.2 µm/47.6 µm wavelength option is suggested for
consideration, with the pros and cons of each approach identified. While
the longer wavelength option has some partially resolved technology
issues which need to be addressed and requires increased diameter
retroreflectors and input/exit mirror, success of the shorter wavelength
option may require a major redesign of the input/exit mirror
arrangement.

The need for a real-time alignment system is reaffirmed, with the
preferred choice being a self-scanning IR/FIR-based system over that of
a visible light system due to potential mirror reflectivity problems
associated with mirror erosion and carbon deposition. The dispersion
interferometer concept, which employs nonlinear optics to frequency
double an input laser beam which then co-propagates with the original
un-doubled laser beam, is examined for applicability to ITER. Finally, a
number of potential research and development areas are identified that
could greatly impact the TIP system design, as well as potential physics
issues that could be addressed using the TIP system. One key area here
is the possible application of hollow waveguides for transporting
signals to/from the ITER plasma.

# A. INTRODUCTION

The development of a multi-channel toroidal interferometer/polarimeter
(TIP) system is desired to measure the electron density profile on ITER
with high reliability and sufficient time resolution (1 msec) and
accuracy (1%) for plasma feedback control. Supplementary data are to be
provided by a poloidal interferometer/polarimeter system.

A short wavelength probing laser beam is used in TIP both as an
interferometer to measure line average electron density and as a
polarimeter to measure the plasma-induced Faraday rota-tion. A plan view
of ITER showing five tangential probe beam paths through the ITER plasma
is shown in Fig. 1, as taken from Sec. 2.6.3.3 of the *ITER Technical
Basis -- Plant Description Document* \[1\]. The beams enter and exit the
plasma from a single port and are reflected back along the same beam
path by plasma facing corner cube retroreflectors (CCRs) embedded within
the ITER blanket shield modules (BSMs). By requiring only a single port
with no large vibration damping structures, the total penetration area
through the BSMs is minimized and the neutron leakage is thus reduced.
Reflective optics are employed throughout to avoid neutron damage issues
associated with transmission optics, with the exception of vacuum
windows which are located well behind the port plug in a relatively
benign radiation environment.

![](media/image3.png){width="6.05in" height="3.390277777777778in"}

Fig. 1. Schematic layout of the ITER toroidal interferometer/polarimeter
system showing the five lines of sight through an equatorial port (from
\[1\]).

This diagnostic will provide the primary line integral density
measurement input to the ITER basic control algorithms. The system must
therefore be designed for near \~100% availability, as any signal
drop-out will most likely result in a controlled pulse termination.
Extreme care must therefore be taken in the multichannel TIP design. The
purpose of this report is to evaluate the current ITER designs, and to
identify critical design issues that are still unresolved.

# B. OVERVIEW OF EXISTING ITER TIP DESIGNS

An initial TIP design was developed for the ITER-98 device by Carlstrom
*et al.* in the late 1990s \[2, 3\]. This design was based on the use of
collinear 10.6 µm (CO~2~) and 5.3 µm (CO) laser probe beams, with the
shorter wavelength beam employed for vibration compensation. A pair of
acousto-optic modulators are employed to generate intermediate frequency
(IF) signals for interferometry and polarimetry. In the example listed
in Ref. 3, right and left circularly polarized beams are generated with
42 MHz and 38 MHz frequency shifts, respectively, from the unshifted
laser beam. The two beams are combined to form an elliptically polarized
beam which rotates at half the difference frequency (4 MHz) between the
two beams. The rotating elliptical beam is passed through the plasma and
interferes with a reference (unshifted) laser beam at a single detector,
from which both interferometry and polarimetry signals are derived. This
general approach (ignoring wavelength) follows the procedure employed
earlier on TEXTOR \[4\], MTX \[5\] and RTP \[6, 7\], and more recently
on a number of other plasma devices including NSTX \[8\].

The Carlstrom *et al*. design was revisited and revised by Kondoh *et
al*. \[9\] for the revised ITER device. It shares many of the same
design aspects, but with some key differences. Firstly, the extremely
short 5.3 µm wavelength of the vibration compensation laser was rejected
due to problems experienced on JT-60 with a two color interferometer
system employing 10.6 µm (CO~2~) and 3.39 µm (HeNe) lasers \[10\]. This
JT-60 interferometer implementation experienced significant stability
and reliability problems arising from mirror damage, window coating, and
mechanical vibrations at the shorter wavelength. Significantly improved
operation was achieved on JT-60 after the 3.39 µm laser was replaced
with a longer wavelength 9.27 µm (CO~2~) laser \[10-12\], as they now
propose for use on ITER. Alternatively, they suggest the use of a 12.1
µm (^14^C^16^O~2~) / 9.0 µm (^12^C^18^O~2~) arrangement should the 10.6
µm/9.27 µm combination have insufficient wavelength separation for good
vibration compensation.

CO~2~ beam position sensors are proposed in the Carlstrom *et al*.
design to control alignment mirrors and to keep the beams focused on the
detectors, whereas a visible light laser and a camera are employed in
the Kondoh design. This visible light alignment system was developed for
the ITER diverter impurity monitor, and would monitor light scattered
from a 3 mm × 3 mm diffuser mounted at the side of each corner cube
retroreflector (CCR). Another area in which the two designs differ is
the size and operation of the CCRs. In the Carlstrom *et al*. design,
the return beam reflected from the CCRs (35 mm × 70 mm clear aperture)
is horizontally offset by 35 mm to prevent the return beam from
re-entering the laser and thereby adversely affecting its stability. The
Kondoh design minimizes the retroreflector design requirement
(essentially doubled in clear aperture in the Carlstrom *et al*. design)
by eliminating the offset in the reflected beam. Here, the Kondoh
retroreflectors have a clear aperture of 30 mm × 30 mm.

Vacuum windows in the Carlstrom *et al*. design were to be made of ZnSe.
Estimates of the Faraday rotation expected in these windows at 10.6 µm
was considered acceptably low (0.25º) assuming a radial field of 0.3 T
at the window. The Kondoh team has proposed replacing these windows with
chemical vapor deposition (CVD) diamond windows since the Verdét
constant of the diamond is 1/17 that of ZnSe, thus significantly
reducing the Faraday rotation. Additional benefits of CVD diamond
include increased hardness, reduced thickness, excellent heat transfer
and ease of vacuum sealing, all of which trade off against the higher
cost of CVD diamond.

# C. CRITICAL ISSUES

## 1) WAVELENGTH

The problems of window damage and coating experienced on JT-60 at
extremely short wavelengths raise serious concerns. An equally serious
concern has to do with the cumulative effects of erosion on the
retroreflectors. A study by Dr. Voitsenya of the Kharkov Institute for
Physics and Technology, in which CCRs were bombarded with deuterium ions
from a plasma source \[13\], has shown that long term sputtering by
charge exchange atoms (CXA) and/or deposition of carbon-based (or
beryllium-based) contaminant layers can affect the optical properties of
the CCR.

Qualitatively, Bennett's formula on surface roughness,
*R*=*R*~0~exp\[--(4π*d*)^2^/λ^2^\], is frequently []{.mark}used to
express the specular reflectance *R* of a rough surface whose ideal
reflectivity is given by *R*~0~ where *d* is the mean surface roughness
and λ is the wavelength of the reflected light \[14\]. This is plotted
in Fig. 2 for both a single mirror surface and a CCR as a function of
*d*/λ. As is pointed out by Voitsenya, the specular reflectance of a CCR
at \~10 µm has decreased by 20% for a surface roughness *d* ≈ 200 nm.

![](media/image4.png){width="3.7319444444444443in"
height="2.6083333333333334in"}

> Fig. 2. The specular reflectance of a metal mirror as a function of
> the mean roughness according to the Bennett formula (from \[13\]).

Although CXA sputtering was shown to seriously degrade the CCR
reflectivity at 10 µm, mounting the CCRs in narrow and long grooves
should eliminate CXA sputtering concerns as long as the CXA flux can be
reduced by at least an order of magnitude as compared to that at the
first wall. The primary concern instead is the role of carbon film
deposition. The amount of carbon film buildup expected in ITER is at
present unknown, but studies have been performed on TEXTOR \[15\] using
a "periscope" with a channel length three times that of the diameter
(35 mm) placed in the scrapeoff layer and oriented along a tangent to
the toroidal direction. Extrapolations from this study (see Fig. 3)
would predict a carbon film layer of 250 nm (such as was built up on
TEXTOR after a total of 1047 sec of plasma discharge) to be reached
within one to two ITER discharges. It should be noted, however, that the
viewing lines on the TEXTOR study were not fully tangential and that the
deposition on ITER may be substantially worse.

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height="1.6236111111111111in"}

> Fig. 3. The thickness of carbon-based film deposited on a test mirror
> in the TEXTOR periscope system (from \[15\]).

Increased surface roughness, as a result of carbon film deposition, has
the effect not only of decreasing the mirror reflectivity but can also
alter the polarization angle of the reflected beam. The polarization
effect depends strongly on the initial polarization of the probing beam
and is maximum when the electric field is parallel to the mirror
surface. Plotted in Fig. 4 are the calculated changes in polarization
angle after a single reflection from a carbon coated rhodium mirror
(rhodium film electroplated onto a metal substrate) at 10 µm. The figure
shows that the polarization rotation effect (due to surface roughness)
depends on both film thickness and angle of incidence, and that the
desired ITER polarization accuracy of 0.01º is achieved with a
relatively thin carbon film thickness of 100 nm only with incidence
angles \< 40º. No data are available on the polarization effect for
triple reflections as would be the case from a retroreflector; further
calculations are therefore required. Of perhaps greater concern to the
short wavelength operation design is the polarization effect of carbon
film buildup on the input/exit mirror. The angle of incidence upon this
mirror will have to be chosen to be as small as possible so as to have
the smallest polarization changes, which implies an increase in the
number of turning mirrors required to provide the five probing beams.
This in turn implies significant changes to the reflective optics which
carry the probing signals to and from the input/exit mirror.

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height="2.504861111111111in"}

Fig. 4. Polarization angle rotation at λ=10 µm calculated for a single
reflection from a rhodium mirror as a function of carbon (C) film
thickness and incidence angle (from \[13\]).

ITER also has a high level of potential synchrotron emission in the 3-5
µm range due to suprathermals. This, together with the earlier expressed
erosion and deposition concerns regarding the input/exit mirror and
CCRs, would make it prudent to discard the 10.6 µm/5.3 µm option in
order to maintain the robustness of the diagnostic. While the 10.6
µm/9.27 µm and 12.1 µm/9.0 µm options could prove viable (the use of
heated mirrors may result in reduced deposition, while research into
mirror mitigating/coping methods via in-situ laser cleaning and/or
heating of the vacuum vessel is ongoing), the former may have
insufficient wavelength separation for good vibration compensation. On
the other hand, the 10.6 µm/9.27 µm CO~2~ laser technology is well
proven and highly dependable while the 12.1 µm (^14^C^16^O~2~) / 9.0 µm
(^12^C^18^O~2~) laser technology is far less advanced and may not be
sufficiently reliable for use on ITER.

As an alternative, it is worth considering longer wavelength operation
since both mirror reflectivity and polarization effects are
significantly ameliorated. Additional benefits include an increased
interferometer phase shift (scales linearly with wavelength, an
increased Faraday rotation (scales with the wavelength squared), and a
decreased phase shift generated by mechanical vibrations (scales
inversely proportional to wavelength). The main disadvantages are
increased diffraction and refraction effects, and a significantly
increased Cotton-Mouton effect (scales with the wavelength cubed).

Diffraction effects, which dictate how rapidly a collimated beam spreads
out as it propagates, result in larger diameter retroreflectors and/or
steering mirrors being required at longer wavelengths. Plotted in Fig. 5
are Gaussian beam simulations of the five TIP beams, calculated at 10.6
µm. Beams that travel the longest distances to their corresponding CCRs
are less tightly focused, and therefore are of the most concern.

![](media/image7.png){width="4.457638888888889in"
height="2.484027777777778in"}

Fig. 5. Schematic of the beam layout and designed beam diameter of a
Gaussian CO2 laser from the laser output to the retroreflectors (from
\[9\]).

A general rule of thumb is that the diameter of each optical element
must be at least 2.2 times the beam diameter (the 1/e width of the
intensity distribution of the laser beam), thereby allowing a 99.2%
theoretical transmission through the optic. In the simulations of
Fig. 5, the Ch. 1 beam diameter is approximately 11 mm at the CCR. This
would lead to a minimum retroreflector diameter of 24 mm to avoid
vignetting the reflected beam. Applying a 25% safety factor, this yields
a CCR diameter of 30 mm as chosen by Kondoh *et al*. \[9\].

CCRs with a 37 mm diameter (37 mm wide by 60 mm tall) are envisaged for
use in the ITER poloidal polarimeter system, as described by Donné *et
al*. \[16\]. A CCR of diameter 37 mm employed in the TIP system would
allow a wavelength increase to (37/30)×10.6 µm = 13.1 µm using the same
diameter input/exit mirror and the same 25% safety factor. The
10.6 µm/9.27 µm and 12.1 µm/9.0 µm options are thus certainly feasible
with CCRs of this size.

The next step up in wavelength with sufficient optical power would be a
57.2 µm/47.6 µm CH~3~OD laser system as is under investigation by
Nakayama *et al*., with output powers of 1.6 W and 0.8 W, respectively
\[17\]. Assuming a 13 mm beam waist diameter at the CCR corresponding to
a CCR with a 37 mm diameter, this would translate to an input/exit
mirror beam diameter of \~90 mm (for λ=57.2 µm) as compared to \~23 mm
(for λ=10.6 µm) in the Kondoh design. A CCR of diameter 50 mm would
couple to an 18 mm beam waist diameter and an input/exit mirror beam
diameter of 66 mm which is somewhat smaller than the \~75 mm diameter
mirror envisaged for the poloidal interferometer/polarimeter system.

Refraction effects will bend the probing beam, resulting in translation
of the beam at the location of the retroreflector. Plotted in Fig. 6 are
the simulated radial refraction and interferometer phase shift at 10.6,
12.1, 47.6 and 57.2 µm for ITER plasmas with two different density
profiles given by n~e~ = 10^19^+a×10^20^×(1--ρ^b^) m^--3^ where ρ is the
normalized radial coordinate. As was done by Donne *et al*. \[16\], both
a flat (ITER reference) plasma with a=1, b=10 and a peaked plasma with
a=2, b=2 have been studied. While refraction is certainly increased at
these longer wavelengths, the worst case level of radial refraction
expected at 57 µm is still small (\~6 mm) as compared to the beam
diameter and will be larger than the corresponding levels of vertical
refraction arising from the shaped ITER plasmas. These small
displacements can be easily corrected by (a) appropriate beam steering,
or (b) extending the size of the retroreflector along the plasma
midplane in a manner similar to that proposed by Donné for extending the
retroreflector vertically in the poloidal system. One item of concern
which has not been evaluated here is the refractive effects that ELMs
might have at these longer wavelengths, which should be an item of
future study.

Based on these calculations, neither diffraction nor refraction are, in
themselves, limiting factors to 57.2 µm/47.6 µm operation. We now turn
our attention to the Cotton-Mouton effect which arises from plasma
birefringence \[18-20\]. For an elliptically polarized beam, the
Cotton-Mouton effect causes the ellipticity of the polarization ellipse
to change, while the Faraday rotation effect causes the ellipse to
rotate with constant ellipticity. If the Cotton-Mouton effect is large,
this can considerably complicate the optical layout needed to acquire
the desired Faraday rotation measurements \[20\]. We note that one of
these techniques, which involves the simultaneous phase measurements of
both Faraday and Cotton-Mouton effects using polarization modulation,
has been investigated numerically \[21\] and tested experimentally at
119 µm \[22\] for possible future use in the ITER poloidal
interferometer/polarimeter system.

\(a\) ![](media/image8.jpeg){width="3.7465277777777777in"
height="2.6118055555555557in"}\
(b)![](media/image9.jpeg){width="3.85in" height="3.0458333333333334in"}

Fig. 6. (a) Refraction effects, and (b) interferometer phase shift, as a
function of wavelength and beam position for simulated ITER plasmas.

Plotted in Fig. 7 are simulated Faraday rotation and Cotton-Mouton
effect predictions for the same conditions as assumed in Fig. 6. The
Cotton-Mouton effect, which is extremely small at \~10 µm, yields a
shift as large as 7º at 57.2 µm. While not insignificant, careful
optical design can limit the amount of Faraday rotation measurement
inaccuracy that this relatively low change in ellipticity produces.
Consequently, there should be no need for a complicated optical layout
and detection scheme to measure the Cotton-Mouton at this level,
particularly when compared to the large Faraday rotation of \~1600º
predicted on this channel for the same plasma parameters.

At 10.6 µm, the role of the Faraday rotation measurement is to improve
the reliability of the system, particularly during long pulse operation,
since the phase rotation is less than one fringe and does not require a
fringe counter based system to keep track of multiple fringe shifts. In
the case of a 57.2 µm/47.6 µm operation, the single fringe condition
(corresponding to 180º Faraday rotation, as most phase detection schemes
detect twice the Faraday rotation) is easily exceeded on all channels.
Nonetheless, an unambiguous determination of the absolute Faraday
rotation level is still possible without having to track multiple fringe
shifts by using Faraday rotation data from both wavelengths. Faraday
rotation scales as λ^2^, which at these wavelengths translates to the
57.2 µm signal undergoing 1.44 times the rotation experienced at 47.6
µm. Another way of putting this is that the 57.2 µm signal undergoes 13
fringes of rotation in the time that the 47.6 µm undergoes 9 fringes
(13/9 = 1.44). Recording the relative rotation experienced at both 57.2
µm and 47.6 µm allows the absolute rotation to be determined within a 13
fringe (at 57.2 µm) window. This translates to a maximum allowable
rotation angle (for unambiguous determination without tracking) of
2340º, which is less than the ITER simulations in Fig. 7 predict. Hence,
robustness is still ensured. The relatively low amount of Faraday
rotation expected at \~10 µm (see Fig. 7) led in turn to the requirement
of a Faraday rotation resolution of 0.1º to keep the phase detection
error less than 1% \[3\]. The Faraday rotation levels at \~50 µm are
increased by a factor of \>20 times; hence, a minimum Faraday rotation
resolution of 2.0º would be appropriate here and relatively easily
achieved.

(a)![](media/image10.jpeg){width="3.91875in"
height="2.5409722222222224in"}\
(b)![](media/image11.jpeg){width="3.8152777777777778in"
height="2.682638888888889in"}

Fig. 7. (a) Faraday rotation, and (b) Cotton-Mouton effect, as a
function of wavelength and beam position for simulated ITER plasmas.

The next question that arises is whether or not there are any technology
issues that could hamper successful operation on ITER at these
wavelengths. The issues here are laser modulation, detection, beam
splitters, and reliability.

Heterodyne detection techniques require that there be a difference
frequency between the probing beam and the reference beam which
interfere on the detector. Techniques to generate this difference
frequency are: (1) acousto-optic (Bragg cell) modulation, (2) Doppler
shift by reflecting radiation off a rotating cylindrical grating or
"Veron" wheel, and (3) beating the output of two lasers at slightly
offset frequencies.

Acousto-optic modulators (AOMs) are commercially available at
wavelengths up to 12 µm. The diffraction efficiency depends strongly on
optical wavelength (scales as λ^--2^), however, making it difficult to
apply to the FIR regime with high efficiency. A study made in the 1980s
identified a number of liquids with potential for long wavelength FIR
acousto-optic modulators \[23\]; however, this work did not progess
beyond a very preliminary study. Rotating gratings have been
successfully applied at wavelengths of 119 µm and above, with frequency
shifts as high as 100 kHz \[22\]. Although more difficult and more
expensive to fabricate at \~50 µm, this is certainly a feasible option
although one whose low bandwidth will limit the temporal response.
Although a fast time response is not a requirement of the ITER TIP
system for density profile determination, a high IF frequency permits
the system to follow rapid density changes without loss of signals. In
addition, recent experiments with the FIReTIP system on NSTX have shown
the usefulness of a high bandwidth TIP system for the study of edge
density fluctuations. One option here for high bandwidth is the
so-called super rotating grating, in which a rotating grating is
fabricated on the top of the rotor of a turbomolecular pump with a high
rotation speed of 43,200 rpm \[24\]. This has been employed on CHS to
achieve a 1 MHz frequency shift at 337 µm with a frequency stability
Δ*f*/*f* = 4×10^--4^ \[25\], althouth the robustness and reliability of
the super rotating grating concept for use on ITER needs to be examined
in more detail.

Optically-pumped FIR lasers have a natural linewidth of a few MHz. A
three laser implementation, such as on the FIReTIP system on NSTX, is
another option for simultaneous interferometry/polarimetry operation.
The natural linewidth of the CH~3~OD laser lines is sufficient to obtain
a 2-3 MHz frequency shift for polarimetry, while the Stark effect
technique successfully employed on NSTX \[8\] can be used to provide a
5-10 MHz frequency shift for interferometry. It should be noted here
that a Stark-effect linewidth broadening technique has not yet been
attempted on a CH~3~OD laser, although the symmetry of the CH~3~OD
molecule should ensure its success similar to that achieved on NSTX with
a CH~3~OH laser at 119 µm \[26\].

Another area of concern for \~50 µm operation is identification of
suitable materials for use as vacuum windows and beam splitters. Here,
Chubu University and NIFS in Japan have been at the forefront of this
research area \[27\]. CVD-diamond and silicon are both shown to be
suitable window materials with high transmittance (\>80%) at \~50 µm.
Silicon etalons and wire meshes (similar to that employed in the FIReTIP
system on NSTX \[8\]) have both been studied for use as beam splitters.
Silicon etalons have shown promise as beam splitters; however, they
require a tight tolerance on etalon thickness (Δt \< 5%). The wire mesh
beam splitters employing simple cross patterns appear to be relatively
easy to fabricate, but it has proven difficult to obtain the same
transmittance/reflectance ratio at 57.2 µm and at 47.6 µm.

Detector technology choices at \~50 µm are (a) Schottky barrier diode
(SBD) corner cube mixers which have been successfully used at NIFS
\[28,29\], (b) Ge:Ga photodetectors with a noise equivalent power
(N.E.P.) \< 8×10^--13^ W Hz^--1/2^ at 1 kHz which have also been
successfully used at NIFS \[28\], and (c) InSb detectors with an N.E.P.
\< 7.5×10^--13^ W Hz^--1/2^ at 1 kHz \[29\]. Ge:Ga photodetectors have
been experimented with at NIFS where they have been used with good
results in conjunction with their 57.2 µm /47.6 µm lasers. Of these,
only the SBD mixers do not require cryogenic cooling and are thus an
attractive option for ITER.

Continued advances in SBD technology have taken place in the past
decade, not yet reflected in the NIFS studies. Whisker-contacted
Schottky diodes with 16 THz cutoff frequencies are commercially
available from NMRC, Ireland \[30\]. Because whisker-contacted diodes
are very sensitive to mechanical influences, and the contacting
procedure is very difficult and time consuming, much of the research in
this field involves the development of planar devices suitable for
monolithic integration. Planar Schottky diodes have been developed at
Virginia Diodes in the U.S.A. with cutoff frequencies of up to 7 THz
\[31\], and by the *Institut für Hochfrequenztechnik* in Germany with
cutoff frequencies up to 9 THz \[32\]. A receiver noise temperature of
8,250 K has been reported at 2.5 THz using GaAs Schottky monolithic
membrane diodes developed at the Jet Propulsion Laboratory (JPL), with
an estimated mixer noise temperature of 3,500 K \[33\].

To summarize, newly reported studies into environmental effects on
retroreflectors and in-vessel mirrors reveal serious problems with short
wavelength operation. Of the short wavelength options previously
considered for the ITER TIP system, the 10.6 µm/5.3 µm option would
suffer the greatest from these problems and it is recommended to discard
it. The wavelength choices are thus between a 10.6 µm/9.27 µm or a 12.1
µm/9.0 µm system as proposed by Kondoh *et al*., and a 57.2 µm/47.6 µm
system being pursued by NIFS. The former has unresolved issues regarding
mirror and retroreflector damage and coating that do not yet have a
solution. The latter requires a significant increase in the size of the
retroreflectors and input/exit mirror, and some technological issues
require further development; however, it appears to be a viable
alternative.

## 2) BEAM PROPAGATION TO/FROM THE ITER PLASMA

One technology not discussed in either of the ITER TIP design papers is
the use of hollow glass waveguides (HGWs). This type of waveguide
typically consists of a reflective metallic layer and a
reflection-enhancing dielectric layer deposited on the inside of a small
bore silica or glass tubing. These waveguides have been developed to
transport high power CO~2~ (10.6 µm) radiation with low loss. Early
papers in this field employed constant-bore guides, with reported losses
of 0.17 dB/m for 1000-µm-bore guides and 0.46 dB/m for 530-µm-bore
guides propagating the fundamental HE~11~ mode \[34\]. The loss
increases to 0.93 and 1.36 dB/m, respectively, when guides are bent to a
radius of 0.25 m.

The use of tapered-bore hollow glass waveguides is now under
investigation. These can be divided into two types: short and long
tapers. Short tapers are tapered at a steep angle 1° over a short
distance, typically \<10 cm, and are often used in conjunction with
nontapered HGWs at the input end to reduce coupling losses. Long,
gradually tapered HGWs are tapered at a very small angle (typically
0.1°) and are intended to be used in place of traditional, nontapered
guides. The large bore at the input end improves coupling, and the small
bore at the output end allows for the enhanced flexibility. Reported
losses are higher than in nontapered HGWs, but may have applicability to
ITER in that the attenuation constants are less sensitive to bending
\[35\].

HGWs could be employed in ITER TIP for either short or long wavelength
implementations. In addition to the obvious advantage of reducing the
beam penetration area in the BSMs, the use of bent HGWs can reduce
neutron streaming while reducing the number of reflective optics
required to bring signals to/from the ITER plasma. The use of HGWs may
also circumvent much of the mechanical displacement that would otherwise
have to be compensated for.

## 3) ALIGNMENT METHOD

A real-time alignment system is required in both short and long
wavelength implementations. The choices here are between a visible light
system using a CCD camera as proposed by Kondoh *et al*. \[9\] or an
IR/FIR system that uses one of the two IF/FIR beams themselves for
alignment. The latter has been chosen by Donne *et al*. for the poloidal
system, who propose "that as soon as the reflected signal for an
individual chord becomes below a certain detection threshold, the chord
is automatically switched from measuring mode to a scanning mode"
\[15\]. In the poloidal system design, one of the turning mirrors is
tilted in two directions by ±0.32º, making it possible to scan the beam
over an area of 58×58 mm^2^ at the retroreflector position. This
technique, applied to the toroidal system, is highly suited to real-time
operation without operator intervention and is the preferred option
independent of the choice of operating wavelength. In addition, the
self-scanning option has the benefit of being able to directly handle
low levels of beam refraction that the ITER simulations predict at \~50
µm.

Previously discussed concerns regarding mirror erosion and redeposition
on the input/exit mirror are also valid at visible wavelengths. Mirror
reflectivity data collected on Tore Supra and TEXTOR, detailed in Ref.
15, are reproduced in Fig. 8. It is shown that reflectivity changes are
most pronounced at the shorter wavelengths, with almost no change
observed in the IR region. This would support the use of an IR- or
FIR-based system rather than a visible light system for alignment.

![](media/image12.png){width="3.3875in"
height="2.347916666666667in"}![](media/image13.png){width="2.9875in"
height="2.3430555555555554in"}

Fig. 8. Plots of (left) mirror sample reflectance before (dotted lines)
and after (solid lines) exposure inside Tore Supra, and (right)
molybdenum mirrors reflectance after exposure to 312 sec of TEXTOR
plasmas in a deposition dominated zone (from \[15\]).

One uncertainty is the role that HGWs may plan in reducing the level of
mechanical displacement that would have to be compensated for. This is
an area for future study, as this could significantly impact the
required scanning area of the real-time alignment system.

## 4) DISPERSION INTERFEROMETER CONCEPT

The dispersion interferometer (DI) concept \[36-38\] is a two wavelength
interferometer scheme in which nonlinear optics are used to frequency
double an input laser beam, which then co-propagates with the original
un-doubled laser beam. Its primary advantage is low sensitivity to
mechanical vibrations. Originally implemented using a Nd:YAG laser
(λ=1.06 µm) \[36,37\], the concept has more recently been implemented on
TEXTOR using a CO~2~ laser (λ=9.57 µm) with good results \[38\].

A CO~2~ laser-based DI system for ITER, i.e. a λ=9.6 µm/4.8 µm or a
λ=10.6 µm/5.3 µm configuration, would suffer from the same mirror and
CCR problems that the Carlstrom *et al*. design had, and should be
rejected for the same reasons. Although the technique could, in
principle, be applied to a longer wavelength laser, the doubling
efficiency decreases rapidly with increasing wavelength, adversely
affecting signal-to-noise ratios. At wavelengths longer than \~10 µm,
doublers are not efficient enough for practical application.

# D. RECOMMENDED RESEARCH AND DEVELOPMENT TASKS

The list of recommended research and development tasks to be undertaken
depend strongly on the choice of operating wavelength, and are therefore
broken down accordingly.

## 1) SHORT WAVELENGTH (\~10 µm) TASKS

### OPTICAL DESIGN

As discussed earlier, one of the greatest concerns to the short
wavelength design is the polarization effect of carbon film buildup on
the input/exit mirror. The TIP optical design should be examined
carefully to estimate the polarization changes expected for each of the
five probing beams as a function of carbon film deposition depth.
Alternatives to the shared, single input/exit mirror configuration
should be explored to minimize these polarization changes.

## 2) LONG WAVELENGTH (\~50 µm) TASKS

### CH~3~OD LASER DEVELOPMENT

There needs to be a successful demonstration of a Stark-tuned CH~3~OD
laser to ensure sufficient bandwidth and stability can be achieved at
\~50 µm. This could be done by constructing a modified version of a
CO~2~-pumped FIR laser (employing CH~3~OD rather than CH~3~OH) for
57.2 µm operation, for comprehensive laboratory testing.

## 3) WAVELENGTH INDEPENDENT TASKS

### INVESTIGATION OF WAVEGUIDE INTERFACE SYSTEM

A study should be made of the selective use of HGWs in the ITER TIP
system. Possibilities here include running long lengths or constant-bore
or long tapered HGWs throughout the ITER blanket shield modules, or
solely using short tapered HGWs in regions such as interfacing joints
where alignment can be critical. One issue which would need to be
investigated is the amount of polarization and phase changes due to
variations in bending curvature due to machine motions.

### SELF-SCANNING ALIGNMENT SYSTEM

A test of the self-scanning laser alignment concept should be
undertaken. This could be undertaken at any laser wavelength desired, as
the concept is in itself not wavelength dependent.

### FIRST MIRROR DESIGN

Continued research into plasma first mirror and retro-reflector design
are essential, as well as research into mirror mitigation/coping
methods. Examples of the latter include baffles, shutters, the use of
in-situ laser cleaning or laser ablation, and heating of the vacuum
vessel (via, for example the use of RF cleaning discharges). These
mirrors must also maintain a good optical quality in the presence of
plasma radiation, neutral particle bombardment, and nuclear heating.

### IN-SITU CALIBRATION

A plan for in-situ calibration and alignment maintenance/verification
should be developed.

# E. PHYSICS ISSUES WHICH COULD BE ADDRESSED

There have been many physics studies conducted using FIR
interferometer/polarimeter systems, and many more proposed for future
study. The wide range of MHD instabilities that can be monitored using
the ITER TIP system range from relatively low frequency energetic
particle modes (EPMs) including alpha particles and "fishbones" (a type
of EPM), through intermediate frequency toroidal Alfvén eigenmodes
(TAEs) and resonant TAEs, to high frequency compressional Alfvén and
global Alfvén eigenmodes. Note that these studies require a considerably
faster temporal response than that required for ITER density control (\~
1 msec), but such system bandwidths are easily achieved in both short
and long wavelength configurations. The TIP electronics should therefore
be designed with a wide bandwidth, to the extent that this is possible
without sacrificing stability and robustness of the line integral
density measurements.

A Fizeau interferometer scheme to measure the toroidal plasma current
has been proposed for ITER, based on work being done on the Madison
symmetric torus (MST) device \[39\]. The Fizeau effect provides a
measure of the electron current along the line of sight of the beam, and
involves measuring the relativistic phase shift of an electromagnetic
wave associated with movement of a dielectric medium. This work is
interesting, but any changes to the detection geometry that detracts
from the basic robustness of the TIP system should be avoided.

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