**Final Report for SOW-536-20060721-JOHNSON-1**

**"Review of the ITER Residual Gas Analysis (RGA) System"**

**Submitted by**

**Walter L. Gardner**

**Fusion Energy Division**

**Oak Ridge National Laboratory**

**March 2007**

**Introduction**

One of the diagnostics tasks for which the US is responsible is residual
gas analysis for determining gas composition. Gas composition
(especially in the divertor and pumping duct regions) is considered to
be an essential measurement parameter as put forth in Table 4.22-1
\[group 1a\] of the ITER Project Integration Document (PID) \[ref 1\].
In addition the PID sets forth measurement requirements in Table 4.22-2.
A subset of these for gas composition is shown in Table 1 below.

**Table 1**

![](media/image1.pct)

To better reflect the column headings the data in the columns was
rearranged and some information added as shown in Table 2.

**Table 2**

![](media/image1.pct)

The remainder of discussion of the Residual Gas Analyzers (RGA's) in the
PID as well as in the Design Description Document for Plasma Facing
Components and Operational Diagnostics \[ref 2\] appears to refer to
standard quadrupole mass spectrometer-based RGA's. The drawback to these
is that one cannot resolve He from D~2~ unless a specially designed
quadrupole with limited mass range is used. Furthermore, resolution of
the hydrogenic (H, D, T) mass peaks is poor. To offset these drawbacks
it has been recommended that a Penning Gauge Diagnostic \[ref 3\] be
incorporated as part of the RGA system. Such a diagnostic cannot be used
alone because it can only respond to species with optical emission
lines. Hence, one should assume that the quadrupole mass spectrometer
(QMS) and Penning Gauge Diagnostic (PGD) are complementary sensors.

Based on manufacturers' specifications for QMS's and operational
experience with PGD's Table 3 shows the expected measurement
capabilities for each sensor type.

**Table 3**

![](media/image1.pct)

**Impact of ITER Environment on Design**

[Pressure]{.underline}

The operating pressures to be sampled in the pumping ducts by the RGAs
are expected to range from a few Pa up to 20 Pa. This pressure range is
greater than can be tolerated by QMS (few x 10^-2^ Pa) or PGD (few x
10^-1^ Pa) sensors. Hence, the RGAs must be differentially pumped. A
preliminary design exists for a Type 2 Diagnostic Vacuum Pumping System
\[ref 4\]. Since that design, the output of the cryopump was changed to
exhaust into the torus service vacuum ring rather than the main cryopump
foreline \[ref 5\]. A further recommended change to the design is to add
a second cryopump. The problem with having only one cryopump of the size
being considered \[ref 6\] is that it would reach its hydrogen capacity
in ≈9 hrs \[pump capacity (\~2000 Pa-m^3^) ÷ {pressure (\~0.01 Pa) \*
pumping speed (6 m^3^/s)\*3600 s/hr}\]. Fast regeneration takes \~1 hr,
and full regeneration takes \~2.2 hrs. As stated in the PID, ITER will
have a run schedule of two 8-hour shifts. Hence, to have the diagnostic
RGA available for a full run day two cryopumps are required. To save
cost and weight, two smaller capacity cryopumps such as the
CTI-Cryogenics Model 8 should be considered. A schematic is shown in
Fig. 1.

[Radiation]{.underline}

Very high radiation fields can degrade ceramic insulators over time,
which would affect both QMS and PGD sensors.

High radiation fields can degrade the electrical components of
commercial QMS femtoamp high-gain amplifiers, which must be located near
to the Faraday cup sensor. It is possible in moderate radiation fields
to use vacuum tubes and other radiation resistant components \[ref 7\].
However, the current recommendation is to locate the diagnostic RGA's
outside the bioshield, so that the QMS should be able to tolerate the
relatively low radiation field found there.

Tritium beta decay will cause a background shift in the QMS secondary
electron multiplier (SEM). This shift will increase over time as tritium
ions become buried in the target material until the SEM is no longer
usable. Currently, the thought is to operate the system at a high enough
pressure that the Faraday cup detector is the only one needed. If
important the SEM could be brought online for limited operation.

The fiber optics that transmit the optical signal from a PGD are
susceptible to the formation of color centers in a high radiation field,
which degrades transmission. This can be ameliorated by continuously
annealing the fiber at 250˚C \[ref 3\]. However, in high neutron flux
areas annealing will not be fast enough to work. This is not a concern
for the standard RGA's, but would be a concern for the "advanced" RGA
concept described later.

[Magnetic Field]{.underline}

There is theoretical evidence that the sensitivity of QMS-based RGA's
begins to degrade significantly in magnetic fields greater than 0.05 T
parallel to the mass filter axis \[ref 8\]. Instrument resolution
becomes better with increasing field. Experimental results in this paper
indicate a qualitative decrease in sensitivity and increase in
resolution with applied magnetic field. For transverse fields \[ref 9\]
an increase in resolution is observed for a field magnitude of 0.017 T,
which is dependent on angular position (x-y direction). Enhanced
sensitivity with applied field is attributed to flux leakage into the
ionizer.

There appear to be no studies on the effects of magnetic fields on the
Penning gauge diagnostic.

**Locations**

Specific locations for the RGA systems on ITER are being investigated.
The PID has identified an RGA system sampling each pumping duct (four
places). Other areas around the machine may be specified at a later date
depending on perceived need and cost.

The proposed location for the RGA systems is currently just behind one
of the main vessel cryopumps is shown in Fig. 2a. A pipe runs forward
from this location to sample the pumping duct. This has the advantages
of being behind the bioshield, and being in a lower magnetic field (≈0.1
T) that can be more easily fitted with magnetic shielding. From this
position a pipe would run up to the pumping duct as shown in Fig. 2b.
Though the diameter of the pipe shown is 30 cm (DN300) this was recently
reduced by the ITER-IT to 20 cm (DN200). The length shown is \~5 m.
Total length to the RGA, while every attempt will be made to minimize
it, could be between 6-7 m. Time response as a function of length for
various masses is shown in Fig. 3. Note that at six meters, masses less
than 40 AMU have a time response less than 1 s, falling to masses less
than 20 AMU at seven meters. It is not clear what drove the
specification on time response. This should be revisited by the ITER-IT
at a later time when the design and configuration are more mature.

An aperture or apertured valve located at the pumping duct of the pipe
has not been factored into the layout at this point (see Design Concept
section). It is highly recommended that the aperture or apertured valve,
and pipe be considered as the "front end" for this diagnostic, and given
sufficient design priority to be incorporated into the basic machine
design early on. In addition, sufficient space behind the main cryopump
(Fig. 2a) should be reserved for the diagnostic RGA system.

**Design Concept**

The design configuration currently being considered is shown
schematically in Fig. 4. It is based on integrating the RGA sensors
directly into the Type 2 Diagnostic Vacuum Pumping System (T2DVPS). In
addition to the QMS and PGD sensors, a calibrated pressure gauge, a
calibration gas system, an aperture or apertured valve to provide
differential pumping, secondary containment, and the T2DVPS are shown.

RGA's require periodic calibration. Once installed it is impractical to
remove these systems to a central calibration facility. In-situ
calibration to known gas mixtures and pressures is the preferred method.
The calibration gas system is comprised of cylinders of pure and
pre-mixed gases, regulators, a mixing volume with calibrated pressure
gauge, a variable leak, and an isolation valve.

An apertured valve is recommended over a fixed aperture so that the RGA
system is available to monitor vessel vacuum conditions where partial
pressures may fall below ≈10^-7^ Pa. **This decision rests with the
ITER-IT.** The aperture or apertured valve is not placed at the RGA end
of the pipe because its conductance is low enough that the pressure over
the length of the pipe from the duct to aperture would be nearly
constant. Gas exchange would then be governed by diffusion. For
deuterium in the pressure range of 5-20 Pa the diffusion time response
(5-m length, D~2~) ranges from \>10 to \>60 seconds.

A second configuration was briefly considered. In this configuration the
pipe from the pumping duct would feed directly into the cryopumps, and
the differentially pumped RGA section would be teed off the pipe near
the cryopumps. If one could sustain the gas throughput, the time
response would be excellent and there would be no need for an apertured
valve at the pumping duct. However, the flow (of order 60 Pa-m^3^/s
hydrogen) given the rated pumping speed and pressure near 20 Pa would
quickly overload the cryopump thermally in a few seconds. Even if the
pump could handle the thermal load the hydrogen capacity would also be
reached in a matter of seconds. In order to reduce the flow to the point
where the pump reaches its capacity in \~1hr (while the other pump
regenerates) a ≈0.3-mm diameter aperture would be required. This
aperture is only slightly larger than the aperture for the first
configuration (≈0.1 mm). Hence, the pressure on the upstream side is of
the same order as that at the duct, meaning that the diffusion time
responses are applicable, and thus this configuration was dropped.

**Development Needs**

There are three areas where development is needed in order for the
diagnostic RGA design to move forward. The information will also be
needed to help generate better cost estimates as the design moves closer
to the preliminary and final design phases.

[Magnetic field measurements]{.underline}

The first development area has to do with the impact of magnetic field
strength and direction on the RGA signal output, particularly the
quadrupole mass analyzer portion of the RGA system. As pointed out
previously, resolution generally improves with magnetic field. However,
sensitivity can either decrease (as one would normally expect), or
increase (as was actually observed due to field effects on the ionizer)
depending on field direction. The effect on sensitivity is also mass
dependent. At the level of field studied in reference 9 the change in
sensitivity was much larger than the accuracy requirement shown in Table
3, and this was only for one mass (argon). The peak height change in
reference 8, though in arbitrary units, appears to give a similar
result.

In order to determine the minimum magnetic shielding requirements for
the QMS, the minimum tolerable field and orientation need to be
determined. \[Note: The quadrupole electric fields are not axisymmetric,
hence, orientation with respect to the transverse magnetic field is
important.\] This will allow determination of the best orientation of
the quadrupole sensor, and the correct amount of shielding, given a good
estimate of the maximum external field and its direction. While the PGD
sensor head should be much easier to shield, because of its smaller
size, as well as having its own permanent magnet field that will
ameliorate the effects of an external field, measurements of minimum
field and orientation are recommended to verify the level of shielding
needed.

[Calibration system conceptual design development]{.underline}

The second area needing development is the calibration gas system. As
discussed previously a calibration gas system is needed, and a
high-level hardware representation was shown. \[Note: a calibration
system was not considered in the original RGA package.\] However, this
representation is based solely on what has been used for PGD systems on
JET and Tore Supra. A calibration gas system suitable for calibrating a
QMS will undoubtedly be different though qualitatively similar. For
instance, it will be desirable to calibrate the QMS with hydrocarbon
(C~x~H~y~) and deuterated hydrocarbon (C~x~D~y~) gases. Contamination of
either sensor type when using such gases will need to be researched to
see if issues exist. Furthermore, whatever the final design of the
calibration gas system it will be interfacing with a tritium handling
system, i.e., the RGA. Thought must be given to how the interface will
be made, which components need to be tritium compatible, and which parts
of the system may see tritium under worst case scenarios, and, hence,
need secondary containment.

[Tritium containment, compatibility and operational concept
development]{.underline}

It is recognized that the diagnostic RGA system will be exposed to
tritium and, as such, will need secondary containment. However, the way
in which this containment will be implemented has not yet been
considered. Furthermore, all of the interfaces will have to be
considered. Such interfaces would include, but not be limited to,
electrical, He supply and return to the cryopumps, RGA input from the
pumping duct, cryopump exhaust, and He vent.

Another thing to recognize in addition to containment is the tritium
compatibility of components. As yet attention has not been paid to
whether or not commercial parts exist in a tritium compatible form for
all RGA related components.

Finally, the operational aspects of the RGA system have not been worked
out. This is important to consider for tritium operations, not only
because of the obvious safety and health implications, but also to make
sure that all components needed for all operational scenarios (cold
startup, standby, normal operations, calibration, troubleshooting, and
shut down \[including emergency\]) have been identified.

**Development needs outside the ITER project**

While gathering information to develop the design specifications and
concept for the DRGA's it was discovered that there are only two RGA
systems currently operating on tokamaks that measure during a discharge
(Tore Supra and JET). These systems have significant issues. The JET RGA
does not work in all types of discharges, and is only capable (it is
unclear why) of free running mass scanning. The Tore Supra RGA has
difficulty with RF noise when operating during RF heated discharges. The
bottom line is that an RGA is not a mature diagnostic for tokamaks. More
emphasis needs to be placed on getting an RGA system working reliably on
a present day tokamak to study the problems that the ITER DRGA systems
may face. To this end collaborative efforts will be pursued to gain
experience and solve issues prior to delivery of the ITER DRGA systems.

**An "advanced" RGA for in-divertor operation**

Sampling the divertor gas composition directly through a long pipe
(≈13-15 m) with the required time response of 1 s is unfeasible for a
reasonable pipe diameter as shown in Fig.5. Ideally one would like to
have a device located in the divertor that would locally measure the gas
composition at pressures up to 20 Pa, in magnetic fields of several
Tesla, and in high radiation fields and neutron doses. There has also
been recent interest by the pellet fueling team in knowing the T:(D+T)
ratio within ≤100 ms response times to control firing of T pellets vs. D
pellets.

It has been determined that an RF glow discharge-based device could
provide such a direct divertor gas composition measurement on the time
scales needed. This [RF]{.underline} [D]{.underline}ischarge-based
[O]{.underline}ptical [G]{.underline}as [A]{.underline}nalyzer (RFD-OGA)
uses a capacitive RF discharge to generate optical emission from various
gas species of interest to ITER (H~α~, D~α~, T~α~, He, \[and perhaps\]
C~2~, CD and CT). One might think of this system as having the same
function as the Penning gauge diagnostic \[ref 3\], yet able to operate
at substantially higher pressures (20 Pa), operate in very high magnetic
fields \[ref 10\], and withstand significantly higher radiation doses.
The system would be comprised of a discharge chamber, a low power (few
tens of watts) 13.56 MHz generator, matching network, calibration gas
system, optical system (mirrors, lenses, fiber), spectrometer,
instrumentation and control electronics, and data acquisition system. A
schematic of the RFD-OGA system is shown in Fig. 6. A 3D rendition of
what the discharge chamber might look like is shown in Fig. 7.

It is proposed that the discharge chambers of the RFD-OGA's be located
at several toroidal locations under the dome in diagnostic divertor
cassettes. Figure 8 shows a potential location. The optical output from
the discharge would be relayed by mirrors back to the bioshield where
collecting optics focus the light into a fiber optic that then feeds a
high resolution spectrometer. The discharge is driven by an RF source
and matching network located outside the bioshield. Connection to the
discharge chamber is by an ITER approved coaxial cable with a diameter
of perhaps twice that used for cable TV.

The calibration gas system, as for the standard RGA's, is comprised
primarily of bottles of pure and premixed gases, a suitable mixing
volume, a calibrated pressure sensor, an isolation valve, and variable
leak. The system will be located outside the bioshield and feed the
discharge chamber through a long length of stainless steel tubing.
Because of the long length of tubing needed and the fact that flow rates
are mass dependent, the remotely operated isolation valve and variable
leak would have to be located near the discharge chamber to keep gas
ratios constant. An isolation valve will be used in conjunction with the
leak valve. The gases (D~2~, He, Ne/N~2~) are assumed to be ejected into
the torus vessel and pumped by the main cryopumps. Because there is not
a suitable window material for in-vessel applications, the discharge
chamber is open to the vessel to allow optical transmission. While this
opening is suitable for letting gas into and out of the discharge
chamber from the divertor region, it is not suitable for maintaining
sufficiently high pressure in the chamber during calibration with
reasonable gas throughput. Hence, a conductance limiting straight tube
is attached to the chamber output.

The discharge chamber would be fabricated of ITER approved metal and
ceramic components, and, as such, should be highly radiation resistant.
The maximum voltage applied across the ceramic insulators should be of
order 100V prior to discharge initiation.

**Because the RFD-OGA concept is undeveloped and unproven, it is
considered new scope, and will not be considered as a part of the ITER
Diagnostic RGA package. If there were strong ITER interest, funding
support would be needed for development, design and implementation.**

**References**

1.  Project Integration Document, Version 3.0, IDM: ITER_D_2234RH, p.
    291, (January 2007)

2.  Design Description Document WBS 5.5.G, Plasma Facing Components and
    Operational Diagnostics, G 55 DDD 38 01-07-17 W 0.1. (July 2001).

3.  Hillis, D. L., et al., Rev. Sci. Instrum. 70(1), 359-362 (1999).

4.  Design Description Document WBS 3.1J, Diagnostic Vacuum Pumping
    Systems, G 31 DDD 10 98-05-27 W 0.2 (May 1998).

5.  Design Description Document WBS 3.1, Vacuum Pumping and Fuelling
    Systems, G 31 DDD 14 01-07-19 W 0.1, page 31 (July 2001)

6.  Design Description Document WBS 3.1J, Diagnostic Vacuum Pumping
    Systems, G 31 DDD 10 98-05-27 W 0.2, page 11 (May 1998). (apparently
    same specifications as for CTI-Cryogenics Model 10F).

7.  Pearce, R., et al., Vacuum 44(5-7), 643-645 (1993).

8.  Tunstall, J. J., et al., Vacuum 53, 211-213 (1999).

9.  Srigengan, B., et al., IEE Proc.-Sci. Meas. Technol., 147(6),
    274-278 (2000).

10. Kaneko, T., et al., Jpn. J. Appl. Phys., 44(4A), 1543--1548 (2005).

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