Appendix D2

**Justification for the Requirements set for the FIRE** **Measurement
Specifications** Kenneth Young (based on a document developed primarily
for ITER in cooperation with Alan Costley and George Vayakis, both from
ITER, Naka).

**Introduction**

This short document provides the reasons for the quality of the
measurements described in the Table of Measurement Specifications for
FIRE. Its purpose is to enable instrument makers and tokamak and
shielding constructors to understand the requirements in developing
their designs. However there are some clear over-simplifications in the
table, so that it is expected that the design effort will be closely
linked to experienced plasma diagnosticians and modelers in determining
the final capabilities of the instrumentation.

In the table, the minor radius of the first wall at the midplane, a, is
used to define spatial locations and resolutions because it is the
engineering value which can be referenced to the machine design.
However, the operational plasmas will vary in size and shape so that,
particularly for edge measurements, and for diagnostics viewing from
off-center-line locations, some additional thought should be given to
defining exactly the number and spacing of the sightlines. As an
example, for measurements near the divertor, the plasma measurements
should extend beyond r/a =1, mapped back to the midplane. For some
diagnostic techniques, e.g. neutron detectors, even spacing between
channels will not optimize the measurement.

Also the requirements are not aimed at the start-up phase for the
plasma, where it is expected to start off relatively small at an edge
limiter. It is thought that the \"full-size plasma\" measurement
specifications are sufficient, but this must be confirmed (e.g. the
number and location of the equilibrium coils for measuring the shape
should be checked for both full operation and start-up).

Note: FIRE is a double-null tokamak, with symmetric divertor
arrangements top and bottom. At this time, it has not been determined
whether measurements in the divertor need to be made in the top, bottom
or both. Thus the needs for divertor, and X-point, measurements are not
qualified by where they might be made.

**Justifications of the Measurement Requirements**

1\) [Gas Pressure and Composition in Main Chamber]{.underline}: The
quality of the vacuum and the constituent elements provides vital
operational information during conditioning and prior to the plasma
pulses. Because of the complexity of the components inside the vacuum
chamber, this measurement should be done at several locations close to
the first wall. Response time to the signals cannot be fast so the
pressure can be sampled at 100 ms intervals, and the elemental analysis
at 1 s. (Note: slower times are possible following maintenance and
between pulses; table presently calls for measurement during the pulse,
and the value of this measurement should be checked.) Absolute
accuracies of 20% and 50% are defined for the pressure and elemental
analysis respectively, though relative accuracy should be much better,
during the pulse. Better precision, over longer time intervals will be
required between pulses.

2\) [Gas Pressure and Gas Composition in Ducts]{.underline}: The
pressure and composition of the gas in the pumping ducts at the divertor
provide information on the performance of the divertor, including the
behavior of the gases injected into the divertor to cause radiation. 20%
accuracy is expected for both measurements, with 100 ms time resolution
needed for the pressure and 100 ms resolution for the elemental
analysis.

3\) [First Wall Visible Image and Wall Temperature]{.underline}: A TV
display of the plasma, and the first-wall in its light provides a
valuable operational tool for the operators and will be very valuable
for long pulse operation. In addition the wall temperature is expected
to rise because of the plasma radiation and also because of losses of
fast ions and alpha-particles to localized regions of the wall. The
temperature of the antennas must also be monitored. High temperatures
may be a reason to modify the plasma operation. The expected range of
temperatures is 200° - 1500°C corresponding to a heat load of order 10%
of the expected typical radiative load, and should be measured with an
accuracy of \~ 20°C. The spatial resolution must be capable of resolving
significant features of the wall - the smallest expected size of
off-normal hot spots is \~ 3 mm.

4\) [Toroidal Magnetic Field]{.underline}: The toroidal magnetic field
is a key element in the magnetic configuration. Its settings must be
extremely reproducible. Its value is used in support of other diagnostic
methods and analysis codes, and it is key to establishing the plasma
location. 0.1% measurement accuracy is required to ensure negligible (\<
5 mm) contribution to the overall reconstruction accuracy. Because of
its role in establishing the -limit, it must be sampled frequently.

5\) [Plasma Current]{.underline}: The plasma current, I~p~ , plays a key
role in providing the magnetic configuration, and in providing the
initial heating of the plasma. Its value is fed directly into plasma
control, but also it is an element in evaluation of other parameters
such as the -limit, also used in control. It must be measured with high
precision and with fast time resolution (1 ms). During the quench
following a disruption, a 0.1 ms time resolution is needed, but the
accuracy can be relaxed.

6\) [Plasma Position and Shape]{.underline}: The plasma position must be
controlled tightly as the current and pressure change. The magnetics
must be capable of maintaining the double-null configuration for the
length of the pulse. Rather tight tolerances are required for analyses
but also for the closeness of the scrape-off to the heating antennas and
to locate the separatrix within the divertor. These data will be used
with an equilibrium code, such as EFIT, for optimizing the plasma. 10ms
resolution will be sufficient for this control, and is expected to be
much faster than other elements in the control loop. Fast vertical
movement can be expected and fast resolution in following this movement,
with the possibility of controlling it, requires fast time resolution
(1ms).

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## 

## 7) [Loop Voltage]{.underline}: The loop voltage, together with I~p~, defines the inductive power input. The measurement can be important at start-up, and also during a disruption when it leads to the acceleration of runaway electrons. It has the same time resolution as I~p~ and it should be able to be measured with precision during the low-voltage 

(\<\< 1V) period of the current flat-top, when a large fraction of the
current drive will be non-inductive.

8\) [Halo Currents]{.underline}: Halo currents through first-wall
components could lead to damage from magnetic forces, as well as
providing significant engineering design information. Data from
currently operating devices show currents up to 0.2 I~p~, sometimes with
considerable variation toroidally. To confirm the peaking factor,
several measurements are required in the toroidal direction with good
(few %) relative accuracy . Absolute accuracy should be sufficient to
determine whether allowable stresses are being exceeded significantly
(\~ 20%). The halo-current specifications are very much dependent on the
first-wall configuration and the numbers should be reviewed when the
first-wall design is complete.

9\) [Error Fields, Locked Modes and RWM Identification]{.underline}:
Many low frequency aspects of magnetic fields affect the plasma
performance. Error fields, due to imperfections in the windings,
locking-modes (slowing-down resistive tearing modes) and resistive wall
modes (RWMs) are examples. Field correction coils should be provided for
the first. MHD modes, especially the (m,n) = (2,1) modes, can slow down
and lock with drastic impact on the plasma. Potentially this locking
could be controlled and prevented. Measurement of the modes\' time
behavior prior to locking should be made with high time resolution
(1ms), and high sensitivity (few %), but absolute accuracy on the field
level of 30% will be adequate. The need to actively stabilize RWMs, even
with copper walls providing significant stabilization, requires
measurement of the plasma fluctuations to feed to an array of powered
stabilizing coils. The precision requirements and the number of
measurements will await final tokamak design information.

10\) [Line-Averaged Electron Density]{.underline}: The relatively simple
measurement of line-averaged density is an indicator of performance for
the operators and has been used as an enabling signal for the operation
of additional heating. It should be measured with high reliability and
availability. Data should be taken on the same time scale as the
profile-measuring system, which will be fast compared to deliberate
changes of the plasma. If a \"killer\" pellet or some other form of
injection of material is employed to mitigate the effects of a
disruption, then the system should be capable of a faster response, but
with a reduced need for precision.

11\) [Radiated Power]{.underline}: The total power radiated from
different regions of the plasma will be spread over a wide spectral
range, but will be dominated by line radiation. Integral measurements
across the core plasma, across the x-point region and in the divertor
are required for regular operation and for assessing the power balance.
Where operation with a deliberately injected gas in the divertor to
protect the divertor plates is planned, high intensities can be expected
in this region. A total estimate of the power radiated at a disruption
is necessary for determining the fraction of the power lost by
radiation. The operational measurements should be made with 10% accuracy
and 10 ms time resolution, for calculations of power balance, while the
disruption power loss requires faster resolution at 3 ms, but not such
high accuracy (20%).

12\) [Z~eff~]{.underline} [(Line - averaged)]{.underline}: The average
impurity content of the plasma in the core is an important measurement,
particularly during conditioning stages of the device\'s operation. For
a burning plasma device, it can also provide clear indications of the
build-up of the helium-ash created by the thermalized alpha-particles.
Since this measurement is key to the conditioning process, it should
have relatively good time resolution (10 ms) for short discharges, but
does not need very high absolute accuracy (20%).

13\) [Impurity Species Monitoring]{.underline}: The plasma facing
materials in FIRE are beryllium-coated copper in the first wall, and
tungsten in the divertor. To effect a radiative divertor, and possibly
for diagnostic use, noble gases such as neon, argon and krypton will be
injected. Oxygen, nitrogen and carbon are standard impurity elements.
Global estimates of these elements must be obtained continuously for
physics reasons, but also for the first-wall protection (Cu spectral
lines would imply damage). Both the absolute quantity of the elemental
components as well as their rate of increase is required. A relative
accuracy of 10% is the minimum required to support such analyses in a
meaningful manner. At least 10 ms time resolution is required to follow
intentional shape changes.

14\) [Low (m,n) MHD Modes, Sawteeth, Disruption Precursors]{.underline}:
MHD has been measured traditionally on tokamaks by an array of magnetic
detectors near the first wall and by internal measurement of temperature
fluctuations by electron cyclotron emission (ECE) or x-ray emission.
These types of external and internal measurement are expected to
continue in FIRE. For the wall measurement, the coil spacing should
allow interpretation of mode numbers up to (m,n) = (10,2) with frequency
up to 10 kHz and 10% accuracy. The requirements are similar on the
internal measurement, but with the addition of spatial resolution of
a/30.

15\) [Neutron Flux and Emissivity]{.underline}: The total neutron flux,
over a very wide dynamic range of neutron production from low-power
deuterium discharges through to burning D-T plasmas, is required with
good accuracy (10%) for its role as a performance indicator and for
comparison to modeled results using other measurements. It is also
important for developing total fluence records and levels of activation.
Fast time resolution of the rise and decay of the flux is required
(1ms). The spatial distribution of the neutrons, and also the
alpha-particle birth (and heating) profile for D-T plasmas, is required
for a wide range of transport analyses. Spatial resolution of a/30 (the
same as for the other profile diagnostics) near the plasma center and
10% accuracy are the minimum required to support such analyses in a
meaningful manner. At the edge, because of the strong sensitivity of the
neutrons on the plasma temperatures, measurement with fewer spatial
points will only be possible. Detailed modeling for reference plasmas
should be used to define the spatial requirements. The fusion power, and
its spatial dependence, are derivable from the neutron data

16\) [Runaway Electrons]{.underline}: Runaway electrons may occur at two
times during the plasma operation. Firstly when the plasma breakdown is
bad; accelerating electrons to high energies can cause damage to
first-wall/divertor. The second high current of runaways can be created
in an avalanche after the thermal quench at a disruption. This serious
potential source of damage may have to be quenched by injection of a
dense source of neutral atoms. The electrons can reach 20 MeV in the
FIRE fields and build up rapidly, requiring 10 ms time resolution. The
measurements do not have to be very accurate.

17\) [Divertor Operational Parameters]{.underline}: The temperature of
the divertor surfaces close to the strike-points is going to provide an
important operational input for managing the core plasma and divertor
interaction. Rapid excursions of the surface temperature due to ELMs or
disruptions are likely (2 ms time resolution) and the full range up to
2500°C must be covered at an accuracy of 10%. The spatial resolution
should be 3 mm in the direction across the strike-point. Net erosion of
the divertor plate surface, with redeposition elsewhere due to
sputtering is expected, and the life of the divertor surface must be
monitored in real-time for the long high-power pulses. Depth
measurements up to \~1 mm with 0.2 mm accuracy in \~ 1 s at locations 1
cm apart across the strike points at a number of toroidal locations are
needed. If there is no real-time response to erosion, then the
measurement can be much slower between pulses. The gas pressure behind
the divertor plates defines the divertor performance and should be
measured at several toroidal and poloidal locations with about 20%
accuracy with time resolution of less than the typical fueling
time-scale, i.e. 50 ms. The gas composition should also be measured
behind the divertor plates with a slower time resolution, necessary for
analysis (1 s). The introduction of high-Z impurity gas in the divertor
will cause detachment of the plasma from the divertor plates. As the
visible ionization front moves up the leg of the separatrix, its
position must be measured to allow control of the gas flow. 20 mm
spatial separation between observation points is required with 1 ms time
resolution because of the expected rapid movement. The time resolution
must be compatible with fast control of the front whose time-scale is
limited by the gas puff mechanism. (The Divertor ITPA group are expected
to make recommendations for ITER shortly and these can be adapted for
FIRE.)

18\) [Dust Monitoring]{.underline}: Quantities of dust have built up at
the bottom of all tokamaks. In TFTR and JET, this dust was mostly
carbon, possibly from the break-up of codeposited layers, and entrained
significant amounts of tritium. While a tungsten divertor should not
provide the same type of problem as far as tritium inventory is
concerned, it is considered prudent to provide a technique for the
measurement of any dust build-up. The specifications will be provided
after ongoing trials on present-day devices, e.g. JET and NSTX, are more
mature.

19\) [In Vessel Inspection:]{.underline} Inspection of the first wall
and divertor, without breaking vacuum, on a regular basis following
unusual plasma events or after high - power operation permits evaluation
of possible damage. Operated in its highest resolution mode, 1 mm
resolution will identify cracks or damage significant with respect to Be
thickness as well as resolve fine detail at the FIRE 1st wall.

[Provision for Capability for Glow Discharge Cleaning and Vacuum Vessel
Illumination]{.underline}: It is anticipated that various probe
mechanisms will be necessary to support operation of the tokamak. These
will have to be integrated with the diagnostics onto the device,
incorporating shielding, etc., so they have been listed here so that
they can be operational for first plasma.

20\) [Plasma Energy]{.underline}: The plasma energy, ~p~, provides a
strong indication of the quality of the plasma performance. Since it is
likely that the tokamak will achieve its best performance close to the
-limit, it is essential that this parameter should be measured
accurately. Its time-scale should follow that of key plasma events and
its accuracy should be within 5%. For disruptions, during the quench of
I~p~, the accuracy does not need to be so high. The -limits are a
function of shape, pressure, rotation and current profiles and the
plasma-wall distance, so that all these parameters will have to be
measured with high accuracy to permit operation close to the limit. If
the measurement of is made diamagnetically, all pressure components due
to fast ions, alpha-particles will be included with the thermal
component; using kinetic parameters, it will be difficult to properly
account for the fast particles.

21\) [Electron Temperature Profile]{.underline}: The electron
temperature, with good spatial dependence, is a major indicator of
plasma performance and a key component of transport analyses. The
profile provides key information in instability analyses. Steep
transport barriers are observed inside the plasma core and electron
temperature pedestals at the edge play a role in analysis of the
transport. A time resolution of 10 ms is short compared to times of
interest and allows for study of MHD. For kinetic control of the stored
energy or internal transport barrier (ITB) gradient, 10 ms time
resolution is expected to be sufficient since this is much faster than
the typical core confinement times and of the same order as the actuator
(heating) response time. The core temperature should be measured with
10% accuracy or better to enable a useful determination of the stored
energy and to support useful analysis of plasma performance with
transport codes. To resolve fine structure on the profile such as ITBs
it should be measured at 30 locations across the profile. The edge, with
much steeper gradients should be measured with only 5 mm between
measurements at about 20 locations. The fast edge temperature
requirement is set for the observation of transients due to ELMs. For
FIRE, the definition of r/a = 0.8 as the core/edge boundary, where a
indicates the wall mid-plane radius, should be adequate since the plasma
boundaries are not expected to vary widely in their distance from the
wall. But the comment in the introduction relating to the use of \"a\"
as the reference location should be taken into account for all edge
measurements.

22\) [Electron Density Profile]{.underline}: The requirements on the
electron density measurement are set for the same reasons as those for
the electron temperature. The tight requirement on the accuracies for
the core and edge density at 5% is established because one of the
reference scenarios for FIRE is the H-mode with rather small density
gradients over a large fraction of the profile. The fast edge density
requirement is set for the observation of transients due to ELMs. The
maximum edge density of 2.10^20^ m^-3^ is somewhat arbitrary and is
meant as a guide for diagnostics, e.g. reflectometry, intended for
following edge behavior during RF heating.

23\) [Ion Temperature Profile]{.underline}: The requirements on the
measurement of the ion temperature profile with its impact on the
transport and plasma stability are very similar to those set for the
electron temperature. Because the dominant additional heating is ICRF, a
higher maximum temperature has been set. A 10% accuracy is needed
because of its significance in transport calculations. A spatial
resolution of a/30 is considered necessary to match the other profile
data being fed to transport codes and to provide good determination of
the pressure across transport barriers; the specification in the edge
matches those of the electron parameters. While the ions are largely
unaffected by MHD, a slower time resolution would be possible but for
calculating transport, the same 10 ms as chosen for the electrons is
used.

24\) [Current Density Profile (q(r))]{.underline}: Precise measurement
of the q(r) profile becomes particularly important in the optimizing of
the plasma performance using magnetic shear and in counteracting the
impact of neoclassical tearing mode (NTM) instabilities. For NTM
stabilization it will probably be sufficient to determine the location
of the q = 1.5 and q = 2 surfaces. Similarly for the control of reverse
shear discharges the location of the q~min~ is probably all that is
required. For such discharges, q \> 5 can be expected in the plasma
core. The full evaluation of the plasma performance requires a complete
measurement of the q(r) profile and this will require measurements of
the internal magnetic field, edge magnetic field, and the use of an
equilibrium code, such as EFIT. For low values of q(r) it should be
determined within 10% while at higher values, where the gradient is
normally steeper, better absolute accuracy, of +- 0.5 is required. 10 ms
time resolution will be sufficient to follow intentional changes to the
equilibrium and to determine the location of the NTMs. The latter\'s
location has to be determined to within an accuracy of 20 mm. For the
purpose of reverse shear control, where the current penetration times
are of order 1 sec, the localization of q~min~ can be obtained
relatively slowly.

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## 25) [Plasma Rotation]{.underline} [Profile]{.underline}: Plasma rotation and velocity shear have been seen to be very significant in affecting plasma stability and loss, and it may be possible to control the plasma motion with an external drive. These measurements provide information about the electric fields in the plasma. Hence the rotation in both the toroidal and poloidal directions should be measured with similar spatial resolution (a/30) as other plasma profile parameters, with a time resolution of 10ms. For comparison of data with theory, the relative accuracy should be high, but an absolute accuracy of 30% is sufficient.

## 

26\) [Z~eff~]{.underline} [Profile]{.underline}: The Z~eff~ profile
contributes to the transport analysis and to the understanding of
impurity localization. It will also potentially provide information on
the build-up of helium-ash in the plasma center. It is not expected to
change significantly with time and so 100ms provides sufficient time
resolution. It is also unlikely to have steep spatial gradients so
spatial resolution of a/10 is appropriate. Rapid transients, on the time
scale of 10s of milliseconds, may be possible so that some higher
temporal resolution with good relative accuracy, but possibly poorer
absolute accuracy is needed.

27\) [Impurity Density Profile]{.underline}: Measurement of the
fractional content of the main impurities is required. This profile
supports the Z~eff~ profile in defining the dominant impurity content
and its transport. The requirements have been split between the light
elements (Z≤10) and the heavy elements with Z\>10. The maximum
permissible level of the latter is much smaller. For both ranges, the
minimum requirements to support a useful analysis are a spatial
resolution of a/10 (10 mm in the edge), with 100 ms time resolution and
20% accuracy.

28\) [H-mode: ELMs and L-H Transition Indicator]{.underline}: The
transition from L-mode to an H-mode has been observed traditionally from
quiescence of D-light fluctuations. This integral measurement needs only
good relative accuracy, but must be sampled relatively fast (0.1 ms) to
catch the fluctuating signals. Edge localized modes (ELMs) appear in the
plasma edge of various sizes and amplitudes. These are normally
determined by integral measurement of the D -light fluctuations when
they occur. Measurement of the electron density and temperature
excursions in the ELMs is proposed (these may be higher-frequency
enhancements of the systems used for the edge measurements 21 and 22).
The transients can be very fast so that 10 s is proposed as the time
resolution. At the L-H transition an edge pedestal in electron density
and temperature is formed. Its measurement contributes to the spatial
resolutions set for the edge density and temperature (21 and 22).

29\) [Radial Electric Field]{.underline}: The radial electric field, and
associated rotation, has been found to provide strong stabilization of
MHD. It appears to play a key role in the plasma confinement for
advanced tokamak (AT) plasmas. It should therefore be measured at the
same resolutions as the other main profile parameters. The range of
fields to be expected has still to be determined. Potential fluctuations
measurement is highly desirable for correlation with density
fluctuations, but that is not currently anticipated.

30\) [High-frequency Instabilities (MHD,]{.underline}
[NTMs,]{.underline} [AEs):]{.underline} There is a large family of
potentially damaging instabilities present in plasmas with steep
gradients. In burning plasmas the instability can be enhanced by the
presence of fast-ions and alpha-particles. To be able to relate the
plasma behavior to theoretical predictions and to assess the impact of
these perturbations on the plasma or individual particle properties, it
is necessary to measure their properties, such as amplitude, frequency
and location. Neoclassical Tearing Modes (NTMs) and much higher
frequency members pf the family of Alfvén Eigenmodes (AE-modes) and
similar modes with n\>10, up to \~ 300 kHz, can be expected. Good
relative accuracy of measurement is required with both external and
internal localizing measurements. Density fluctuations of about 10^-5^
of the local density may be significant. Preferably, these should be
measured at both low-k and high-k for relating to electron transport and
validating turbulence models. NTMs, which are a particular hazard for
ATs, can be stabilized with electron heating or current drive once
observed. These modes need to be localized within about 50 mm radially
and the local q measurement will also be used in their identification.

31 [Edge Turbulence]{.underline}: Studies of edge turbulence in current
devices is a rapidly growing field with many different types of
turbulence being identified. While relatively high frequencies have been
observed and fast visible imaging has been applied, it is not yet clear
what the measurement most applicable to transport studies should be
defined for FIRE.

32\) [Radiation Profile]{.underline}: In the overall power balance of
the plasma it is necessary to determine the power being radiated away
from the different regions of the plasma. This may be quite non-uniform,
particularly near the X-point and in the divertor. In the presence of
deliberately introduced local radiation, as in operation of the
radiative type of divertor, it is particularly important to determine
the principal radiation sources. Hence radiation must be measured
throughout the main plasma, in the X-point region and in the divertor.
For the latter, there will be a strong variation along the separatrix
legs during formation of the radiative divertor, and this measurement
will be used to validate modeled behavior. This last measurement should
have 50 mm spatial resolution along both the inner and outer legs with
good relative accuracy (absolute accuracy 30%) with 10 ms time
resolution. The same time resolution applies to the core and X-point
measurements, but with a reduced spatial resolution of a/15, because of
the expected larger size of radiation features, and accuracy of 20%.
(Note that in FIRE the X-point is nearly on a horizontal plane with the
inner-divertor strike-point so that the spatial resolutions may be hard
to achieve and should be re-examined.)

33\) [Plasma Parameters at the Divertor Targets]{.underline}: The plasma
parameters close to the divertor plate surfaces have a huge dynamic
range depending on the operational mode of the divertor and of the core
plasma. A drop in density, or saturated current, will indicate plasma
detachment. This information is very important because of its role in
understanding the divertor operation and in relating to predictive codes
for the divertor performance. Measurements should be made along the
divertor surface across the strike-point with \~ 3 mm spatial resolution
to account for the expected narrow scrape-off region. Measurement is
required for both the inner and outer strike points. The electron
density and temperature, and the ion flux toward the divertor should be
measured with high time resolution (1 ms will enable effects of ELMs to
be seen) and with 30% accuracy over the full range.

34\) [Heat Loading Profile in Divertor]{.underline}: Full-power
operation of the device challenges the capability of the divertor
surface to withstand the heat loads. Hence radiative operation of the
divertor may be necessary. Also, on a rapid time-scale, the ELMs can
dump a lot of energy locally onto the divertor. Measurement will play a
key role in the operation of the divertor, including operational
controls. The surface temperature needs to be measured with high spatial
resolution (3 mm) and fast temporal resolution (10 ms is probably good
enough) and with good accuracy (10%) at least close to the strike-point.
The power load can probably be obtained from this measurement with about
the same accuracy. Under disruption conditions and for ELMs, the
measurement needs faster time resolution at 0.1 ms, but with lower
accuracy (20%).

35\) [Divertor Electron Density]{.underline}: Performance of the
divertor, and comparison of its principal parameters with code
projections, requires accurate measurement of the electron density and
temperature along, and across, the legs of the separatices in the
divertor. There will be a very dynamic environment with big changes in
both density and temperature in relatively short times. Hence the time
resolution has been chosen to be 1 ms with 20 mm. spatial resolution
along the legs and 3mm across the legs, at planes to be chosen, at 20%
accuracy.

36\) [Divertor Electron Temperature]{.underline}: See 35) above.

37\) [Ion Temperature in the Divertor]{.underline}: The ion temperatures
in the divertor also feed strongly into the performance and transport
determination. The measurement specifications are the same as for the
electrons in 35) above.

38\) [Divertor Plasma Flow]{.underline}: Another key parameter in
setting the divertor performance is the flow along the legs toward the
divertor plates. The measurement specifications on this flow are the
same as for 35) above. Note that the requirements for this measurement
are not yet well established.

39\) [Impurity and D,T Influx in Divertor]{.underline}: Measurements of
the fluxes of fuel ions and of some impurity ions into the divertor are
required in order to optimize and evaluate the divertor performance. It
is proposed to use noble gasses, injected through the divertor to detach
the plasma from the strike-points and provide radiation to distribute
the power. This measurement should also be able to resolve the impurity
influx rate from the target into the legs of the separatrix.

## 40

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## ) [Fuel Ratio in Plasma Core; n~H~/n~D~, n~T~/n~D~]{.underline}: The device will start up using pure hydrogen plasmas and will operate for some time in deuterium before moving over to burning plasma studies with D-T fuel. The fuelling will be done in different ways with pellets and gas-feed. ICRF heating will operate on H-minority, 2~D~ or ~T~ (and He^3^-minority) dependent on the field. Once in the burn phase, the fueling of the main body of the plasma must be controlled. The burn rate is not very sensitive to the ratio of the fuel ions for most of its range so that a measurement accuracy of 20% is sufficient and the plasma fueling time-scale is of order seconds so slow time integration of 100 ms is sufficient. A much faster capability for following the incoming fuelling pellets may be required. Note that during D-T operation hydrogen in the plasma core acts like an impurity which reduces the reacting fuel ions while not impacting Z~eff~, so the n~H~/n~D~ ratio must still be measured on the same 100 ms timescale and spatial resolution (a/10) as the fuel species with an accuracy of 20%, at least for the maximum expected ratio of 1/10.

41\) [Fuel Ratio in the Edge; n~H~/n~D~, n~T~/n~D~]{.underline}: FIRE
will operate with all three hydrogen isotopes as its fuel, hydrogen in
early phases and ultimately with a D-T mix. Fueling will be both by
gas-injection and pellets, with recycling from the first wall an
additional source. After the hydrogen operational period, hydrogen will
act like an impurity dilution of the fuel from recycling and other
sources, such as small vacuum leaks. This measurement will also be
valuable for the management of the ICRF heating. Both to understand the
fuel mix and to provide input for control, the hydrogen isotope ratios
should be measured in the edge with reasonable time resolution (100 ms)
and accuracy (20%) where better relative accuracy might be expected.

42\) [Fuel Ratio in the Divertor; n~H~/n~D~, n~T~/n~D~]{.underline}: To
understand the fuel mix and to provide input for control, the hydrogen
isotope ratios should be measured in the divertor, averaged over space,
with reasonable time resolution (100 ms) and accuracy (20%) where better
relative accuracy might be expected. Since it is anticipated that the
divertor will have a wide operating range from fully-attached to
fully-detached, the accuracy applies to the measurement technique, not
necessarily the interpreted ratios.

43\) [Neutral Density between Plasma and First Wall]{.underline}: FIRE
will have a very complex arrangement of tiles and diagnostic port
structures. With relatively narrow gaps between and behind them, these
could provide \"trapped\" volumes for gas, which could provide a source
of fuel. In order to allow control of the fueling by external means, the
influx of neutral D and T atoms in the space between the tiles and the
plasma must be measured. The performance of the ICRF and the LH heating
will be affected by this neutral pressure. The measurement should be
done to 30% accuracy and with 100ms time resolution at a number of
relevant locations distributed toroidally and poloidally. (Is this
really an issue for FIRE?)

[Diagnostic Neutral Beam]{.underline}: In order for many of the key
spectroscopic diagnostics presently used for making ion temperature,
rotation or q(r) measurements to function a source of fast neutral
particles is necessary. A diagnostic neutral beam operating at about 100
keV/amu will optimize the measurements. This beam must be initially at
high intensity, modulated to improve signal/noise performance of the
diagnostic techniques, and point at the plasma axis. It should be of
small size at the plasma axis, e.g. 200 x 200 mm^2^ and operate
continuously for the full length of the pulse. The high density
operation planned for FIRE leads to a very high initial intensity which
can be set after analysis of the diagnostic signal strengths with their
proposed viewing geometries. There are issues with using a too intense
steady beam because of its effect in heating the plasma. Hence a
short-pulse very intense beam is probably more suitable but will require
significant development.

44\) [Escaping Fast Ions and Alphas]{.underline}: Fast ions and
alpha-particles can be lost from the plasma because of their local
source and birth-orbits or because of the impact of instabilities. Fast
fuel ions will be created by the RF heating and, during the H/D
operation, will provide some test of the physics and equipment necessary
for identifying the loss of alpha-particles. Potentially the alphas
could cause local wall damage because the losses are relatively
localized on the outer wall (above or below the center-line depending on
the toroidal field direction). Hence fluxes to the wall should be
measured with 100 ms time resolution (transients at 10 ms resolution to
relate to MHD instabilities) with spacing about a/10 along the wall in
the outer quadrant established by the normal toroidal field direction.

45\) [Confined Alphas]{.underline}: In order to understand the behavior
of the dominant heating particles in the burning plasma, and in the
physics studies leading up to this optimal performance, it is necessary
to measure the spatial distribution of the total number of
alpha-particles and their energy spectrum. The transport and slowing
down of the alphas will be compared with transport and stability models.
The drive-term for alpha-driven AE modes is dependent on the gradient of
the alpha-pressure. For studies of the alpha-particle transport, and the
impact of plasma stability on it, the spatial resolution should be a/10
for profile data with 100 ms time resolution. The energy spectrum should
cover the range from full energy at 3.5 MeV down to close to thermal.
The relative accuracies of the measurements should be high, with
absolute accuracy of 20% proposed.

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46\) [Core Helium Density]{.underline}: Helium ions will build up in the
core of a burning plasma device as the alpha-particles thermalize. Their
transport away from the core, to be pumped away, is significant in the
continuation of the plasma burn, and, if too slow, could potentially
quench it. Hence a measurement of this impurity ion density has to be
measured with precision (10%) and with sufficient spatial resolution
(a/10) and temporal resolution (100 ms) to permit evaluation of its
production and transport within a particle confinement time-scale (\~
s).

47\) [Divertor Helium Density]{.underline}: The thermalized helium-ash
from the plasma core must be transported out through the divertor. A
measurement of this flux of helium will enable the complete history of
the alpha-particles through the plasma to be developed. It needs to be
measured relatively precisely (20%), and with the same time resolution
as in the main plasma (100 ms) to support the analysis.

48\) [Tritium Retention]{.underline}: The tritium inventory will be a
key regulatory issue and understanding the location of the retained
tritium will be very important. While FIRE should not have the issues of
a device with carbon in its first wall, there is still likely to be
tritium retained in void spaces associated with the first wall and
divertor. Quantitatively, while no exact estimate has been made, the
integral quantity in the vacuum vessel should be known within \~ 30%.

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48\) [Neutron Fluence]{.underline}: The neutron fluence at the first
wall may be of value in assessing activation properties of different
materials. For FIRE, where there is no present plan to make use of the
neutrons for any separate studies, e.g. in tritium-breeding, there may
be no need for this measurement and there is certainly no need for very
high accuracy (e.g. 30%), or fast time requirement (10s). The locations
of the measurement can be defined when a purpose is clearly identified.

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