Due to its remarkable engineering flexibility, the Joint European
Torus (JET) has been providing cutting-edge results in fusion research
for two decades. Naturally, there is an outstanding concern: can we
maintain this flexibility and yet significantly enhance JET's capabilities?
In every successful research centre, there is always a combination
of at least three ingredients:
-
a first-class scientific and technical team
-
determination to carry out good experiments
-
investment in new state-of-the-art equipment
Blending these three efficiently is not simple. In a machine of JET's
complexity it is unthinkable to run experiments and install new equipments
simultaneously. So experiments and enhancements compete for programme
time. To keep JET up to date, shutdown periods cannot be avoided.
At the time of writing (December 2004) JET is in the middle of one
of its busiest shutdown periods ever, the main purpose of which is
to further extend the plasma performance and diagnostic capabilities
of JET before we undertake experimental campaigns that will be completely
focused on ITER-relevant studies.
This shutdown period is particularly challenging for our Remote Handling
group, as most of the modifications inside the vessel (including welding)
are carried out by the Remote
Handling manipulator (robotic arm, see Fig. 1). In parallel to the in-vessel
operations, new instruments are being integrated into JET systems and
thorough maintenance is being undertaken (Fig. 2). Additionally, all new components
have to be rigorously tested according to the JET quality assessment
rules.
Here are a few examples of the new installations :
During this shutdown, an upgraded divertor will be installed (Fig. 3 and 4). The divertor
is a region to which the magnetic field deflects the exhaust particles
that have escaped from the hot plasma. In both JET and ITER this divertor
region is positioned at the bottom of the vacuum vessel. The divertor
structure has to be designed with great care as it is exposed to the
high power flux
carried by the lost particles.
In ITER, the plasma cross-section shape will be slightly more triangular
than the shape originally conceived for the JET design. As JET
is very flexible, its magnetic field can be set up to test the ITER plasma
configuration. However, in recent experimental campaigns the JET divertor
structure was not ideally arranged to accommodate this magnetic field
set up, so the plasma
power
in
the ITER-like configuration had to be limited. Within the present shutdown
period, new plasma facing components (carbon tiles) are to be installed
in the
divertor so that JET can operate high-power experiments with ITER-like
plasma configurations.
Current enhancements also aim to complement the ITER-relevant
capabilities of JET in plasma diagnostics,
i.e. in developing hi-tech equipment that allow us to reliably observe
and precisely measure the processes in experimental plasmas.
At JET, special attention has always been given to the measurement of
neutrons. Neutrons carry vital information on the rate and location of
fusion reactions in burning plasmas. With its ability to produce neutrons in both deuterium-deuterium
(D-D) and deuterium-tritium (D-T) fusion reactions, JET provides a unique
opportunity for development of neutron diagnostics and data analysis
methods for future fusion reactors.
To further improve JET's neutron diagnostics, the
Magnetic Proton Recoil Spectrometer (MPR - see Fig. 5) will be upgraded and a new
Time Of
Flight for Optimised Rate (TOFOR - see Fig. 6) facility will be installed during
the current shutdown.
MPR measures energies of protons released
from a special target in head-on collisions with the tracked neutrons.
The kinetic energy
of protons is then almost precisely equal to the energy of incident
neutrons. Protons have the advantage of being electrically charged,
so that their
energy can be precisely measured via their deflection in a well-defined
magnetic field. Of course, to avoid interference, MPR needs heavy shielding
against JET's powerful magnetic fields. Whereas the former MPR was
limited to neutrons produced by D-T fusion (deuterium-tritium fusion
that produces
high energy neutrons), the new version of MPR will also be able to
measure lower energy D-D fusion neutrons. It will be rigorously calibrated
for accurate absolute measurements of neutron energy and neutron flux.
A new neutron diagnostic known as TOFOR is to be installed
in the JET roof laboratory. It will be used to measure energy spectra
from D-D fusion neutrons only. Unlike MPR, the principle of TOFOR does
not rely on rare head-on proton recoils so that the latter has higher
count rate capability. In TOFOR, every proton recoil is registered
in a small scintillation detector in the bottom of the device. Some
of the recoiled neutrons are registered again in the top "umbrella-like" set
of detectors. All pulses are seeded by a system of automated data analysis
so that only the incidences of both bottom and top counts are followed
up. The original energy of each neutron is then derived from the time
that elapsed between the first count in the bottom detector and the
second count in one of the top detectors.
For any reader keen on brainteasers here is a quick
exercise: using the energy conservation law and simple geometry rules,
show that time of flight between the bottom and the top units is not
a function of the recoil angle, but purely a function of neutron energy
and TOFOR size. Three hints:
-
only a narrow beam of neutrons, coming along the
TOFOR axis, can arrive at the bottom detector,
-
the difference between proton and neutron mass can
be ignored,
-
and, most importantly, all the detection units
are installed on an imaginary sphere.
Another vital diagnostic tool for fusion plasmas is
bolometry, which provides absolute measurements of total radiation
losses of a plasma discharge, regardless the radiation wavelengths.
A bolometer is just a tiny piece of metal with precisely defined thermal
properties that heats up due to plasma radiation. The radiation comes
through a narrow slit (pinhole) that defines a "viewing line" of
each bolometer (Fig. 7). Plasma radiation losses along the viewing line are
then derived from the increase in the bolometer temperature. With a
sufficient number of viewing lines (i.e. with a set of suitably positioned
bolometers) it is possible to find out the radiation emissivity pattern
on plasma cross-section. The process of calculating cross-section patterns
from viewing line projections is commonly known as (computer-aided)
tomography or CAT.
During the current shutdown, several new sets of bolometers (Fig. 8) are being installed that will allow for precise mapping of both plasma
emissivity and surface radiation. High spatial resolution is required
in the divertor region in order to correctly localise large radiation
losses caused by particle exhaust. That is why the array of viewing
lines is denser in the divertor region (see figure below) and why four
other bolometric cameras dedicated purely to divertor observations
will also be refurbished during the current diagnostics enhancement
phase.
Active diagnostic methods are subject to considerable changes too.
The LIDAR diagnostics
will be complemented by a new independent High Resolution Thomson Scattering
system (HRTS) with better temporal and spatial resolution of steep
changes in electron temperature and plasma density. This will be very
important
in detailed characterisation of both edge and internal transport barriers.
Charge Exchange Recombination Spectroscopy (CXRS - see Fig. 9) will be equipped
with faster CCD cameras and two new spectrometers. This system has
been used to study the behaviour of impurities along the plasma radius
by analysing the characteristic light emission of impurities after
collisions with neutral beams. With the new CCD cameras the system
will provide five to ten times better temporal resolution of these
processes, and thanks to the two additional spectrometers it
will be possible to observe six different impurity elements simultaneously.
Carbon, Helium, Neon, Beryllium, Nitrogen, Oxygen, Argon and/or beam
emission can be analysed by CXRS at JET.
In addition, an independent CXRS system designed entirely for diagnostics
of the colder edge region of plasma is being refurbished. This "edge
CXRS" will observe the plasma-beam interaction from the top and
bottom.
A new unique diagnostic is under construction to make
detailed measurements of magnetic field line oscillations, known as
Alfvén waves. The diagnostic, known as Toroidal Alvén
Eigenmodes (or TAE - see Fig. 10) antennas, consists of two sets of four antennas.
Some of the TAE antennas (four in maximum) will emit electromagnetic
waves to actively modify the Alfvén waves, while the others
will passively observe the response of TAE. With this diagnostic, JET
will be the fusion facility
best equipped to study and interpret interaction between Alfvén
waves and alpha particles (i.e. Helium nuclei that are born in fusion
reactions).
This interaction is believed - from computer simulations - to play
a significant role in confinement of the alpha particles and also
in the overall stability of plasmas in future fusion reactors.
Two new diagnostics systems will be dedicated to the direct
measurement of lost fast alpha particles. These particles, produced
in fusion reactions, will provide the main heating power for plasmas
in future fusion reactors - the power needed to sustain extreme temperatures
of plasma. Therefore, studies of transport and confinement of fast
alpha particles are of prime importance for our research.
Due to its size and capabilities, JET can confine fast alpha particles
produced in two ways: either in actual D-T fusion, or by injection
of Helium beams into the plasma and consecutive acceleration of Helium
ions by suitable radio frequency power. The new diagnostics - Faraday
cups and scintillation detectors - will be capable of monitoring those
alpha particles that are lost, measuring their fluxes, crude spatial
distributions and velocity components; the former diagnostic tool in
total (integral, average) particle fluxes, the latter by sampling individual
particles.
Plasma-wall interaction is another important topic
for our ITER focused research. At JET, several diagnostics will be
upgraded or newly provided in order to better understand and quantify
the heat distribution on walls as well as erosion and deposition of
wall materials. A new state-of-the-art wide-angle infrared camera will
be installed to overview the heat load on plasma-facing components
and to estimate their temperature during experiments (Fig. 11). Five new Quartz
Micro-Balances and five Rotating Collectors will be installed in the
divertor region to register material deposition. Special coated or
profiled "smart tiles" will provide another precise tool
to identify regions of erosion and deposition.
In total, the current JET enhancements include several
tens of new or upgraded installations. Most of them are highly specialised
one-off products. The actual extent of the works, challenges and physical
principles involved cannot be covered in a single webpage. This is just an illustration
of a busy shutdown period, with equipment being installed that is full
of promise for increased performance in future JET campaigns. |
