J. Plasma Fusion Res. SERIES, Vol.5 (2002) 12-21

Review of D-T Experiments Relevant to
Burning Plasma lssues
HAWRYLUK, R.J.
Princeton Plasma Physics Laboratory
Princeton, NJ 08543(Received: 7 January 2002

/ Accepted: 10 July 2002)

Abstract
Progress in the performance of tokamak devices has enabled not only the production of significant
bursts of fusion energy from deuterium-tritium (D-T) plasmas in the Tokamak Fusion Test Reactor(TFTR) and the Joint European Torus (JET) but, more importantly, the initial study of the physics ofburning magnetically confined plasmas. TFTR and JET, in conjunction with the worldwide fusion effort,
have studied a broad range of topics including magnetohydrodynamic stability, transport, wave-particle
interactions, the confinement of energetic particles, and plasma boundary interactions. D-T experiments
differ in three principal ways from previous experiments: isotope effects associated with the use of
deuterium-tritium fuel, the presence of fusion-generated alpha particles, and technology issues associatedwith tritium handling and increased activation. The effect of deuterium-tritium fuel and the presence ofalpha particles are reviewed and placed in the perspective of the much larger worldwide database using
deuterium fuel and theoretical understanding. Both devices have contributed substantially in addressing
the scientific and technical issues associated with burning plasmas. However, future burning plasma
experiments will operate with larger ratios of alpha heating power to auxiliary power and will be able toaccess additional alpha-particle physics issues. The scientific opportunities for extending ourunderstanding ofburning plasmas beyond that provided by current experiments are described.

Keywords:
deuterium-tritium, alpha-particle physics, isotope scaling, ICRF heating, JET, TFTR

1. lntroduction
The worldwide technical progress in tokamak

experiments, to the mid-scale experiments to the largest
ones, scientific issues associated with size scaling werevery important and perhaps remain among the mostexperiments ranging from the pioneering experiments onimportant issues for future burning plasma experiments
operating with even higher fusion power. An excellent
review ofthis topic can be found by the ITER Team [l].
This paper will concentrate on what was learned when

T-3, to mid-scale experiments, such as PLT, PDX,ASDEX, DIII-D and Alcator C-Mod, to the largest
experiments JT-60, JET, and TFTR has enabled not only
the achievement of significant levels of fusion power in

deuterium (D) experiments but has also provided

the transition from deuterium to D-T operation wasmade on TFTR and JET. The results from the TFTR andJET D-T experiments have been extensively publishedand summarized in other review and overview papers

detailed scientific understanding. This has set the stage

for proceeding with deuterium-tritium (D-T) experiments on JET and TFTR, which began the study ofburning plasmas. In going from the early tokamak@2O02 by The Japan Society of Plasma
C o rre sponding aut hor' s e - mail : rhaw ry luk @ p pp l. gov

Science and Nuclear Fusion Research

12Hawryluk R.J., Review of D-T Experiments Relevant to Burning plasma Issues

12-241. This paper will not attempt to reproduce that
Iarge body of work but will highlight some of the key
results and identify some of the remaining scientific andtechnical issues for future D-T experiments.
Both JET and TFTR were designed to perform D-T

2. lsotope Effects in Confinement andTransport
A large number of different operating regimes havebeen studied on both JET and TFTR. JET hasconcentrated their confinement studies on H-mode

experiments. JET is an elongated tokamak with adivertor, with a plasma cuffent of 4 MA and a toroidal
field of 3.6 T during the D-T campaign. JET produced
l6.l MW of fusion power and the ratio of fusion powerplasmas both ELM free as well as with ELMs [4]. This

is of particular relevance to ITER, since ELMy H-mode
operation is the proposed baseline-operating scenario. In
ELM free discharges, the confinement of D-T plasmas isslightly less than in D plasmas, whereas in ELMy
discharges it is slightly higher. TFTR has studied
isotope scaling in ohmic, ICRF L-mode [26], NBI Lmode 1271, Supershot 128-321,limiter H-mode [33] andto heating po\\,er, which is often called Qp1, was up to
0.64 [12,251. For comparison, TFTR discharges had acircular cross-section defined by an inboard limiter. The
plasma cuffent during the D-T campaign was 2.7 MAwith a toroidal field of 5.6 T, which produced 10.6 MWof fusion power and a value of Qot of 0.27. In bothmachines, the plasma-facing components were composed of a combination of graphite and carbon-carbon
composite tiles;. The D-T experimental campaigns were
performed after years of operation in deuterium, which
established the high performance regimes. JET has
produced 0.68 GJ of fusion energy during their mostrecent campail5n in 1997, whereas TFTR produced 1.7
GJ during D-T operation from late 1993 to early 1997,
when TFTR operation ceased. Both devices processed

reverse shear discharges 1271. The supershot regime was

the highest performance operating regime in TFTR,
though near the end of the operating period explorationof reverse shear discharges had begun. With the
exception of reverse shear discharges, a favorable
scaling with isotopic mass was observed. The focus of
this paper will be on JET ELMy H-mode and TFTR
supershot studies.

has continued to operate in deuterium and is preparing
plans for another D-T campaign in 2006. The plan to

The JET and TFTR D-T experiments provide
another test of our understanding of transport andturbulence. Theoretically, transport is usually believed
to be governed by small-scale turbulent processes
determined by local plasma physics parameters, which
would imply gyro-Bohm scaling and a weak adverse
scaling with mass, x'rth e <At-02 [t+]. tns scaling

have another extended deuterium campaign in JET

would have a favorable scaling with device size.

limited the extent of the D-T campaign in 1997.

However, the ITER energy confinement time databaseindicates a slightly favorable confinement scaling withisotope mass.

99 grams of tritium to support operations. Since
conducting their D-T campaign in 1997, the JET facility

There are three principal ways in which D-Texperiments differ from D experiments. First are differences associatod with isotope effects and, in particular,

The JET team has determined a weak isotope
scaling for ELMy H-mode discharges in good
agreement with the ITER scaling. In particular, they

those which aflect transport and ICRF heating. Second,
the production of fusion-generated alpha particles fromhave concluded that 'tr'"th - <4to t6toou [14]. For comparison, the current ITER scaling (ITER98pby2) is thatthe D-T reactions enables the study of alpha-particle

physics issuer;, including the observation of alphat# - <4>0 Is. To develop an understanding ofthe source
for the weak isotope scaling, Cordey et al. ll4l studied

heating. Third, the technology associated with operation,

maintenance and decommissioning addresses issues ofcontamination and activation never previously addressed

the scaling of the pedestal compared to the core. Manyat this scope within the fusion community. Only a brieftransport models are sensitive to the edge conditions anddiscussion of this important topic will be given.
However, in nrany ways this was a watershed issue,

the question here is the favorable isotope scaling

demonstrating that the fusion community could safely
conduct these experiments, using the fuel needed for areactor and successfully address the issues of activation,
contamination, and waste disposal.

The JET Team has found that the stored energy
associated with the pedestal increases with isotopic

associated with changes in the pedestal or the core.

mass, nearly linearly [4]. In addition, the power loss
associated with ELMs decreases [4]. Thus, part of theeffect of the isotope effect is associated with changes inpedestal parameters. In the core region, the thermal

confinement excluding the energy associated with the13

Hawryluk R.J., Review of D-T Experiments Relevant to Burning Plasma Issues<A>08e10 1 and the ion heatpedestal has a weak adverse scaling with isotopic mass,
16t0 t,very similar to gyro-Bohm scaling?th"o." * q,{1-0confinement, ?E'n -[4]. When the ion heat conductivity is evaluated in thecore taking into account changes in power deposition,

factor of -2 and had a strong adverse scaling with mass,/tot € <A>-r'8102 for fixed plasma parameters (21) and,the ion heat conductivity is found to be significantly
higher in tritium experiments compared with deuterium.
/1e <,42o73to a, which is slightly higher than that given
by gyro-Bohm (Xt* <A>ut); though within the error
bars. Modeling studies by Bateman et al. l34l further
support that the core transport is consistent with a gyro-

Xtot

conductivity decreased substantially in the core by a

* <A>-2'610 3 for fixed engineering parameters (P6).
Ernst e/ al. 136l developed an interesting andsuccessful model to understand the isotope effect in
TFTR. In particular, these authors observed that there is
a competition between the linear growth rate from agyro-Bohm transport model, which decreases with ion
mass, and the shearing rate, which increases with ion
temperature gradient. On TFTR, the sharp density andion temperature gradients contributed to a large
component of the radial electric field, which was notoffset by toroidal rotation, since balanced injection waslargely employed. By including the radial electric field
shear, they were able to model the strong isotope effectobserved in TFTR supershots. The implications of thiswork is that isotope effects depend strongly on operating

Bohm turbulence model. This study found good
agreement with the experimental data when the pedestal
height from experimental studies was used to define theedge conditions. Recently, Budny et al. [35] has fit theconfinement time from the JET experiments in terms of
dimensionless parameters. The authors have inferred amore positive scaling with isotopic mass <A>0ar andhave concluded that the scaling for the ion heat

conductivity is between Bohm and gyro-Bohm, thoughlarge relative errors were inferred in their fitting

regimes with differences in pressure gradients, current
or rotation profiles. However, more work is required toapply this model to the various operating regimes andfor different devices and, in particular, to reversed shear

procedure. Perhaps more interestingly, these authors
suggest that a power law fit may not be adequate todescribe the variations in isotopic mass andcollisionality.
Righi et al. ll5l has found that the power requireddischarges.

experiments than in deuterium, scaling inversely withhydrogenic mass. This may also be influenced by theregimes, with the exception of reversed shear dis-Though the largest isotope effect was observed insupershot discharges, it was prevalent in other operating

to obtain an H-mode transition is less in tritium

charges. Many of the characteristics of the reverse shear
discharges were similar to supershot discharges, with the

behavior of ELMs in which the frequency of ELMs
decreases with isotopic mass. The power required toconsideration for the design of future burning plasma

exception of the current profile. The stored energy in DT and deuterium reversed shear discharges was similar
and the favorable isotope scaling was not observed [27].

experiments.

More significantly the power threshold for internal

obtain an H-mode transition is an important
The results on TFTR show some marked
differences from those on JET, which will be importanttransport barrier formation increased with isotopic mass.
The reason for this difference remains to be understoodfor both projecting the performance of future

and may be important to understand how future

experiments as well as understanding the alpha heating
experiments. With the benefit of hindsight, it appears
that the TFTR supershot plasmas had many signaturesof a transitionless internal transport barrier [36]. Steepdensity and ion temperature gradients in the core, lowadvanced performance regimes behave.

3. lsotope Effects in ICRF Heating
ICRF heating is one of the heating options forconfinement characterized these discharges. In TFTR
supershot discharges, a significant increase in the ion
temperature, a modest increase in the electron temperature, and a correspondingly significant increase in the

ITER and a preferred heating option for compact highfield burning plasma experiments, such as FIRE and
Ignitor. ICRF heating of D-T discharges has beenstudied first on TFTR and then on JET. Second
harmonic tritium heating successfully increased the ion
temperature on TFTR and a combination of directconfinement time was observed [28-32]. This was a very

electron heating and 3He minority heating was observed

reproducible effect conducted in a large number of

to increase the core electron temperature. In these

conditions. The triple product, n1@)Ty@)re, increased up
to -807o going from deuterium to D-T. The thermal

experiments, the ICRF power was applied to a neutralbeam heated discharge [37-38]. Measurements of therecycling at the edge, and improved core and global

T4

Hawryluk R.J., Review of D-T Experiments Relevant to Burning Plasma Issues

Table 1 Comparison of the parameters for the highest
performance discharges on TFTR and JET withthe predicted performance on ITER [zl4].

power deposition were found to be in good agreementwith theoretical models [39]. Near the end of the D-T
campaign on I'FTR, mode conversion heating with themode conversion layer in the plasma core, where the ion
temperatures u'ere in excess of 30keV, was studied. Ion
heating was clearly observed l4O-411.
JET routinely used ICRF heating during their D-T
campaign and in their highest performance discharges

TFTR

JET

Pft"i""(NtW)

10.6

16.1

ITER
400

p"(OxN4Wm)

0.28

0.08

0.43

F"(O)Eo

0.30

0.4

0.8

-R.v(B)Ea

2.O

L.)

4.0

t121. A tritium plasma was heated by deuterium%(O/vAtrG"(0)

t.72

2.52

1.76

minority heating resulting in a record Qp1 for steady

Pol Ph"t

0.03

0.09

0.66

operation, of 0.22 113,16,17l. This was accomplished by
optimizing the energy of the energetic deuterium tail.
Tritium minolity heating was also studied on JET.
Strong ion heating was observed with 3He minority

The alpha heating power density in the core of thehighest performance TFTR and JET discharges iscomparable to the projected values in ITER, despite theheating. The wave absorption was weaker in second
harmonic tritium heating, as would be expected, and theorbit loss associated with acceleration of the ions to highenergy reduced the efficiency. 3He minority scenario
was recommended for ITER during the heating phase on
the way to ignition, since the 3He concentration could belarge difference in fusion power [44]. In addition, p"(0)and -R.V(B), which is a measure of instability drive
term for TAE modes, is also comparable, as shown in
Table l. Thus on both TFTR and JET, it was possible tosmall and an optimized mix of D and T could be

difference between current machines and future higher
performance burning plasma experiments. The ratio ofstudy many important issues associated with alpha-particle physics. However, there is one important

maintained, unlike with either deuterium or tritium
minority heating [16]. In conclusion, ICRF heating wasboth successful and well understood in D-T discharges,
though technology issues associated with the antenna
remain for future burning plasma experiments.

Pu1o6ulP6"u1

is considerably less in JET and TFTR

compared with ITER or other future burning plasma

experiments, and limits the range of alpha-particle
heating studies.

4. Alpha-Pafticle Studies

5. Gonfinement and Slowing-down of Alpha
Particles in MHD Ouiescent Discharges

The behal ior of alpha particles was studied in both

MHD quiescent discharges and in discharges with MHD
activity [24]. MHD quiescent discharges facilitated the

The escaping alpha particles were measured in
TFTR by means of detectors inside the TFTR vacuum
vessel near the first wall [45]. The escaping alpha flux
to the detector near the bottom of the vacuum vessel
decreased with plasma current in agreement withclassical first orbit loss calculations. At 2.5 MA, the firstorbit loss was about 37o in supershot discharges.
The study of the confined alphas was a major

study of the confinement and slowing down of the alphaparticles, since the interaction with MHD activity can beimportant. The effect of alpha particles on MHD stability
was also studied. Finally, the first experiments of alphaparticle heating were performed. A key ingredient inthese experiments was the successful development of
new diagnostics to study the fusion-generated alphaparticles. Many of these diagnostics were tested during
the D-T experiments for the first time and were in theirown right a major technical accomplishment.

experimental challenge due to the low concentration andlarge range in energies spanning from thermal particles

to 3.5 MeV. The high-energy alpha particles (0.5-3.5
MeV) were measured by means of a double charge
exchange technique on TFTR 146,411. A lithium pellet

To benchmark the models for the alpha-birth
profile, neutron-emission measurements were used. The
chord integrated radial profile measurements for the D-T
neutron emission as well as the time dependence was ingood agreement with TRANSP calculations based on
plasma parameters in high performance discharges onboth TFTR and JET [12,42,431. Thus, the TRANSP
calculations provide a good model for the alpha birth
profile.

was injected into the discharge. From the cloud of singly

ionized lithium ions. two electrons were transferred tothe alpha particles. The neutralized alpha particles werethen detected by a charge exchange diagnostic, which
detected deeply trapped alpha particles. To model thisnarrow range of the distribution function, a Fokker
Planck calculation was used to overcome the limited
statistics in the Monte Carlo treatment used in TRANSP.

l5

Hawrvluk R.J.. Review of D-T Exoeriments Relevant to Burnins Plasma Issues

particles, which has been reviewed by Heidbrink andSadler [59]. Discharges with reversed shear or highcentral shear were less intensively studied in TFTR. The

The energy spectrum of the alpha particles near the axisof the discharge is in good agreement with TRANSP
and Fokker Planck calculations [46,48.491.

Since this diagnostic measured the deeply trappedalpha particles, the effects of stochastic ripple diffusion
were evaluated on the confined alphas. Stochastic ripple

loss due to stochastic ripple diffusion was greater but
other possible loss mechanisms were not fully excluded.

diffusion predicts that particles at larger major radii,
where the effect of the toroidal field ripple is greater,

6. Effect of MHD Instabilities on
Alpha-particle Confinement
MHD activity can affect the confinement ofshould exhibit greater loss as was clearly measured [50].

In high performance supershot discharges, stochastic

energetic particles and also alpha particles. Sawteeth
were observed to redistribute the intermediate energy
alpha particles (in the range of 150-600 keV) in minor

ripple diffusion was only a couple of percent; however,in reversed shear plasmas, which were located outboard
in major radius in a higher toroidal field ripple region.
the stochastic ripple diffusion was about 207o [51,52).
Experiments on JT-60U measured the heat deposition
due to ripple loss of fast ions to the plasma-facing
components using an infrared camera, extending thework on TFTR [53].

radius [60]. Though this effect was quite pronounced on
TFTR, the effect of such a distribution may be moremodest in a high performance burning plasma. In suchplasmas, the heating dynamics would be affectedtransiently. Since the alpha slowing down time would beshort compared to the energy confinement time and thesawtooth period, the impact may be modest.

The intermediate energy alpha particles, 150 to 600
keV were measured by charge exchange recombination
spectroscopy [54]. This technique relies upon charge
exchange from neutral beam particles to provide anelectron to an excited energy state of alpha particles,

In addition to the redistribution due to sawteeth,
other MHD instabilities can cause enhanced loss ofalpha particles. On TFTR, neoclassical tearing modes
resulted in enhanced loss to the probes at the vacuum
vessel wall [9]. A strong toroidal anisotropy is observed
as the mode rotates. Enhanced loss is also observed due
to major and minor disruptions and kinetic ballooning
modes [61,62]. Because the loss is localized, it can place
additional requirements on plasma-facing components.

which then undergo a transition. The absolute intensityof the emitted light provides an absolute measurementof particles in this energy range. A very high throughput
optical system was developed for this task. Thisdiagnostic on TFTR provided a sensitive measure of theradial transport of the alpha particles. The absolute

7. Alpha-particle Effect on MHD Behavior

intensity and the radial distribution showed that thealpha particles were very well confined with Du1olu <While MHD can affect the behavior of energetic
particles, energetic particles can also affect MHD
instabilities. A review of previous experiments wasgiven by Heidbrink and Sadler [59]. During the past
decade, motivated by experiments on JET and TFTR

0.03 m2ls [55,56].

The thermal helium ash was measured with aconventional charge exchange recombination system.
The helium density profile is determined by radial
diffusion and recycling from the limiter and walls. For
these supershot discharges, the radial diffusivity iscomparable to the ion thermal diffusivity and the ratioof ro+/rB = 8, which would be acceptable for a reactor,
operating under similar conditions [57]. The issue forand also future burning plasma experiments, there has
been extensive theoretical and experimental work onenergetic particle-wave interactions. One of theinstabilities, which have been extensively studied is thetoroidal Alfv6n eigenmode (TAE) instability, which isfuture machines will be the ability to pump particles ondestabilized by energetic particles resonant with thethe edge and on the details of impurity transportcompared with that of the heat [58]. In particular, will

Alfv6n velocity. This instability is predicted to beamong the most dangerous instabilities for burningtransport barriers affect the relative confinement ofplasmas. Neutral beam and ICRF experiments on TFTR

alpha ash?

and also on the other large facilities have shown that alarge fraction of the particles can be ejected from theplasma at sufficiently high power. Compared with thehigh power neutral beam and ICRF experiments used toexcite the toroidal Alfv6n eigenmode instability, theOn TFTR, the evolution of alpha particles fromtheir birth energies all the way to thermal energies wasmeasured and found to be in accord with expectations in
MHD quiescent discharges with positive shear and 4(0)-1. This is consistent with other studies of enerseticalpha power was relatively low and this was not a
I6

Hawryluk R.J., Review of D-T Experiments Relevant to Burning Plasma Issues

violent instability in either TFTR or JET D-T

as has been observed with ICRF energetic particle

experiments [Lt,20,241. In particular, in the highest
fusion power clischarges, this instability was not driven
by alpha particles and did not have a deleterious effect.
The relatively modest alpha-particle density together
with a large damping rate from the beam ions stabilized
this instability in high performance TFTR discharges.

formation. An interesting experiment was performed on
JET which showed that the sawtooth period increased

with tritium concentration but did not peak withmaximum fusion power production t711. A detailed
analysis of these experiments indicated the effect wasdue to the increase in slowing down time of the tritium
ions compared with deuterium ions and the resulting
increase in perpendicular energy density in the fast
beam ions. The same analysis suggests that for thehighest performance D-T discharge in JET the alphaparticles had a significant stabilizing effect and may
have contributed to the increased sawtooth period. Thus,

Subsequent theoretical calculations showed that thepredicted alpha-driven toroidal Alfv6n eigenmodethreshold is sensitive to the q-profile [63-66]. Reversed
shear discharges and discharges with high 4(0) on axis
were predicted to be favorable to the onset of theinstability. The phase after beam turn-off, when thealpha particle drive was still present but ion damping
was reduced was predicted to offer an opportunity toobserve the instability. TFTR experiments, with highit may be possible that future higher power D-T experiments on JET will be able to demonstrate this effect.

q(0) after beam turn-off, observed the onset of the

8. Alpha-particle Heatingalpha-driven rnstabilities and confirmed theoreticalpredictions for the onset of the instability [67,68]. Bymeans of microwave reflectometry, the radial
eigenmode of the n = 4 TAE was seen to be inExperiments on JET and TFTR have begun the
study of alpha-particle heating U2,731. On TFTR, thealpha heating power was about3Vo of the heating power,
though abott l57o of the power through the electron
channel, enabling a measurement of electron heating. In

approximate agreement with linear theory, though issues
remain for some modes. These experiments confirmed

these supershot experiments, the sawteeth werethat the onset condition for the toroidal Alfv6n

stabilized and there was no detectable adverse MHD. In

eigenmodes was relatively well understood. Future work

TFTR deuterium experiments, the central electron
temperature was observed to scale as the square root of

on nonlinear consequences of alpha-driven toroidal
Alfv6n eigenmodes remains, especially under conditions
when many modes would be destabilized and higher
amplitude instabilities are developed, which may occur
in future burning plasma experiments.

the energy confinement time. Since the isotope effectwas significant on TFTR, plasmas were matched withsimilar energy confinement time to account for theisotope effect on the central electron temperature. A
measurable increase in electron temperature (AZ"-0.8

Though the alpha-driven toroidal Alfv6n+0.3keV for Plu"ion=4.5MW) was observed, in reasonable accord with expectations both in terms of themagnitude of the increase and the radial dependence
U2l. The increase in central electron temperature wasobserved to be greater in discharges with higher fusion

eigenmodes on TFTR were very weak, evidence existsthat these modes spatially redistributed part of the alphaparticles. The pellet charge exchange diagnostic
measured the radial profile of .53 MeV and 1.7 MeV
particles. Compared with a discharge which did not havepower (A7"-2keV for Prusion up to 7.5MW); though,there were few shots at these higher values of fusion
power with comparable D shots for a baseline.
JET was able to operate at higher ratios of Pu1o1,u/P6.4=0.12 with 304OVo of the power input to the coreelectrons than TFTR and conducted an experiment toan observable t.oroidal Alfv6n eigenmode instability, thealpha particles are redistributed radially and the low
energy particles are depleted from the core. This wasobserved when the central 4(0) was -2 but not for lower
values of q(0) t501. These were very interesting results
near the end of the TFTR D-T campaign. Further
experiments and more theoretical modeling is required
to understand the alpha-particle redistribution in detail.

measure alpha heating by varying the tritium concentra-tion over a wider range than TFTR [73]. This was done
to isolate the isotope effect, which as discussed above

Since these experiments on TFTR, strong radial
redistribution of energetic beam ions has also been

was inferred to be weak on JET. The effect ofobserved during negative-ion neutral beam experiments
on JT-60U [69-70].

differences in heating power from shot to shot was taken

One of the potential benefits of alpha particles isthat they might help to stabilize the sawtooth instability,

experiment was the variability due to sawteeth, whichchanged with tritium concentration, as noted above [71].

into account. A significant systematic effect in thist7

Hawryluk R.J., Review of D-T Experiments Relevant to Burning Plasma Issues

10. Technology Associated with DeuteriumTritium Experiments

Though there were only a limited number (5) of pulses
used in these experiments, clear indications of alphaheating (LT I.3 !0.23 keV for Pru,ion= 5-6 MW) were"observed on JET. Future higher power experiments are
required to do a comprehensive study of alpha heating.

Both JET and TFTR successfully used on-site
tritium processing, a key technology for future experiments. Both machines observed that tritium retention ingraphite is a serious concem. After extensive efforts toremove the tritium, l6Vo of the tritium, which went into

In historical context, the early neutral beam and rf
heating experiments in the early 70's demonstrated that
it was possible to heat a tokamak; however, subsequent

the torus, remained inside the TFTR vacuum chamberand l2%o in JET t89-931. This is an unacceptableretention rate for future-burning plasma experiments andexperiments at much higher power enabled the detailed
scientific study of confinement and stability. In many
ways, the experiments on TFTR and JET made the firststep by obtaining clear indications of alpha heating. The
next round of higher performance experiments is

has motivated the development of novel in-situ
techniques. A recent collaboration between PPPL and
JAERI has studied the removal of tritium from TFTR
limiter tiles used during the D-T experiments. The tiles
were scanned with a laser to heat the surface and release
the tritium from the co-deposited layers [94-95]. Initial
results look promising and the possibility of using thistechnique on future JET experiments is being discussed.
This also motivates the search for alternative plasma
facing component materials as recognized by the design

required to exploit this new heating technique andevaluate its effect on confinement and stability.

9. Potential for Utilizing Alpha Particles toFurther lmprove the Prospects for a
Reactor
Theoretical work on alpha channeling 174-781,

teams for future burning plasma experiments. Thisremains an active area of research.

frequency sweeping of waves U9), and stochastic ion
heating t80,8ll raise the tantalizing prospects of:increased ion heating, alpha ash control, modificationsof the alpha heating profile, reductions in alpha pressure
to decrease instability drive, and current drive. While itis unlikely that any technique would simultaneously
provide all of these potential benefits, the attainment of
one or more of these objectives could be beneficial. Thisis an area in need of further experimental study, first inhydrogen or deuterium discharges and then in D-T

JET has demonstrated that it is possible to maintainin-vessel components with a remotely operated
manipulator t96-971. After the conclusion of the D-T
campaign, they used this technique to replace theirdivertor tiles. This is a critical technology for future
experiments. However, it is important to realize that in
ITER the fluence will be up to 104 greater, which placesdischarges. Fundamental wave-particle physics studiesof alpha channeling were begun on TFTR near the endof operations t82-851 and showed some encouraging
results but raised some important theoretical issues [86871. Recently, Thomas et al. [88] suggested that theincreased ion heating accompanying the alpha-particle
heating experiments is not well explained and raised thequestion as to whether unexplained transfer of alphaheating to the ion channel occurred. The data for this islimited and further analysis is underway. Gates et al.

even greater requirements on this technology and theability to remotely repair all components, which was notpossible in JET. Nonetheless, the JET results are a verysignificant demonstration of this important technology.
TFTR is in the process of being decommissioned
t98-991. This is a three-year effort and all of the external
structure around the vacuum vessel and toroidal fieldcoils has been removed, though the neutral beam boxes
are being retained for future experiments. At this time
(December, 2001), the vessel is being cut apart using a
diamond wire cutting technique and already three of the[81] have predicted that stochastic ion heating fromten vessel sectors have been removed. On TFTR,

compression Alfv6n instabilities driven by neutral beamions may contribute to enhanced ion heating. Experiments on NSTX are underway to investigate this. This iscontamination control is the largest radiological issue
along with waste disposal and handling. In futuremachines, activation will be much greater anddisassembly will have to be performed remotely.

an area awaiting further experimental results todetermine the viability of these techniques for futureburning plasma experiments. While a demonstration of
the impact of these concepts will require a burning
plasma, experiments to develop an understanding of

11. Conclusions
Results from TFTR and JET enhanced the technicaland scientific basis for proceeding with a burningthese concepts can be performed in current experiments.

plasma experiment. These experiments reduced the risk

18

Hawryluk R.J., Review of D-T Experiments Relevant to Burning Plasma Issues

for a future experiment by exploring the effects

performed. These experiments also identified scientificmotivated the construction of experiments on TFTR
with high central values ofthe safety factor and reduced
beam ion damping, which confirmed that the onset of
the instability is in reasonable accord with theory. Eventhese weak toroidal Alfv6n eigenmode instabilities

and technical opportunities in a future burning plasma
experiment.
H-mode isotope scaling studies on JET, together
with the worldwide physics database, provide a good

redistributed the deeply trapped alpha particles. Future
experiments with larger amplitude modes will focus on
the nonlinear consequences of these instabilities. Alphaparticle heating was observed on both TFTR and JET,

associated with the use of a tritium, by studying theconfinement and heating of alpha particles and bydemonstrating that D-T experiments can be safely

technical basis for the baseline operation of a burning

setting the stage for future higher power andplasma experiment. The JET ELMy H-mode

performance experiments in which the ratio of Pu1o5u/

experiments support the confinement time and power
threshold projections with respect to isotope mass.

P1,"u1will be higher.

Critical aspects of the technologies for a burning
plasma experiment were utilized for the first time,

Analysis of the core transport indicates that theconfinement in the core is in reasonable accord withdemonstrating that it is possible to operate, maintain anddecommission a device operating with D-T fuels. The
technological requirements will be far more demanding
in future burning plasma experiments due to the higher
neutron fluence and tritium processing requirements andpotential tritium retention in the vacuum vessel.
The TFTR and JET experiments, together with the

gyro-Bohm scaling, though the overall energy confinement time has a weak positive isotopic dependence. An

important observation was that the plasma energy
associated with the pedestal increased and the ELM
frequency and power loss by ELMs decreased withisotopic mass. Nonetheless, the understanding of isotope
scaling is incomplete. The dimensionless scaling studies
are not in full accord with gyro-Bohm scaling in thescientific and technical results from the worldwide
fusion effort, have developed the basis for proceedingwith higher performance burning plasma experiments.

core. There is a clear variation in scaling results indifferent operating regimes. The radial electric field

Those experiments will enable not only ashear in TFTR appears to play a significant role insupershot discharges, in which the confinement time has
a strong positive scaling with isotopic mass. Further
work is required to understand the isotope scaling inother operating regimes and in particular, discharges

comprehensive study of alpha-particle physics but also
the assessment of the scientific issues associated withalpha-particle heating on stability and confinement.
Furthermore, a burning plasma experiment will further
the development of the technology required for fusionenergy development.

with reversed shear.
The physics of ICRF heating is well established.
Second harmonic tritium, He3 minority. mode conversion
to ion Bernstein wave, deuterium minority and tritium
minority heating were all explored. He3 minority andsecond harmonic tritium heating have been identified as
the preferred approach for ITER and FIRE. Deuterium
minority heating produced the highest values of Qpl insteady state JET discharges. For future burning plasma
experiments, the technology challenges associated withICRF antenna lemain due to the higher neutron fluence.
Alpha-particle confinement, transport, and slowing
down are well understood in MHD quiescent discharges,
though not as extensively studied in reversed shear
experiments. Alpha particles can be expelled from thedischarge and radially redistributed by MHD instabilities. Alpha particle driven instability was studied. In
the highest performance discharges on both TFTR and
JET, the alpha-particle pressure was insufficient todestabilize tbe toroidal Alfv6n eieenmode. This

Acknowledgements
The experiments reported here are a brief summaryof a great effort by the TFTR and JET teams. It has been
my pleasure to work with both groups during the D-T
campaigns. In preparing this paper, I wish to thank R.
Budny, C. Gentile, R. Goldston, N. Gorelenkov, R.
Majeski, D. Meade, S. Medley, R. Nazikian, E. Perry,
C.K. Phillips, M. Redi, C. Skinner, J. Strachan, E.
Synakowski, M. Watkins, and M. Zarnstorff for useful

discussions and valuable input. This work was
performed under U. S. DOE Contract No. DE-AC02-76-

cH03073.

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2l