J. Plasma Fusion Res. SERIES, Vol. 5 t2002) 143-148

FIRE, Exploring the Frontiers of Burning Plasma Science
MEADE Dale, JARDIN Stephen, KESSEL Charles, MANDREKAS Johnl,
ULRICKSON MiChACI2, HEITZENROEDER PhiI, TITUS PCtCr3 ANd SCHMIDT JOhN
Princeton Plasma Physics Laboratory P. O. Box 451, Princeton,, NJ 08543, USA
I Georgia Institute of Technology, Atlanta, GA 30332, USA
2 Sandia National lnboratory, Albuquerque, NM 87185, USA
3 Massachusetts Institute of Technology, Cambridge, MA 02139, USA(Received: ll December 2001 / Accepted: 2 October 2002)

Abstract
The buming plasma regime will exhibit a number of complex phenomena that must be studied andunderstood if magnetic fusion is to be successful. The FIRE design study of a burning plasma experimenthas the goal of developing a concept for an experimental facility to explore and understand the strong

non-linear coupling among confinement, self-heating, MHD stability, edge physics and wave-particle
interactions that is fundamental to fusion plasma behavior. This will require plasmas dominated by alphaheating (Q = lO) that are sustained for duration longer than characteristic plasma time scales. The FIRE
pre-conceptual design activities, carried out by an U. S. national team, have been undertaken with theobjectil'e of finding the minimum size (cost) device to achieve the essential burning plasma science goals
while building on the physics understanding of today. FIRE is envisioned as an extension of the existing
advanced tokamak program leading to an attractive magnetic fusion reactor (e.g., ARIES-RS). FIRE
activities have focused on the physics and engineering assessment of a compact, high-field tokamak withthe capability of achieving Q = l0 in the Elmy H-mode for a duration of - 1.5 plasma current
redistribution times (skin times) during an initial burning plasma science phase, and the flexibility to add
advanced tokamak hardware (e.g., lower hybrid current drive and feedback stabilization) later. The
configuration chosen for FIRE is similar to that of ARIES-RS, namely a highly shaped plasma 16- = 0.7,
K* = 2), with double-null divertor and aspect ratio = 4. The key "advanced tokamak" features are: strong
plasma shaping, double null poloidal divertors, low toroidal field ripple (<0.3Vo), intemal control coils
and space for wall stabilization capabilities. Only metallic plasma facing components (tungsten divertor
ta.rgets and Be coated first wall tiles) are used to reduce the in-vessel tritium inventory. The reference
design point is Ro - 2.14 m, a = 0.595 m, B,(Ro)- l0 T, Io=7.7 MA with a flat top time of 20 s for 150

MW of fusion power.

Keywords:
fusion, buming plasma experiment, ignition, tokamak, alpha particles

1. Introduction

and the development of low-activation, neutron-resistant

Magnetic fusion is technically ready to proceed to
the next stage of fusion research: the study of burning
plasmas dominated by self-heating due to the fusion
process, the optimization of the magnetic configurationmaterials. A national team consisting of scientists andengineers from more than 15 institutions in the U.S.
fusion community is designing an advanced tokamak@20O2 by The Japan Society of Plasma

Corre sponding author's e -mail : dmeade @ pppl. gov

Science and Nuclear Fusion Research143

Meade D. et al., FIRE, Exploring the Frontiers of Burning Plasma Science

device known the Fusion Ignition Research Experiment(FIRE). The primary mission of FIRE is: to attain,
explore, understand and optimize magnetically-confinedfor these experiments and should be specified in termsof the natural plasma time scales such as tE = | s, t*e =5 s and ?.ti, - 10 - 15 s. The goal for FIRE pulse
duration of 20 s at l0 T and 30 s at 8.5 T is sufficientfor >10 rp for pressure profile evolution, > 4 q1" foralpha ash transport and burn control, and - 2 ?r1;n forplasma current profile evolution in advanced regimes.

fusion-dominated plasmas [,2]. This will require
plasmas dominated by alpha heating (Q = l0) that are

sustained for a duration longer than characteristic
plasma time scales such as the energy confinement time(?E), the alpha ash confinement time (rg"), and thecurrent redistribution or skin time (46n). The FIRE preconceptual design activities have been undertaken withthe objective of finding the minimum size (cost) device
to achieve the essential burning plasma science goals.
The work is managed through the Virtual Laboratory forTechnology and is funded by the U. S. Department ofEnergy. The FIRE experiment is envisioned as part ofan intemational multi-machine strategy that consists of:

3. Optimization of a Burning Plasma
Experiment
A systems study was undertaken to find theminimum size burning plasma to access the physics

requirements discussed above. This study was
specialized for inductively-driven tokamaks withtoroidal field (TF) and poloidal field (PF) coils that are

pre-cooled to LN2 temperature and then heatedadiabatically during the pulse. The systems codeburning plasma experiment(s), very long-pulse nonburning experiments in advanced configurations(advanced tokamak, advanced stellarator, etc,) and ahigh fluence fusion materials irradiation test facility[3,4]. These individual facilities could be in the -$lB
scale or less, and could be led by parties sited aroundthe world. This distributed international program has

includes constraints for stress, resistive and nuclear
heating of the coils and volt-sec requirements. The
geometry can be chosen to have TF and PF coils
unlinked as in FIRE or linked as in low aspect ratio or
spherical tokamaks. The code optimizes the allocation of
the space in the inner coil stack between the freegreater flexibility, less technical and management risk

standing ohmic solenoid and the wedged TF coil. The
confinement is taken to be H-mode with ITER98 (y,2)
scaling. For these studies, the plasma density had asmall density peaking of n(0)/<n> = 7.2, nlncw 3O.75
and 37o Be impurities due to sputtering of first wall. The
systems code varied the major radius, R, and aspect

than a single large facility such as ITER which isestimated to cost $5B.

2. General Physics Requirements for a
Burning Plasma Experiment, FIRE
The first goal of FIRE is to carry out burning
plasma experiments to address confinement, MHD

ratio, A, with H(y,2) = 1.1, Kss = 1.8, 4"yr = 3.1 andPru"ion = 150 MW to obtain plasmas with a Q

stability, fast alpha physics and alpha heating and edge
plasma issues expected in fusion reactor scale plasmas.
For these experiments alpha heating must dominate theplasma dynamics, therefore fo, the fraction of plasma- l0 and a

20 s burn time. For these constraints, the smallest size
device to achieve the burning plasma requirements for acryogenically-cooled inductively driven tokamak with
unlinked TF/PF coils has a shallow minimum around A= 3.6, B = 10 T and R = 2.1 m as shown in Fig. 1. The
normalized burn time measured in plasma current
redistribution times, t1 = ?bu.n/T.kin, shown in Fig. I
increases significantly as the aspect ratio is increased.
The minimum aspect ratio that satisfies the physics
requirement of 2 q is A > 3.4. The advanced tokamak

heating due to alpha particles, must be greater than 50Vo.

This in turn requires that the minimum Q = Prurnnl) 5. The goal for the design is to achieve Q =10, with ignition not precluded under slightly more
optimistic physics.
FIRE is also being designed to study burning
plasmas in advanced configurations in a later phase as
an extension of the existing advanced tokamak program.
For these experiments, it will be desirable to study
regimes that are bootstrap current dominated,/5"> 50Vo
(P* > 2.6) with the possibility of exploring /, . trp to 7 5Eo(pN - 3 6) These regimes will require strong plasma
Pext. heatfeatures of significant bootstrap current are alsoenhanced at higher aspect ratios. Indeed, the fusion
power plant design studies based on advanced tokamak
scenarios have all chosen A = 4 and relatively highmagnetic field such as 8.2 T for ARIES-RS [5] and I I T

for ASSTR2 [6]. These considerations have lead to the
choice ofA = 3.6 for FIRE that is higher than the aspectratios chosen for other burning plasma experiments(ITER-FEAT; A = 3.1 and IGNITOR: A = 2.8).

shaping and stabilization of the n = 1 kink by aconducting first wall or feedback.
The pulse duration is a very important requirementr44

Meade D. et al., FIRE, Exploring the Frontiers of Burning Plasma Science

Table 1 FIRE, Parame,"r. 61 @ = 10

2'8R(t) ,.,

R (m), a (m)2.r4,O.595

4'

2.0, 1;77

r6s

6. 6*

0.7, = 0.4-{.55>3QN

B,(&) (T), /o(MA)10.7.72.2

O=Pr.';."/(P""^+Pos)

10

2.4

H98(y,2)l.l?r' (s)

1.04

P"

1.81

P*,/Pr", t

r.3

1.8

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Aspect Ratio, A
Fig. 1 Optimization of an inductively-driven copper coilburning plasma experiment.

2., (3Vo Be + He (5 r"))*r.4

RV p"(Eo)

3.8

t Margin for H-Mode Power Threshold* He ash accumulated with 2""= 5 7t4. FIRE Gonfiguration and General
Parameters
Wedged TF Coils (16)The FIRE engineering activities have focused on
the physics and engineering assessment of a compact,

TI

copper-alloy conductor, high-field tokamak with thecapability of achieving Q = l0 in the Elmy H-mode fora duration of - 1.5 plasma current redistribution times(skin times) during an initial burning plasma science
phase, and the flexibility to add advanced tokamak
hardware (e.g., lower hybrid current drive) later. The
configuration chosen for FIRE is similar to that ofadvanced tokarnak designs such as TPX [7], KSTAR [8]and ARIES-RS [5], namely a highly shaped plasma,with double-null divertor and aspect ratio = 4. The key"advanced tokamak" features are: segmented central
solenoid for f'lexibility and strong plasma shaping.
double null poloidal divertors, low toroidal field ripple(< 0.3Vo), internal control coils and space for wallstabilization capabilities. The reference design point is
Ro = 2.14 m, a = 0.595 m, B,(R.) - l0 T, /e = 7.7 MAwith a flat top time of 20 s for 150 MW of fusion powerwith the parameters and cross-section shown in Table I
and Fig. 2.

rCompression Ring
Wall
Vacuum Vessel

ShieldingControl Coil

StabilizersnW-pin Outer Plale
actively cooled

Fig. 2 FIRE Configuration

investigating the onset of alpha-driven modes at thelower densities and to optimize the edge plasma for
confinement studies and optimal divertor operation. The
performance of FIRE can be projected by selecting JET
data with parameters similar to FIRE, namely BN > 1.7,
Z"rr<2.0, K> 1.7 and2.7 1egs13.5. The average H(y,2) and density profile peaking, n(0)/<n>y for these data

5. Plasma Performance Projections for Elmy
H-Mode Operation

was found to be 1.1 and 1.2, respectively. This isconsistent with the analysis of JET H-mode data
presented by Cordey et al [9] that shows that the H

The physics issues and physics design guidelines

for operating modes and projecting burning plasma
performance in FIRE are similar to those for ITER-

factor is higher at higher plasma cross-section

FEAT. However, the natural operating regime for FIRE
is better matched to the existing H-mode database, andFIRE can access the density range from O.3 < n/n6y1 <1.0 through a combination of pellet fueling and divertor

triangularity, and has a maximum at nln6y1= 0.6. A 0-D
power balance calculation was used to determine the Qvalue in FIRE as a function of H-factor as shown in Fig.
3. The density profile was assumed to have n(O)/<n>, =pumping [1,2]. This flexibility is important for
t45

Meade D. et al., FIRE, Exploring the Frontiers of Burning Plasma SciencePo*€r (MW)

30

50

25
Pfusion = 150 irw,tO/'s I I t"*w

f*.

nln6yy = 9,7

20

20-+- n(o)/<n>v E 1.5+ n(o)/<n\=1.2

o
15

10

10

I
'o

I

I
[*."

Q * 11.&
Alph

P$war

I

t\

I

I

Ailx i{ary PNst
Jt

,

$*

-5

o 4 I

12

nj:t*)

20 24 28 t2

Fig. 4 Evolution of an H-mode plasma.

0

0.7

0.8 0.9 I

1.1 1.2 1.3 1.4

Neoclassical tearing modes (NTMs) pose aH98(y,2)potential threat to the achievement of the required Blr

Fig. 3 Fusion Gain for FIRE

values in tokamak burning-plasma experiments such as
FIRE, since the polarization-current stabilization model
predicts that the critical Bp for their onset scales like p*.
The value of p* in FIRE is intermediate between that inpresent-day tokamaks such as JET and that in ITER-

1.2 (x points) or 1.5 (A points) with 3Vo Be and selfconsistent alpha ash accumulation. The slightly more

peaked profiles are consistent with density profiles
observed during neutral beam heating in DIII-D Hmodes and more recently on JET. FIRE would aim toachieve this modestly peaked profiles using pellet
fueling in ICRF heated discharges. On this basis, FIRE
would be expected to achieve Q > lO for JET-like Hmodes. Typical plasma parameters for a Q = l0 plasma
are given in Table 1. Physics based models using
marginal stability transport models such as GLF23 also
predict Q values around 10. These models depend
sensitively on the value of the H-mode temperaturepedestal which is projected to be higher for plasmas
with high triangularity, and pedestal density low relative
to the Greenwald density. A next step experiment, suchas FIRE, would provide a strong test of these models
and improve their capability for predicting reactor
plasma performance. A I ll2 -D Tokamak Simulation
Code (TSC) [0] simulation of this regime with H(y,2)= 1.1 and n(O)l<n>y = 1.2 indicates that FIRE canaccess the H-Mode and sustain alpha-dominatedplasmas for a duration > 20 tE, ) 4 t'" and - 2 ur11n as
shown in Fig. 4. In addition, time is provided for plasma

FEAT, and NTMs might arise in FIRE for the reference

values of Bp (1.5 - 2.0). For this reason, NTM
suppression by feedback-modulated LHCD is being
evaluated. Calculations with a LHCD model in the TSC

code have shown that a 10 MW 5.6 GHz system with

50/50 on/off modulation should be capable of
suppressing the mln = 312 mode up to B'o = 2.9.

6. Advanced Modes in FIRE
The standard regime in FIRE without wallstabilization is limited by kink instabilities to ft.r < 3 andbootstrap factions, /5. 3 50Vo. Exploitation of advanced

tokamak regimes requires stabilization of the low nkinks as recently demonstrated on DIII-D [ 1,12]. If thez = 1 kink could be stabilized by a conducting wall or
feedback in FIRE, then advanced tokamak regimes with
B* < 3.6 and/6, < 757o are possible. TSC has been used
to determine the current drive, plasma heating powerand energy confinement required to dynamically accessthese advanced regimes in a burning plasma. The
example shown in Fig. 5 has B = 8.5 T, Ir= 5.5 MA,
which confines alphas very well, and the coils would
allow burn times up to 35 s. LHCD is calculated selfconsistently using LSC for density profiles with n (0)/<n> = 1.5. This quasi-steady reversed shear discharge
attained ft.r = 3.0,,6. = 64Vo and Q = 7.5 for moderately
enhanced confinement of H(y,2) = 1.6, and was 1007o
non-inductively driven after 11 s. Exploitation of thesestartup and controlled shutdown to avoid plasma
disruptions. The burn phase is sufficiently long to allow
investigation of plasma profile evolution due to alphaheating, accumulation of alpha ash, testing of techniques

for controlling fusion power output and the initialstudies of plasma current evolution due to alpha heating.

t46

Meade D. et al., FIF.E, Exploring the Frontiers of Burning Plasma Science

50

Power (MW)FIRE experimental program will utilize a variety ofpulse lengths and field levels with the pulses per year(-2,000) and duty cycle (-0.001) similar to existing

Current (MA)40large experiments.

FIRE will provide reactor relevant experience for30
RF +divertor and first wall power handling since theanticipated thermal power densities on the divertor
plates of -5 MWm"2 for detached operation and -20
MWm-2 for attached operation exceed present

LHCD-

20

experiments and approach those anticipated for ARIESRS. The D-T experiments on TFTR and JET observed
tritium retention fractions of = 15 to 3OVo with carbon
limiters and divertor plates. This large retention fraction

10

01020
Time, swould have a significant impact on the operationalschedule of a burning plasma experiment and reactor,
and would require periodic shutdowns to remove theFig. 5 Advanced Mode in FIRE

excess tritium inventory. FIRE would use only metallic

regimes will require stabilization of the n = 1 kink,either by feedback from coils mounted in the first wallof the FIRE vacuum vessel or by a method to rotate the
FIRE plasma, and an improved long-pulse capability for
the FIRE internal components. An important feature of
the FIRE cryogenic copper alloy magnets is that thepulse length increases rapidly as the field is reducedwith flattops of - 40 s at 8 T, - 90 s at 6 T and 240 s at
4 T. The primary limitation to exploiting this long pulse

materials for plasma facing components, and carbon
would not allowed in the vessel due to tritium inventorybuild-up by co-deposition. The divertor plasma-facing
components are tungsten "brush" targets mounted on
copper backing plates, similar to a concept developed by

the ITER R&D activity. The outer divertor plates andbaffle are water-cooled and with a time constant of afew seconds come into steady-state equilibrium during
the pulse [4]. The first wall is comprised of Be plasmasprayed onto copper tiles. The neutron wall loading in
FIRE is -2 MWm-2 and produces significant nuclear
heating of the first wall and vacuum vessel during the
20 s pulse. The inner divertor targets and first wall arecooled by mechanical attachment to water-cooled
copper plates inside the vacuum vessel and have athermal time constant of about 40 s. This is the primary
limitation to exploring longer high power pulses on
FIRE. Sixteen cryo-pumps - closely coupled to thedivertor chambers, but behind sufficient neutron
shielding - provide pumping (> 100 Pa m3/s) for D-T
and He ash during the pulse. Pellet injection scenarioswith high-field-side launch capability will reduce tritium
throughput, and enhance fusion performance. The indevice tritium inventory will be determined primarily by
the cycle time of the divertor cryo-pumps, and can range
from < 2 g for regeneration overnight to -10 g forweekly regeneration. The tritium usage per shot andinventory is comparable to that of TFTR and therefore
will not require a large step in complexity beyond
previous US fusion program experience in regulatory
approvals for tritium shipping and handling.
The construction cost of the tokamak subsystem(magnets, divertor, plasma facing components andmagnet capability is the generic magnetic fusion
problem of handling the plasma exhaust power underreactor relevant conditions.

7. Engineering Description of FIRE
The baseline magnetic fields and pulse lengths canbe provided b1'wedged BeCu/OFHC toroidal field (TF)

coils and OFHC poloidal field (PF) coils that are precooled to 77 "K prior to the pulse and allowed to warm
up to 373 oK at the end of the pulse [13]. 3-D finiteelement stress analyses including electromagnetic, andthermal stress due to ohmic and nuclear heating haveshown that this design has an additional margin of 307obeyond the usual engineering allowable stressrequirements. Large (1.3 m by 0.7 m) midplane ports
provide access for heating, diagnostics and remotemanipulators, while 32 angled ports provide access tothe divertor regions for utilities and diagnostics. FIRE,
like the previous BPX design, is being designed

mechanically to accommodate 3,000 full field, fullpower pulses and 30,000 pulses at 213 field with a total
fusion energy production of 5.5 TJ. The repetition timeat full field and full pulse length will be < 3 hr, withmuch shorter times at reduced field or pulse length. The
t47

Meade D. et al., FIRE, Exploring the Frontiers of Burning Plasma Sciencemechanical structure) has been estimated by U. S.
industry to be = $350 M (FY02US) including $71 M ofcontingency. Another = $850 M would be required for
auxiliary heating, startup diagnostics, power supplies
and buildings to put the project at a new site. If an
existing site is found with about $200M of site credits

divertor may also significantly reduce the frequency andintensity of vertical displacement disruptions which is a

(FY02US). The construction schedule for FIRE would

critical issue for the feasibility of a tokamak based
reactor. The high power density in FIRE poses a
significant challenge for the divertor and first wall
designs, but this is a generic issue for magnetic fusion.
The success of FIRE in this area would provide yield
important benefits for technology development forbe 6 to 8 years depending on the annual funding

future fusion devices.

available. The first year of operation would utilizehydrogen plasmas, followed by two years of deuterium
plasmas with tritium operation beginning in the fourth
15 years.

A critical issue for all next step experiments is tosupply auxiliary heating power at high power densities
to a fusion plasma. FIRE proposes to use ICRF heating
which has been demonstrated on existing experiments
but the high power densities and neutron wall loading

8. Assessment of FIRE

present in FIRE will require significant plasma
technology R&D. This R&D will be needed if ICRF isthen the overall project cost would be about $1Byear. The total experimental program would last about

FIRE is a natural extension of the existing state of
the art tokamaks, and is based on the extensive
international H-mode data base for projecting

to be used in a fusion application.
The FIRE design study is a U. S. national activity

performance to the burning plasma regime. Due to theTechnology. PPPL work supported by DOE Contract #DE-4C02-76CHO3073.

managed through the Virtual Laboratory forhigh magnetic field, the extrapolation required to attain

Q = l0 is a modest factor of 3 in terms of thedimensionless confinement time (@c?E - Bre). While

this reduces the uncertainty in attaining a burningtll

plasma, it does not extend some plasma parameters(e.9., p*) to full reactor values. The MHD stability
characteristics of FIRE, with qe5 = 3.1 and fi' = 1.8 for
initial burning plasma experiments, are similar to thestandard MHD regimes in existing tokamaks and will
explore the synergistic effects of energetic alphas andReferences
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t2l D.M. Meade et al., l4th ANS Topical Meeting on
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begin to explore the important areas of neoclassicalsorrento, IAEA-CN-77 ETP2I 14 (2000).
[7] J.A. Schmidt et al., J. Fusion Energy 12, No. 3, 221

tearing modes (NTM) and resistive wall modes (RWM).
Lower hybrid current drive and feedback stabilization is
being evaluated as an experimental tools to investigate
the control of NTMs and RWMs. Divertor pumping andpellet fueling will allow FIRE to vary the density, hence( 1 993).

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TAE modes.
The dcluble null divertor configuration produces thestrongest plasma shaping which is critical for resolving
and exploiting a number of physics effects related toconfinement and MHD stability. The double null148