The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Concept of the IFMIF Accelerator System
Masayoshi SUGIMOTO, Michikazu KINSHO, Masayoshi KAWAI, Michael CHERNOGUBOVSKY
Department of Reactor Engineering, Japan Atomic Energy Research Institute,
Tokai-mura, Naka-gun, lbaraki, 319-11 JAPAN
Abstract
The conceptual design activity of the International
Fusion Materials Irradiation Facility has been carried out forthese two years and the final design report was completed.
The design will be reviewed to proceed to the next step inboth the domestic and the international frameworks. A
simple system dynamical model is helpful to understand thebehaviors of the IFMIF accelerator system including theinjector, the rf linacs, and the transport lines.
1 Introduction

The International Fusion Materials Irradiation Facility(IFMIF) provides the intense neutron field to simulate thebehavior of the materials in the fusion reactor environment,
like the prior proposals as FMIT or ESNIT. The users'
requirements for the neutron irradiation field are given inTable 1[1]. The Conceptual Design Activity (CDA) of theIFMIF is proposed to the Fusion Power Program Committee(FPCC) and has been carried out between February 1995
and January 1997. The activity consists of four subgroups(accelerator, target, test cell and integration groups), andJapan, US, EU and Russia participate the design tasks.
The final design report [2] is completed in December 1996,
however the fully technical details are removed from thereport to keep the report size short. As a separated task,
the cost estimation is also carried out by each participant
country and presented as the informal documents. The
report discusses the interface issues among the subsystemsof the facility and treats the safety/reliability issues in moredetails relatively.
Table 1. Requirements for an Intense Neutron Source
Neutron Flux/
Equivalent to 2 MW1m2 in 10 L
Volume Relation
volume (MW/m2 = 4.5xl0 17 n/m2s)
Neutron Spectrum
Similar to First Wall neutron
spectrum (PKA!fransmutations)
Neutron Fluence
DEMO-relevant Fluences of
Accumulation
150 dJl!lNRT in a few years
Neutron Fluxs 10 o/o/cm based on dimensions of
Gradient
test specimen
Machine70%
Availability
Time Structure
Quasi Continuous Operation
Accessibility
Irradiation Volume for experiments
and Instrumentation
As a consequence of employment of such report style,it is necessary to refer each reference document for thetechnical design details. In the first part of this article thecomprehensive summary of the accelerator design conceptof the IFMIF is discussed, and in the second part the simple
system model to analyze the behavior of the accelerator
system is described. As the conclusion some possible
alternatives of the accelerator design concept are presented.
2 Accelerator System
From the long-term discussion since FMIT proposal,
the best neutron source reaction for the IFMIF is the d-Lireaction, which has a broad peak in the neutron spectrum
near the half of the incident deuteron energy,. Ed, at theforward angle. This property can be used to tailor theneutron spectrum simulating the fusion reactor neutron field
for each material at the various reactor environments.
However, the energy integrated neutron yield at the forward
angle from the thick lithium target behaves like Ed1.s, thehigher deuteron energy is desirable to obtain the higher
neutron intensity effectively. The required energy andintensity of the deuteron beam is estimated as 40 MeV and
250 rnA, respectively, to fulfill the volume/intensity
guidelines: 0.1 L for> 50 dpa/fpy; 0.5 L for> 20 dpa/fpy;
and 6 L for > 1 dpa/fpy. The 40 MeV deuteron provides agood neutron field for most metallic components but it isdesirable to reduce the energy for other materials likeceramics. From the result of the accelerator system design,
the energy step of 4 MeV is preferred to achieve the best
performance and the output deuteron energy can be selectedfrom 32, 36 and 40 MeV.
The cw rf linac is chosen as the deuteron accelerator
and two identical 125-mA linac modules are employed asproviding the beams to the targets independently. Thisconfiguration eliminates both the 250-mA injector and thefunneling scheme, which involve the highest technicaldifficulties.
It also improves the overall system
availability if the irradiation test can be continued during
the beam-off events of either of two linacs. Because thetemperature control system of test cell should be shut down
to prevent the annealing of the damage when the irradiating
neutrons are disappeared.
The frequency ofthe rflinacs is 175 MHz to eliminate
the frequency change in the system and to achieve the best
performance for 8-MeV RFQ and 40-MeV DTL. The
high power rf source in this frequency range can beavailable from Thomson CSF and EIMAC, however thelong range (> 100 hr as a primary goal) operation is notguaranteed yet.
The beam footprint at three target stations (2 lithiumtargets and 1 beam calibration dump) should be identical for-127-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

the beams. from two linac modules and the distribution isrectangular (20 em horizontal and 5 em vertical) with a flat
top region. The sharp edge in the horizontal sides and thesmooth tail in the vertical sides are required to keep theliquid lithium flow stable. The energy dispersion of ±0.5
MeV FWHM. is also required to make a margin from themaximum temperature to the boiling point inside the lithium
target. These requirements influence the HEBT design:•two-dimensional beam expander using the high ordermultipoles which has the different characteristics in
the horizontal and vertical directions;•space periodic transport system to carry the beam tothree stations with identical manner;•final bend to place two beams at the same footprint;•energy dispersion cavity (EDC) at the end of transport.
No combined function bend is considered to simplify thetransport design and, as a result, the two-story beamtransport with 5m level difference should be designed.
The levels of the corresponding linac modules have thesame difference. The 5m difference comes from the 10°injection at the final bend and the protection of the beamtransport elements from the neutron back streaming.
The availability goal is 70 % including the scheduled
maintenance (one month per year and 8 hours per week),so that the inherent system availability is calculated as
80.7 %. By taking into account the availability goals of
the other subsystems, the accelerator system availability
is required to be 88 %.
The reliability/availability/maintainability (RAM) model has been developed toallocate the availability budget to each subsystemcomponents: the major subsystems of the accelerator
module are the injector, RFQ, DTL, RF system, and HEBT,
as shown in Fig. 1. The failure modes of the components
are considered and then, for each failure mode, the meantimes between failures (MTBF) and the mean times torepair (MTTR) are estimated in the steady state operation.
The summary table of the RAM model of the accelerator
system is given in Table 2.

Table 2. RAM Model ofthe IFM1F Accelerator System
Subsystem
Injector
Linac
RF system
HEBT
Total

MTBF
[h]
159416
191
417
59.5

MTTR

[h]
218913
8.5

Reliabilit
~ [%]
34.8
76.1
42.3
67.7
7.6

Availabilit
~ [%]
98.7
95.8
95.5
97.0
87.6

For each component in the subsystems, the p.egative
exponential distribution and the independence among thefailure modes are assumed as-I

MTBF- ( ~ (MTBF; )') • MTTR - MTBF. ~ ( MTTR) ,II
MTBF iReliability: R _lJ R; _ lJ (exp( _mis~;ime)} ,Availability :A-IT A - rr( I +MTTR/MTBF. 1 ).'
i

Ii

I

where the suffix irepresents the failure mode.

3 System ModelIn the actual operation of such a high current
accelerator system, the beam loss control is the mostimportant problem to keep the hands-on maintenance during
the machine lifetime and to maximize the machine
availability. From tltis point of view, some design issues
are untouched in the CDAyet:(1) The current instability of the injector may produce themismatch to RFQ due to the change of the neutralization
process in the LEBT. A beam loss along the RFQ (andDTL, HEBT) can be occurred.
(2) The pulse mode operation is proposed at the startup timeand a transient part of the beam pulse may be lost. AnHEBT

Flowing Li Target

To the OtherTarget Stations

Fig. 1. Layout of the IFMIF accelerator system-128-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden ·City, 1997

alternative approach to use the cw beam with startingfrom the very low intensity has also a risk to loose theall beam before reaching the operation current. It isrequired to minimize the time-integrated beam loss by
optimizing the startup operation.
(3) The better availability is obtained by removing theunreliable components from the design. In this context,
the number of components in the HEBT system is not
optimized and may be reduced if the beam requirements
at the lithium target are reconsidered. For example, theEDC is required to increase the beam energy spread to±0.5MeV. Since the natural energy spread during thetransport will be ±0.2MeV, it can be unused if theboiling point margin in lithium target is moderate
although it depends on the progress of the target
technology.
These issues are closely related to the accelerator system
dynamics and a simple_ model to describe its behavior isconsidered. The top-level block diagram of the dynamicalsystem is shown in Fig. 2. The objective is the beamspecification requested for the irradiation tests at eachinstance. The maximum availability should be kept byminimizing the beam loss and the associated activation,
which are caused by the disturbance of the parameters andthe component failures.

Objective

Availability

Fig. 2 Top-level Block Diagram of Accelerator System.
3.1 Single Module Operation Model

Two modules of the IFMlF accelerator are operated
independently as far as either module is running normally.
At the off-normal events like beam trip, the following
recovery process can be taken:(a) a full intensity beam is immediately restored by using anautomatic control (recovery time< 10 sec),(b) the beam is restored by ramping the current with a minor
parameter adjustment(< a few min), or(c) an inspection or a repair is necessary and sometimes
both accelerator modules must be shut down to do so.
If it is evident that the recovery process will take more thana minute, the signal is sent to target and test-cell groups toadjust their temperature control systems.
The beam trip
is caused by failures of accelerator components and by theinterlock signals from the other subsystems. However, thelatter is neglected in the present analysis.
The component parameters are treated as the system
state variables and their behavior is modeled by various

stochastic processes (Poisson, Gaussian, etc.). The beamloss is also a state variable to be derived by the parametervalues and the beam dynamics calculation. In the case oflarge beam loss, the accelerator is shut off to avoid theexcessive activation. The obtained availability is strongly
influenced by the criterion of the acceptable loss level, and a
3 nNm goal is proposed at the CDA. At present, we aredeveloping the beam dynamics simulation code with highprecision to estimate the correct beam loss for the IFMlF
accelerator module. The results of the rough simulation
code indicate essentially no beam loss after RFQ and only
the component failures are taken into account in this region.
As a result the 88% availability can be achieved at theassumed failure rates.

a

3.2 Dual Module Operation Model

As described before, the operation of two modules has
a correlation when the repair is necessary for thecomponents placed except in the accelerator vaults, which
are separated by the heavy radiation shields. So the failure
rates of the HEBT or RF transport components are treatedas the correlated events, such as buncher rf power, EDC,and quads/bend chain at the end of HEBT. Although. thereliability of these elements is estimated to be very high andthe loss of availability becomes small, we need to pay anattention to the sensitivity of the input parameters (e.g.
MITRIMTBF ratio) to keep the good performance.
3.3 Extended Operation ModelAs an upgrade option, the additional two accelerator
modules are installed, and the beams from each two
modules are paired and transported into the same lithium
target. Therefore two lithium targets and test cells areused in parallel. The availability of this extended
operation becomes lower than 88% because the correlated
operation mode has a larger effect.

Conclusion
The conceptual design of the IFMIF accelerator
system is overviewed and the system availability model isdeveloped to analyze the effects of the component failure
and the resulting beam loss. The precise simulation of
the beam loss is urgent to obtain the confident activationlevel during the operation period.
References[1] Editors: J.E. Leis et al., Report on International Fusion
Irradiation Facility, Workshop San Diego, USA, Feb.
14-17, 1989.
[2] IFMlF CDA Team, ENEA Frascati Report, RT/ERG/FUS/96/11 (1996).

-129-