DSP Application to Parallel Processing in JT-60 Plasma Control
T. Kimura a, K. Kurihara a, Y. Kawamata a, K. Akiba b and H. Adachi aa

Japan Atomic Energy Research Institute, Naka Fusion Research Establishment
801-1 Mukoyama, Naka-machi, Naka-gun, Ibaraki-ken 311-01, Japanb
Mito Software Engineering Co.
1-3-1, Ohmika-cho, Hitachi-shi, Ibaraki-ken 319-12, JapanAbstract
A real-time plasma shape visualization system has beendeveloped for plasma control in the JT-60 tokamak. Fast
digital signal processors (DSPs) having inter-processor
communication ports are applied to parallel processing inthis system. The optimum number of the DSPs wasestimated according to the degree of the parallelism andallowable delay in the process of the plasma control. Thissystem can visualize time evolution of the plasma shapeclearly every 60 ms in real time. It has been fully used formonitoring the plasma behavior in the tokamak
experiments.
1 Introduction
Plasma shape control in tokamak-type fusion devices
like JT-60[1] and ITER[2] is one of the important issues
for improving plasma performance. For example, plasma
confinement performance depends on the shape of theplasma column cross section like ellipticity andtriangularity[3]. A gap between plasma surface and anantenna of a radio-frequency (RF) wave heating system ismore essential factor for increasing its plasma heating
efficiency than the plasma column center position. It isvery important for protecting in-vessel components likedivertor tiles and for producing desirable plasmas tocontrol strike points of separatrix lines on divertor tiles.
From the above points of view, we have developed areal-time plasma shape visualization system for plasma
control in the JT-60 tokamak. Fast digital signal processors(DSPs) having inter-processor communication ports areapplied to parallel processing in this system.
Section 2 of this paper describes an overview ofcomputational procedure in the plasma shape identification.
The structure of the parallel processing for identification isshown in section 3. The configuration of the real-time
plasma shape visualization system and its performance aredescribed in section 4. Final section has concluding
remarks including the prospects of the system inapplication to real-time plasma control.

outermost magnetic surface of the plasma using the fluxfunction, which is expressed byφ(r, z) = ∑ ai(r, z, h)⋅φi + ∑ bj(r, z, h)⋅Bj+ ∑ ck(r, z, h)⋅Ick + d(r, z, h)⋅Ip ,where φi, Bj, Ick and Ip are the measured values of magnetic
flux, magnetic flux density, poloidal field coil current andplasma current respectively, and ai, bj, ck and d are thecoefficients given by both the observation point incylindrical coordinates (r, z) and the parameter vector h. In
the FCP approximation method, for example, this vector isdefined as the locations of all the filament currents. We cancalculate the coefficients in Eq.(1) for all possibly utilized
positions (r, z) in the 2-dimentional space of the vacuum
vessel cross section and the parameter vector h in advanceof obtaining the flux function values. We can store thesecoefficients on a large-capacity memory as a tabular form
corresponding to the positions. Hence, we can execute fastcalculation of the flux function values by the table look-up
method.
The algorithm to obtain the outermost magnetic surfaceof divertor plasma is expressed as follows [5]:•Step 1: The position of the divertor X-point issearched as a saddle point on the three-dimensional curved
surface (r, z, φ(r, z)).
•Step 2: The flux values at the fixed limiters on thefirst wall are calculated for investigating whether theplasma configuration is limiter one or divertor. Thesmallest flux value φmin among those at the limiters and theCL

PoloidalField
Coil
Filament
Current

Divided into
5 Regions(a)-(e)(b)

Plasma(a)

2(1)

X(c)

Computational procedure of the plasma shapeidentification

So far various methods like the filament-current-plasma(FCP) approximation, Legendre-Fourier expansion andboundary integral equation-based methods have been
proposed for the plasma shape identification [4], where
magnetic sensors such as flux loops, magnetic probes andRogowski coils are used. We can obtain the shape as the(d)Flux Loop(15)X-point(e)Fig. 1 Search for the plasma boundary.

VacuumVesselX-point indicates the flux value of the plasma boundary.
•Step 3: The contour of the curved surface having φminis searched. This contour corresponds to the outermost
magnetic surface of the plasma, i.e. the plasma boundary.

DigitalInputs

KG40S1

SMT-305

It is possible to divide each of the computational steps forthe plasma shape identification into plural and parallel
processes. For example, the computational step for theboundary search (step 3) is divided into five parallel
processes corresponding to four regions of the main plasmaboundary (a)-(d) and a region of the divertor separatrix lines(e) as shown in Fig.1. Hence, we applied parallel processing
to the real-time plasma shape visualization system so as torealize fast computations. We adopted the TMS320C40
DSP (Texas Instruments Co., U.S.A.) as processors in thisparallel processing [6], because this DSP has six
communication ports available for fast inter-processor datatransfer. Another reason why we selected this DSP is that we
can use a compiler named "Parallel C" (3L Ltd., UK) on thisDSP-based platform.
We can easily define theconfiguration of this parallel processing system and assign
the computational processes to each DSP using thiscompiler.
Two kinds of VME bus-based DSP boards namedDSP8040 and DSP8044 (MTT Corp., Japan) are applied tothis system. The DSP8040 board has two KG40S1 modules(MTT Corp., Japan) with VME bus interface. Each module
is equipped with one TMS320C40 DSP running at 40 MHz
and three 256 kB SRAMs. On the other hand, the DSP8044
board has two SMT305 modules (Sundance Multiprocessor
Technology Ltd., UK) without VME bus interface. Each ofthese modules, however, provides two 16-MB DRAMs andtwo 1-MB SRAMs in addition to one TMS320C40 DSP.
These DRAMs are used for installing the look-up table
described in section 2.
The performance of the TMS320C40 DSP is summarizedin Table 1. Data of fifteen flux loops, four poloidal field coilcurrents and a Rogowski coil are used for calculating theflux function values by Eq.(1). 6.5 megabytes of memory
are allocated for a set of the coefficient table corresponding
to one pattern of h in application of the FCP approximation
method to JT-60. The number of flux computations required
for the plasma boundary search amounts to about 1500.

ItemTime (µs)

Multiply and add
Scalar product ofvectors with 20components
Interpolation
betweentwopositions
Comparison oftwo values
Datatransfer(4 Bytes)

0.05
1.60
1.50
0.35
0.85

KG40S1

SMT-305

Root

Node 2

Node 6

FC&DSDC

DTG

FC&DS

Node 5

Node 1

Node 3

Node 7

FC&DS

PBS

PBS

FC&DS

SMT-305

KG40S1

KG40S1

SMT-305

8044

8040

8040

8044

DC:Data Communication, DTG:Data Transfer to the Graphic Processor
FC&DS:Flux Calculation & Data Storage,PBS:Plasma Boundary Search

Fig. 2 Topological structure of the parallel processing system.

Taking the degree of parallelism in the identification
processes into consideration in addition to the above data,we estimated the optimum number of the DSPs for thissystem [7]. Fig. 2 shows the topological structure of thesystem, where eight DSPs are connected with each otherthrough the communication ports. Four SMT305 modules
having two 16-MB DRAMs (nodes 4-7) are dedicated forthe parallel processing of the flux function value
computations. One KG40S1 is assigned as the root
processor for data I/O. Another one (node 1) is used forsending the plasma boundary data to a graphics processor.
The other two KG40S1 modules (nodes 2 and 3) are used for
deciding the plasma boundary positions from the results
calculated by the SMT305 modules.
4 Configuration and performance of the real-time
plasma shape visualization system
The real-time plasma shape visualization system for theJT-60 operation is a VME bus-based multiprocessor system.
As shown in Fig. 3, two DSP8040 boards and two DSP8044
ones are installed in a VME rack. A reflective memory (RM)
board named VMIVME-5576 (VMIC Co., U.S.A.) is also
installed for fast communication of input signals of themagnetic sensors and the coil current data. For drawing theplasma shape on a monitor, we adopted a TMS34020-based

CPU

DDDSSSPPP
M1 M2 S1VME Rack
DSP
S2

GSP

Communication Ports
SymbolDSP/M1, M2
DSP/S1, S2
GSP
CPU
DIO
RM

Graphic
Processor

Node 4

Ethernet
HostWSCPUMVME147

VME-bus3 Structure of the parallel processing

Table 1 Performance of the
TMS320C40 DSP

Reflective
MemoryType
DSP8040
DSP8044
UDC6000TI
MVME147
DSP8230
VMIVME-5576

Manufacturer (Nation)
MTT Co. (Japan)
MTT Co. (Japan)
Univision Tech. (U.S.A.)
Motorola Inc. (U.S.A.)
MTT Co. (Japan)
VMIC Co. (U.S.A.)

RM

DIO
Monitor
Clock
Electromagnetic Data
Functions / Specifications
TMS320C40, 40 MHz (KG40S1)
TMS320C40, 40 MHz (SMT305)
TMS34020-based graphics accelerator
MC68030, 25 MHz, 32-MB DRAM
64-bit digital inputs/outputs
256-kB reflective memory, 6.2 MB/s

Fig. 3 Configuration of the real-time plasma shape visualization system.

the stack area to the internal registers of the DSP.
By taking these measures, we can improve theprograms so that the DSP will give its fullperformance.
5 Concluding remarks

Fig. 4 Example of the reproduced plasma shape.

graphics processor named UDC6000TI (Univision
Technologies Inc., U.S.A.). A 16-bit digital inputs/outputs
board named DSP8230 is used for input of a clock signal. A
32-bit MC68030-based CPU board is dedicated forsupervising this VME system and communicating with its
host workstation via an Ethernet LAN.
The real-time plasma shape visualization system has been
applied to the JT-60 divertor operation since the last June.
This system can display the plasma shape on the monitor
about 16 times per second. Then, we can easily observe thesequential behavior of plasmas such as formation of limiter
plasma and configuration transition from limiter to divertor
with the X-point in addition to the oscillation of the plasma
position. An example of the reproduced plasma shape isshown in Fig. 4. Time evolution of the plasma current andthe horizontal and vertical positions of plasma column
center can be displayed as well as that of the plasma shape.
The functions of slow playback and enlargement of thedivertor region are also available for detailed observations.
It takes about 47 ms to identify the plasma shape andabout 13 ms to draw it. In the design of the visualization
system, however, we had estimated the time required for theidentification to be 3 ms. The reasons why the actual
execution time required for the identification is about 16
times longer than the estimated one are supposed to be asfollows. First the program assignment to each DSP has not
yet been optimized. Second, we have not yet made effective
use of the function of pipeline processing which the DSP
itself possesses. For example, we have not yet changed theorder of decision and branch instructions in the programs sothat they may not interrupt the pipeline processing. When
we simply compile source programs using Parallel C, atemporary save area for the data in the middle ofcomputation is allocated to the SRAM on the global bus.
Hence, it is necessary for efficient pipeline processing tomodify the compiled programs manually so as to allocate

For the operation of the JT-60 tokamak, wedeveloped the parallel processing system for realtime plasma shape visualization, where VME busbased DSPs, a graphics processor and a reflectivememory are applied. This system can reproduce theplasma shape about 16 times per second and it has
been fully used for analyzing plasmas in theexperiments since this June. The real-timevisualization system can also display the time
evolution of the plasma current and the plasma
horizontal and vertical positions along with thetime evolution of those used in the feedback control
system for the plasma position and current. We cancheck the integrity of the magnetic sensors andtheir signal conditioners used in the feedback
control system.
Hereafter, we are going optimize the programs of thepresent system in order to increase its processing speed. Inaddition, we are going to develop the algorithm for
improving the computational stability in the identification.
Our goal is to apply this system to real-time feedback
control of plasma position and shape for improving theplasma performance.
Acknowledgments
The authors would like to thank the other members of theControl Group of the JT-60 Facility Division I at JAERI for
their cooperation in developing the system.
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Incorporation (1991).