Control of plasma parameters by the externally
applied toroidal field in RFX-mod
Euratom – ENEA
Association
M. Cavinatoa, R.Cavazzanaa, A. Luchettaa, G. Manduchia, G. Marchioria, A. Soppelsab,
M.Spolaorea, C. Taliercioa
a Consorzio RFX, Associazione Euratom ENEA per la fusione, Corso Stati Uniti 4, I-35127 Padova, ITALY
b Università degli studi di Padova, Electric Engineering Department, Via Gradenigo 6/a, I-35100, Padovs, ITALY
ABSTRACT
The recent modifications brought to the RFX experiment, now RFX-mod, pose a strong focus on the active control of the
plasma magnetic configuration. The new machine layout features a thin stabilizing shell which makes active control feasible
and necessary at the same time. The new set of actuators includes a complex saddle coils system and a redesigned power
supply system driving the toroidal field coils.
The new toroidal power supply system allows a great flexibility since each sector, composed by four coils, is driven by an
independent d.c./d.c. switching converter, in series with the main power supply providing the bias current. The current control
on the load, performed by the four quadrant fast switching converter, allows producing the desired waveforms of toroidal field.
To fully exploit the capabilities of such a system, two different controllers have been designed enabling to track
automatically a selected evolution of either the toroidal field at the plasma edge or of the F parameter respectively.
The new control systems allow to perform experiments in specified plasma configurations targeting particular physics issues and
improve the overall reproducibility of the RFX-mod discharges.
CONTROL MODELS
Equivalent poloidal
current producing Bφ(a)
Two possible approaches were considered for the control of Bφ:
voltage control and a current control.
In the voltage input model the system can be seen as a
transformer with a primary circuit whose current is the toroidal
winding current and two equivalent secondary circuits whose
currents are the poloidal components of the vessel and plasma
currents, respectively.
The advantage of this voltage input model lies in the possibility
of retaining a description of the physical interactions between
the elements contributing to the toroidal magnetic field
configuration.
Toroidal windings
Resistance & stray
inductance
Loop to account for
shell penetration time
The model for the current control is derived by identification
of the transfer function linking the toroidal winding current
reference to the actual current flowing in the winding.
Vθ Experimental
Vθ Simulated
Two analytical expressions in the variable s domain were
evaluated: the former by a simple exponential fitting, the
latter by an identification procedure based on the ratio
between the Fourier transforms of the current and the current
reference.
Experimental validation of the voltage model
CONTROL SYSTEM DESIGN AND EXPERIMENTAL RESULTS
INTRODUCTION
5
6
The peculiarity of such configuration lies in the fact that the toroidal field in the
plasma outer region reverses respect to its direction in the axis region.
During the pulses, the toroidal field applied by the external windings is varied in
time to enhance and control the field reversal.
The main parameters characterising an RFP configuration are the pinch
parameter Θ=Bθ(a)/< Bφ > and the reversal parameter F= Bφ(a)/< Bφ >
Firstly the open loop control of the reversal parameter F was implemented
and tested.
Ip [A]
4
Even the simple open loop F control scheme proved very effective in
enhancing the experiment reproducibility and it was routinely used in
experiments since its introduction.
2
0
0
0.05
0.1
The feedback control of the reversal parameter is based on the closed loop
control of the toroidal field at the wall.
RFX-mod is equipped with a set of 48 toroidal field windings which guarantees a
field ripple below 4% of the total value.
The single coil is rated for a peak current of 18.3kA and a flattop current of
11.5kA.
The coils are grouped in twelve sectors each composed by four coils series
connected by copper busbars.
0.2
-0.2
-0.4
The active control of the F parameter has allowed operation with shallow
reversal, with F between 0 and 0.02, usually subject to a rapid reversal loss
and consequently to an abrupt plasma termination.
Shot # 20368
Shot # 20372
0
0.05
0.1
Time [s]
The described control scheme allowed to obtain rapid F variations (∼1ms) as
well as inducing periodic F oscillations, required during the Oscillating
Poloidal Current Drive operations, where the periodic variation of F are used
for the study of improved plasma confinement regimes.
-0.05
-0.1
-0.15
Step
-1/R_vt
Bt wall ref
-K-
k_reg*z_reg(s)
num(s)
Itor_ref
p_reg(s)
It
den(s)
K1
Saturation
Regulator
G1(s)
Controlled F
0
Disturbance
Model
Signal 6
TOROIDAL FIELD SYSTEM
0.15
0
F
The toroidal field at wall, Bφ(a), was controlled by means of preprogrammed current waveforms in the
toroidal windings. In such a situation since the total toroidal flux is mainly driven by the plasma current,
any toroidal current variation induces a change of plasma regime. This strongly reduces the reproducibility
of the experiments.
0.2
0.2
Uncontrolled F
MOTIVATIONS
0.15
Time [s]
F
RFX-mod is a Reverse Field Pinch (RFP) experiment with R/a=2m/0.46m.
x 10
-0.2
-0.25
-K-
Transport
Delay
-0.3
-0.35
Sine Wave
-0.4
0.05
K2
0.1
0.15
0.2
Time [s]
-K-
1
Out1
Reference F
Measured F
Bφ control system scheme
CONCLUSIONS
TOROIDAL FIELD: PULSE SEQUENCE
The new control systems implemented permit to fully exploit the capabilities of the new RFX-mod toroidal field system.
TFAT
TC DB
TCAC
At the pulse start the TCIS switches are closed and the TFAT
converter charges up the windings for the bias toroidal field.
Label
Description
TFAT
3 kV @ 16kA ac/dc
thyristor converter
TCDB
4 kV @ 5.5
blocking diodes
kA
TCCB
4 kV @ 16
capacitor banks
mF
TCIS
TCCB
TCCH
TCCH
3 kV @ 3/4.6 kA
IGCT chopper
TCAC
3 kV @ 6 kA, dc/ac
IGCT inverter
TCIS
4 kV @ 16 kA @ 128
MA2s, switches
TCIS
TCIS
The plasma current rises, the TCIS switches are opened and
the current flows to the TCCB capacitor bank with the TCCH
choppers controlling the voltages (max 3kV)
The dynamic control the F parameter allowed to explore areas in the F,θ parameters space impossible to reach with the pre-programmed
setting of the toroidal windings current
The tracking capabilities of the F controller allow also to follow fast oscillations used for the study of improved plasma confinement regimes.
Once the reversal current reaches the desired level, the
TCAC inverters sustain it and the TFAT converter is used as
a primary supply to the six inverters.
During the flattop the TCAC inverter control the sector
currents. This is the phase in which the designed controllers
are active.
Sixth IAEA Technical Meeting on
Control, Data Acquisition, and Remote Participation for Fusion Research
Research
4-8 June 2007, Inuyama,
Inuyama, Japan