Overview on the power supply systems for plasma instabilities control

Fusion Engineering and Design 86 (2011) 565–571

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Overview on the power supply systems for plasma instabilities control V. Toigo ∗ , E. Gaio, R. Piovan, M. Barp, M. Bigi, A. Ferro, C. Finotti, L. Novello, M. Recchia, A. Zamengo, L. Zanotto Consorzio RFX – EURATOM – ENEA Association, C.so Stati Uniti 4, 35127 Padova, Italy

a r t i c l e

i n f o

Article history: Available online 31 May 2011 Keywords: Plasma instabilities control Power supply systems Tokamak RFP

a b s t r a c t The paper presents an overview on the power supply (PS) systems for plasma instabilities control in fusion experiments, based on active control coils. First, the MHD instabilities and the approach to their control in Tokamaks and Reversed Field Pinches (RFPs) are described. Then, the features of MHD modes controls presently used in fusion experiments are reviewed. For the control systems based on active coils fed by fast power supplies, the typical requirements in terms of power, dynamics, accuracy and delay are summarized and discussed. Then, a survey on the technology available to design these types of PSs is given, together with the most suitable circuit topologies and guidelines for the design, on the basis of solutions adopted in existing experiments. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The control of plasma instabilities has a key role for achieving advanced scenarios both in Tokamak and Reversed Field Pinch (RFP) experimental machines [1,2]. In RFX-mod, the largest RFP in operation, the improvement of the plasma boundary through feedback control of the MagnetoHydroDynamic (MHD) activity has been an essential element for enabling a new, self-organized plasma state called Single Helical Axis (SHAx) [2]. In Tokamaks, besides the axis-symmetric control of plasma position and shape, the optimization of the overall plasma performance and the capability to avoid plasma disruption or to mitigate its consequences require a large variety of additional controls, such as local control of error fields, sawtooth oscillations, Neoclassical Tearing Modes (NTM), Resistive Wall Modes (RWM) and Edge Localised Modes (ELM) [3,4]. The implementation of tools for the control of the MHD activity is increasing in the majority of the existing experimental devices, with particular emphasis on identifying the most viable solutions for ITER and DEMO. In general, they are based on heating (NBI, radiofrequency) and fueling (pellet injection) systems and/or active controls with external or in-vessel arrays of non-axis-symmetric coils. Typically, in the latter case, the coils are independently fed by individual fast power supplies (PS), digitally controlled in real-time. After a description of the main plasma instabilities and the approach to their control in Tokamaks and RFPs, the paper summarizes the main data of the active control coil systems and of the requirements and features of the relevant PS. Then, some guide-

∗ Corresponding author. E-mail address: [email protected] (V. Toigo). 0920-3796/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2011.04.075

lines for the design are given and some specific issues are described highlighting how they have been solved in existing applications.

2. MHD instabilities in Tokamaks and RFPs 2.1. MHD instabilities in Tokamaks Tokamaks are affected by a variety of MHD instabilities. The Resistive Wall Modes (RWMs), which are of major concern for ITER, arise from ideal kink instabilities in the presence of a conducting structure surrounding the plasma. RWMs grow on the resistive time scale of the structure; their feedback stabilization depends on the size of the machine: the larger is the machine, the slower the growth rate and the easier is the control. NTMs [3] are driven by the local reduction of the bootstrap current due to the pressure flattening across the magnetic island; they appear in high-beta plasmas. The suppression of NTMs with m/n = 2/1 (m, n: poloidal and toroidal mode numbers respectively), is particularly important, since they cause mode locking and finally generate disruption. The ELMs [4] are short, recurrent instabilities of the edge plasma. ELMs are fast (some ms) bursts of energy and particles occurring only in high confinement mode (H-mode), which are transported to the divertor plates along field lines causing the plate erosion. There are various types of ELMs with different characteristics; type I or ‘giant’ ELMs, observed in most H-mode plasmas, present high amplitude, which requires to be reduced. Sawtooth oscillations, arising when the safety factor decreases below one, may couple with NTMs and ELMs, resulting in serious loss of plasma energy and confinement degradation.

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V. Toigo et al. / Fusion Engineering and Design 86 (2011) 565–571 Table 1 Main control tools implemented in the different experiments.

AUG E

NTM

A A

RWM P

ELM

A

FTU

JET

E

EA

-

-

-

TEXTOR

DIII-D

NSFTX

E

-

-

-

A

-

-

-

A

E

A

KSTAR EAST

RFX

JT-60U

-

A

E

A

A

A

A

A

A

-

-

JT-60SA E

ITER

A

E

A

A P

A

Sawteeth

-

E

-

E

-

-

-

-

-

-

-

E

Error Fields

-

-

A

-

A

A

A

A

A

-

A

A

A

Active coils

P

D Pellets

E

ECCD -ECRH NB Injectors

Finally, error fields are also important as they can cause modes locking and associated disruptions. Disruptions can also be induced by fast vertical instabilities; their limitation is particularly important in high elongated Tokamaks where the growth rate is higher. 2.2. MHD instabilities in RFPs In the RFP experiments, MHD instabilities have different peculiarities with respect to Tokamaks. In RFPs, a large spectrum of unstable Tearing Modes (TMs) grow and saturate very rapidly during the setting up of the configuration, and then their amplitude remains more or less constant. TMs are present even with an ideal magnetic boundary; their reduction is crucial in order to reduce the plasma-wall interaction, which limits the plasma current performances. RWMs are present in the RFPs also, but, differently from Tokamaks, where they are pressure driven instabilities, they are current-driven, and are therefore observed at all values of ˇ. In RFPs the unstable RWM spectrum consists of a range of m = 1 in a wide range of n values, thus requiring multi-mode control. 3. Control of plasma instabilities in Tokamaks and RFPs While in RFPs the plasma instabilities control is mainly based on conducting walls and active coils, in Tokamaks auxiliary heating systems are mainly used; nevertheless, recently the use of sets of coils actively controlled has been increasing very much in Tokamak too. Table 1 summarizes the main control techniques implemented or foreseen in the different experiments, for different classes of instabilities. 3.1. Control of plasma instabilities in Tokamaks The effects of Electron Cyclotron Current Drive (ECCD) in stabilizing NTMs have been investigated and demonstrated in several experiments. ECCD is considered one of the most effective methods [5–8,11]; with this technique, in fact, it is possible to drive a localized current exactly in the narrow region of the plasma where the instabilities occur. In addition to the active stabilization with ECCD, avoidance of NTM onset can be performed through optimization of the current and pressure profiles, using Neutral Beam Injection (NBI). In case of mode locking, the ECCD action can be effectively combined with actively controlled coils to induce NTM rotation, both for reducing the growth rate and for optimizing the locking position for the ECCD action. The RWMs can be controlled by plasma rotation induced by NBI or by actively controlled coils placed on the inner or the outer side of the vacuum vessel [9,10]. Active RWM stabilisation in Tokamaks for performance enhancement is a rather new field; it aims

at producing a helical magnetic field to counteract the field perturbation produced by the RWM, or to induce mode rotation which generally has a stabilising effect [11–14]. Besides fueling systems, Resonant Magnetic Perturbation (RMP) field produced by active coil systems, suitably designed and controlled, can either cause type I ELMs disappearance or reduce their amplitude [16,20]. Error fields are also reduced by means of active coils; often a set of radial coils is designed for more control actions as shown in Table 1 [13–15,17–19]. Low resistivity shells and dedicated axisymmetric coils fed by fast PS are the tools for controlling fast vertical instabilities [20–23]. 3.2. Plasma instabilities control in RFPs In the RFPs, the control of MHD instabilities strongly relies on the presence of a conducting wall, which provides passive stabilization of TMs and reduces the growth rate of RWMs. However, the finite resistivity of the conducting wall and the associated diffusion time constant of the radial magnetic field limits the duration of this effect. This limitation can be overcome only by active control of the edge radial field. The idea of a net of coils which can mimic an ideally conducting wall by locally opposing the radial field was theoretically proposed [24] and has been recently developed for RFP devices [25,26]. 3.3. ITER In ITER, a very wide set of tools for MHD modes and fast vertical instabilities control is foreseen; the design and the solution of many open issues are still in progress. ECCD is assumed to be the primary tool for controlling sawteeth and NTMs, while actively controlled coils are chosen as main way to control RWMs, error fields and ELMs [27]. For the latter, also pellet injection will have an important role to increase the ELM frequency, thus reducing the amplitude. As for the active coils, the present design foresees a set of in-vessel coils for ELMs and RWM control and a set of external correction coils to control Error Field Locked Modes instability [28,29]. 4. PS requirements and features Generally, in Tokamaks, the active coils for MHD modes control are rings of toroidally distributed saddle coils installed inside or outside the vacuum vessel at different poloidal positions (top, middle and bottom) and supplied by dedicated PS systems. The main data of the active control coils and requirements and features of the PS systems in experiments in operation are summarized in Table 2 while Table 3 shows the main data of the set of coils presently foreseen for the future experiments.

Table 2 Main data of the active control coils and requirements and features of the PS systems in the present experiments. DIII-D [20]

Extrap T2R

JET [20,22,31,32]

AUG [33]

NSTX [34]

TEXTOR [35]

RWM NTM

RWM

RWM NTM

VS

RWM NTM ELM

RWM or ELM

NTM

I-coil

C-coil

Saddle Coils

Saddle Coils

Saddle Coils

Radial Field Circuit

Saddle Coils A Type

Saddle Coils B Type

Saddle Coils

DED – coils

Position (respect vessel) N. coils

Inside

Outside

Outside

Outside

Inside

Outside

Inside

Inside

Outside

Inside

2×6

1×6

4 × 48

4 × 16

4

2

1×8

2×8

1×6

16

Current

7 kA 10 s

5 kA

400 A

–

3 kA

5 kA

1 kA

1 kA

5 kA

15 kA dc 7.5 kApk –10 kHz

RWM

Coil

ELM

Voltage

0.6 kV

–

1.5 kV

12 kV

1 kV

0.5 kV

–

6.2 kVpk

SPA

Audio

CPS

PR

DFAS

ERFA

TBD

TBD

SPA

TBD

(’99)

Amplifier

(’94)

(‘05)

Audio Amplifier (’04)

(’92)

(’09)

Type

Inverter

–

Thyristor Rectifier

IGBT

–

IGBT Inverter

IGBT Inverter

IGBT Inverter

IGBT Inverter

IGBT

Resonant

N. units

4

24

5

192

16

4

1

8

16

3

8

Config.

3 H-bridges parallel or independent

–

6-pulse Thyristor Rectifier

H-bridge

–

H-bridge

4 Unit in series

Cascade (2 bridges in series)

H-bridge

3 H-bridges parallel

H-bridge Q comp. at load side

Voltage

300 V

140 Vpk

350 V

0.65 kV

–

1500 V

12 kV

1 kV

0.5 kV

1 kV

600 V

Current

1700 A for bridge

200 Apk

5 kA

400 A

20 A

5 kA

1 kA

1 kA

3.33 kA

1.5 kA

Duty cycle

10 s/600 s

cw

10 s/600 s

0.5 s/600 s

cw

3 kA f < 1 k Hz 3 kA/f 1 kHz < f < 10 kHz 1 s/600 s

60 s/600 s

10 s/600 s

10 s/600 s

6 s/300 s

10/360 s

Power supply

Inverter

Inverter

Bandwith

–

≤40 kHz

<100 Hz

<1 kHz

≤25 kHz

0–10 kHz

<10 kHz

3 kHz

500 Hz

–

0–10 kHz

Switching frequency

300 Hz full I 2 kHz red. I

–

–

10 kHz

–

–

–

–

–

7.5 kHz

–

V. Toigo et al. / Fusion Engineering and Design 86 (2011) 565–571

RFX [30] Error Field

Control

567

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V. Toigo et al. / Fusion Engineering and Design 86 (2011) 565–571

Table 3 Main requirements for the active control coils in JT60-SA and ITER. JT60 SA [17]

5.2. Selection of the power supply type

ITER [27–29]

RWM

Error field

RWM ELM

Error field

RWM Coils

EFCC

ELM RWM Coils

Top Bottom Coils

Side Coils

Inside

Inside

Inside

Outside

Outside

3×6

3×6

3×9

6+6

6

2.5 kA

2.5 kA

20 kA

±10 kA

±10 kA

0.6 kV

0.1 kV

0.13 kV

TBD

TBD

Looking at Tables 2 and 3, significant spread in terms of voltage, current and bandwidth requirements can be appreciated, which has led to a large variety of PS systems and topologies. The design of these PS systems is not straightforward; some guidelines are given in the following, together with a review of the main suitable technologies for plasma instabilities control.

5. PS technology and guidelines for the design 5.1. System design A system approach is important to design these PSs, which also requires a synergic cooperation between PS engineers, physicists, magnet and control engineers in order to optimize performance, cost and size. The effectiveness of the coils on the instabilities control increase with their size and their closeness to the plasma; the material of the conductor sheaths, case and surroundings passive structure can also significantly affect their action. All these aspects greatly influence the power level to be delivered by the PS. In addition, the increase of the coil turn number that means of the applied voltage, could allow a much better exploitation of the power capability of the semiconductors; in case of in-vessel coils, however, this could not be always possible. On the physics and control side, a deep reciprocal understanding on the relation between the desired plasma instabilities control and the specific PS requirements is a key step to reach successful results.

5.1.1. Understanding and defining the requirements It is very important to underline that a quite complete set of requirements is necessary. Current, voltage and frequency of the reference output current waveform are not enough; bandwidth, response delay, ripple of the output current, quality of the output waveforms needs to be defined. Recent experiments on MHD control are showing the importance of minimizing the delay time in the PS response to limit the power level necessary for the control itself. This means that even when thyristor line commutated converters could be thought adequate in terms of required bandwidth, they can be not in terms of delay.

5.1.2. The load characterization Another major aspect is the load characterization: the mutual coupling among coils, plasma and passive structures implies induced currents and energy that has to be recovered from the load. In addition, the uncertainties on the plasma impedance complicate an accurate electromagnetic characterization of the load with consequent high impact on the control for both the individual PS and the overall system.

After having studied an overall system optimization and clearly understood the requirements, the PS design is addressed at identifying the suitable architecture to guarantee the required performance on the single coils but also to optimize the global features in terms of modularity level, size, cost, impact on grid. The first choice is related to the PS type. 5.2.1. Linear amplifiers Linear amplifiers represent a highly desirable solution for these applications; in fact, they allow the widest bandwidth of the output waveforms and best dynamic performance and do not produce the noise typical of the switching converters. They are based on power Mosfets and in general, the highest power applications do not exceed some tens of kW due to the limits in the semiconductor power losses. Unfortunately, the power level for plasma instabilities control is generally too high to allow adopting this type of amplifier. 5.2.2. Line commutated converters The line commutated converters in the basic configurations are two quadrant amplifiers supplying unidirectional current. Four quadrant operation is achieved with bridges connected in antiparallel and suitably controlled in particular around the current inversion region; this is the only configuration allowing energy recovery to the grid but in times not less than some ms. This family of converters is widely used in fusion experiments to feed the main magnets due to the requirements of very high power and relatively slow dynamic behaviour; for the same reasons, their use is rather limited for MHD modes control. 5.2.3. Switching converters The switching converter (SC) is the most widely utilized type of PS for MHD control; a broad variety of topologies exists, but Voltage Source Converters composed of input rectifier, capacitive dc link and inverter are the best choice for the following considerations. Besides the better dynamic behaviour with respect to line commutated converters, the switching ones allow decoupling the load from the grid. In this way, the input rectifier can be unidirectional and rated just for the load losses, while the fast bidirectional energy exchange with the load is handled by the capacitive dc link. The cost is probably the major drawback: switching converters are always considered more expensive than line commutated converters but the comparison should be carefully made, case by case, taking into account many aspects, including size considerations. In fact, the basic structure above described can allow providing a common input rectifier and dc link for many inverters. 5.2.4. Considerations on common dc-link design The cost per kVA of transformers and thyristor bridges decreases with the increasing of the power: their number reduction can imply not negligible cost and size savings. Moreover, the common dc-link permits reducing the capacitor bank rating if the energy exchange with the different loads is not contemporary, as typically occurs in the MHD modes control generating rotating magnetic fields. An example of application of this concept is the RFX PS system for MHD control; the main data are summarized in Table 1 and an electrical scheme is shown in Fig. 1. The 192 inverters (400 A–650 V) are arranged in 4 independent subsystems each one composed of a step-down transformer, a rectifier bridge and 48 inverters. These are in turn divided in 4 groups, each with a capacitor bank protected by fuses and 12 H-Bridges (H-Bs) in parallel; these groups are decoupled by diodes to limit the overcurrent amplitude in case of capacitor internal fault [30]. This scheme allowed strong reduction of the number of transformers and thyristor bridges and their

V. Toigo et al. / Fusion Engineering and Design 86 (2011) 565–571 AC/DC Converter

569

DC/DC converter

Group 1

×12

Transformer

Grid 21.6kV

Capacitor Bank

Group 2

Group 3

Group 4

Fig. 3. AUG PS for MHD modes control – conceptual scheme. Fig. 1. Scheme of the RFX PS for MHD modes control.

Power [MVA]

10

1 IGCT ABB 35L4512 IGCT ABB 42L6500 IGBT Wescode T2400E IEGT Toshiba ST2100

0,1 1

10

100

1000

10000

Frequency [Hz] Fig. 2. Most powerful full-controlled semiconductors.

power rating, which corresponds to about a quarter of the total power required by the inverters.

equivalent frequency of the output voltage higher than the switching frequency. The main disadvantage of series connected H-B is that independent dc link and input stage has to be provided for each one, loosing the advantages described in the previous section. An interesting application of these concepts can be found in the conceptual design of the Asdex-Upgrade PS for MHD modes control, recently developed [33]. Here the adoption of a single HB is suitable just for one set of coils (B-coils, see Table 1), not for the A-coils due to the more demanding requirements. In a cascade topology [37] with two H-Bs modules in series and suitable control, the frequency of the instantaneous output voltage is four times the switching frequency of the converter semiconductor devices. In addition, the basic module for A-coils is suitable for B-coils, thus the design is customized for the different needs of the two sets of coils, but in the same time maintains also a satisfactory modularity level; the overall scheme is shown in Fig. 3. It is worth noting the independent dc-links, but also the adoption of a multi windings step-down transformer and of simple diode rectifiers, choice justified by the power level and the verification that the output voltage regulation can be left just to the inverters. 5.4. Control

5.3. Switching converters: choice of semiconductors and topologies 5.3.1. Semiconductor selection As for the semiconductors, the present state of the art is summarized in Fig. 2, which shows the output power versus frequency that can be handled by a single H-bridge inverter component. The most powerful devices available in the market are considered [36]. Components are available on the range of power and frequency below the plots. 5.3.2. Topology selection If switching converters are necessary and a single H-B is not sufficient, suitable series/parallel combinations have to be employed. For the parallel connection, necessary to cope with the current requirements, the cheapest and simplest solution would be paralleling the semiconductors inside the bridge thanks to the positive temperature coefficient. However, the current limitation in case of internal short-circuit often makes preferable bridges in parallel, which can be provided with passive protection devices. As far as the series connection is concerned, putting devices in series is cheaper than H-Bs in series but require provisions to assure the voltage sharing during the commutations; in addition, devices in series allows just fulfilling the voltage specification. On the contrary, H-Bs in series suitably controlled also allows producing

The control is a key part of the design; here just some main concepts are recalled. First of all, it is important to have in mind that these PSs are equipped with their own control, nested within a higher level control system, which calculates the reference waveforms (either current, voltage or open loop reference waveforms) in order to perform the desired magnetic configuration. 5.4.1. PS internal control: current or voltage? From the recent experimentation in MHD modes control in RFX, it was learnt that the PS response speed is an extremely important requirement; the impact of this conclusion is very high and not trivial to be solved. In fact, the desired quantity to be controlled would be the current, which correspond the produced magnetic fields; thus the PS internal current feedback seems the most natural design choice. This choice, nevertheless, does not optimize the response speed, which would require voltage feedback or open loop (direct gate control) operation. When the effect of the mutual couplings is dominant, the PS voltage or open loop control can operate properly only if the external feedback is provided with a decoupling matrix capable to generate proper references and to compensate for the uncertainties on the plasma. This is not always available and probably not at the beginning, therefore it is very useful to provide the internal control for both the quantities. Today, the digital implementations allow improving the

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6. Conclusions The paper underlines the importance of an accurate and complete definition of the requirements and of a “system approach” for the design of the PSs for plasma instabilities control. Switching converter is the most used type for this control; the state-of-theart technology both for semiconductors and circuit topologies is presented. Some specific issues of the design of the power and control sections are described and discussed, showing also how they have been faced in MHD modes control applications in existing machines.

References

Fig. 4. Scheme of the JET enhanced radial field amplifier.

flexibility level of the PS internal control if this is clearly required at the conceptual design level. 5.4.2. Current control Two basic approaches are commented here for those aspects mainly affecting the most important requirements for the applications: the control based on Pulse Width Modulation PWM and the hysteresis control [38]. With PWM control the switches work at fixed frequency that allows a deterministic calculation of the losses. This control requires the presence of a regulator; feedforward compensation of the inverter losses and dc-link voltage variations can be provided. The PWM modulator introduces a delay, which also depends on the hardware implementation, but cannot be less than a switching period. However, the response speed is mainly affected by the regulator, which operation is affected by the load and by the mutual coupling. On the contrary, feedback control based on hysteresis is intrinsically faster, because it is completely independent from the loads; however, the frequency is variable and unknown and needs to be limited to avoid exceeding the allowable switches losses with consequent reduction both of the speed and accuracy. Moreover, it is not possible implementing compensations. 5.4.3. Voltage control If PWM modulators are adopted, feedback control of the voltage is not usually implemented; the overall performance achievable in terms of accuracy is not much better with respect to the operation in open loop with feedforward compensation, which is also faster. In PS units with bridges in series, hysteresis control of the total voltage can be utilized. A good example of this control approach is that implemented in the JET Enhanced Radial Field Amplifier (ERFA, ±5 kA, 12 kV); the scheme is shown in Fig. 4. ERFA achieves the very fast response applying to the load different voltage levels obtained switching on and off the stages in series at a frequency restricted only by the intrinsic limits of the amplifier [33]. The switching optimisation strategy was worked out observing that the extreme values of the output voltage (+12 kV and –12 kV) are obtained only with one combination of the switches (all bridges on), whereas the intermediate levels can be delivered in more than one way, depending on which of the inverters is on: consecutive switching at the same voltage can involve different semiconductors, thus reducing the switching frequency of the single component down to 1/4 of the instantaneous output voltage frequency.

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