Communication Protocol for Plasma Position and ...

Communication Protocol for Plasma Position and Shape Control for the COMPASS Tokamak 1,2

F. Janky, 2 J.Horacek, 3 T. V. Pereira

1

Charles U niversity in P rague, F aculty of M athematics and P hysics Institute of P lasma P hysics, AS CR, v.v.i., Association EU RAT OM/IP P.CR 3 Centro de F usao N uclear, IST, Lisbon, P ortugal 2

Abstract. The Compass tokamak, reinstalled at IPP Prague, will have a new digital plasma position and shape control system. Development of a communication protocol is necessary for the connection between the energetics devices1 built by CKD company and the operator - either human or computer together. The present work is focused on development of such communication protocol. Human operator can send a request for status of energetics, time synchronization, counting errors, etc. through communication protocol. Predefined currents for individual power supplies will be send through the communication protocol. In the future this system will be replaced by feedback control system integrated in CODAC (Control Data Acquisition and Communication system).

Introduction Tokamaks are devices confining plasma inside a toroidal vessel using magnetic fields. These devices can be used in fusion power plants in the future [Wesson, 2004]. Fusion power plants offer the prospect of a sustained thermonuclear fusion and generating electricity, substantial and practically inexhaustible supply of energy with acceptable environmental, health and safety risks. Tokamaks have proved to be the most successful devices in confining plasma with parameters close to that required for fusion to the one required for ignition [Keilhacker et al., 1999]. Tokamak is device a producing toroidal field for confining a plasma inside the vessel. Toroidal field is generated by coils surrounding vessel (marked ”Toroidal field coils”) (Figure 1). Poloidal field is generated by a current flowing through the plasma which is induced by transformer action. The combination of poloidal and toroidal field components generates a resultant helical magnetic field. COMPASS (COMPact ASSembly) is a medium size tokamak with D-shaped vacuum vessel. COMPASS has been moved from Culham and is now being reinstalled in Prague. One of its aims is to study prevention of loss of plasma position which is common in smaller tokamaks but must be absolutely avoided in large tokamaks in future. Different plasma shapes can be studied using easily varyable arrangements of shaping coils [Sartori et al., 2006; Ambrosino and Albanese, 2005; Ariola and Pironti, 2005]. COMPASS has the same geometry and shape as ITER, however, its dimensions are smaller by an order of magnitude (Figure 2). Its small size makes it suitable for study and demonstration of experimental ideas because of the ease and flexibility of operations and thus significantly reduced consequences of loss of control. Lost of control can leads to a plasma disruption, and these have been observed in all tokamaks. Disruptions involve a complicated sequence of events. Their main feature is always a sudden loss of plasma energy. For a major disruption this event is almost always irreversible and leads to a rapid decay of the plasma current and termination of the plasma discharge. The consequences of disruptions in a fusion reactor would be severe, leading to forces possibly of the order of mega-Newtons [Wesson, 2004] exerted on the machine structure, and local over-heating due to parasitic current loads. Plasma shape and position control The plasma position and shape is determined by the poloidal magnetic field generated by poloidal field coils outside of the vessel. On COMPASS three separate systems poloidal coils (and power supplies) control the plasma shape, radial position and vertical position. The plasma shape is open loop controlled by driving a preprogrammed current Is through shaping coils. The set of coils used for shaping can be changed between shots to generate different plasma 1

energetics - flywheel, power supplies for coils e.i.

JANKY, HORACEK, PEREIRA:

Figure 1. Toroidal and poloidal magnetic fields [Wesson, 2004].

Figure 2. Comparison of the tokamaks

shapes. The main component of the shaping field is quadrupole field creating vertically elongated plasma. Changing the ratio of Is to plasma current changes the shape of the plasma, with lower values generally resulting in less elongated plasma. The radial position is open loop stable but (slow) feedback is still necessary to control the position. The coils marked Bz (Figure 3) generate a vertical field controlling the radial position. The BR coils generate a radial field controlling the vertical position. The plasma vertical position is usually open loop unstable with relatively fast time scales. The shape and position loops are effectively decoupled from each other. The shaping field is preprogrammed and usually constant throughout a shot. The vertical and radial field coils are chosen to generate magnetic fields as perpendicular as possible to minimize coupling between the vertical and horizontal stability, thus the two problems can be treated independently. The quadrupole shaping field used to generate vertically elongated plasma, causes the vertical position instability. A parameter characterizing the shape of a plasma is the elongation κ which is the ratio of the plasma half-height b to the plasma half-width a. Vertically elongated plasma (Figure (a) 4) are often used in tokamaks because they permit transition into both higher confinement H-mode and plasma detachment from the divertor tiles. The disadvantage is that such plasma is vertically unstable


Figure 3. COMPASS poloidal field system. S-shaping coils for typical Single Null Divertor - SND plasma, Bz -vertical field coils, BR -radial field [Vyas, 1996].

as demonstrated in (Figure (b) 4): the upper part of the plasma column pushes upwards, the bottom part downwards. As such, it gets unstable if moved away from the symmetry point at the tokamak near midplane. The force is linearly increasing with the vertical displacement, Fz = α · z, resulting t into exponential growth of the instability in time, z = z0 · exp τ . The counter force is, however, delayed by the control system reaction time and then grows linearly due to finite speed of current grows (due to coil inductances) in the external poloidal coils. Stabilization is therefore possible only within certain margins, given by the speed of the reaction. As the plasma moves, currents are induced in the tokamak vessel wall and surrounding structures, acting against the motion. These currents stabilize the fastest displacements of the plasma and therefore helps to control plasma. As such for this reason, the tokamak wall must be conductive as much as possible. This passive damping reduces the instability growth rate, but active feedback is required in addition to stabilize the system. This inertial instability growth rate (before the active feedback starts to act) depends on the ratio of the destabilizing force vertical gradient due to the curvature of the field lines and the damping force gradient due to coupling to the vessel wall. Both these factors depend on the current profile and position of the plasma. Destabilizing forces in the J × BR direction push the plasma away

Figure 4. (a): Plasma elongation [Wesson, 2004]; (b): Destabilizing forces acting on plasma [Vyas, 1996].

JANKY, HORACEK, PEREIRA: from equilibrium. The radial component of the equilibrium field is approximated using a first order Taylor series so that the destabilizing force Fd is ∂BR z (1) ∂z where Ip is plasma current, Rp is the radial position from the centre line of the torus (major radius), ∂BR ∂z is gradient of the radial magnetic field in the vertical direction z. A positive direction of plasma current is assumed. As the plasma moves, currents are induced in the vessel wall which act to oppose the motion of the plasma. This stabilizing force is Fd = −2πIp Rp

Fs = Iv

∂Mvp Ip ∂z

(2)

where Iv is the current induced in the vessel wall and Mvp is the mutual inductance between the plasma and the vessel wall. Communication Protocol A passive shell and active feedback coils for vertical stabilization (VS) associated with the use of feedback for plasma position control are crucial. The signal processing of the VS system must be as fast as possible. In our case the conductive vessel shell stabilizes passively up to 500µs [Vyas, 1996]. COMPASS at Culham used analogue control system so COMPASS needs new control system, fully digital based on ATCA (Advanced Telecommunication Computing Architecture) technology. New digital plasma control provides us advantage for reconstruction of the magnetic equilibrium (plasma shape) in real-time and it also provides a flexible platform to explore new non-linear and adaptive control schemes. The requirement for the new control system is that it must function as a drop-in replacement of the original system. The system dscribed here has been developed for testing power supplies from CKD company, power amplifiers and communication. The hardware was built in CFN, IST, Lisbon. The ATCA system will run feedback software controlling (in real time) currents in tokamak poloidal coils and thus stabilizing the plasma position and shape, based on signals from external magnetic pick-up coils. Before the full ATCA system will be developed, we need to test energetics for toroidal field, equilibrium, shape and magnetizing field power supplies, amplifiers and communication with the energetics. The testing software is based on the RS-232 protocol. RS-232 is a standard in telecommunication, for serial binary data signals connecting between DTE (Data Terminal Equipment3 ) and DCE (Digital Circuit-terminating equipment4 ) [wik, 2008]. Protocol targets point-to-point link for fast and reliable communication. It specifies the following features, which were implemented using a reserved number of bits of the telegram. 1. Hard real-time control of the hardware 2. Error detection 3. Telegram flow-control

Software solution The software is developped in C and we are using the microcontroller Microchip dsPIC30f4013 for controlling and communicating with energetics and PC. The program uses two UART5 (Universal Asynchronous Receiver/Transmitter), one for communication with PC trough serial cable and the second 3

DTE is an end instrument that converts user information into signals for transmission or reconverts received signals into user information. A DTE device communicates with the data circuit-terminating equipment (DCE). 4 DCE is a device sitting between the DTE and a transmission circuit. In a data station, the DCE performs functions such as signal conversion, coding, and line clocking and may be a part of the DTE or intermediate equipment. Interfacing equipment may be required to couple the data terminal equipment (DTE) into a transmission circuit or channel and from a transmission circuit or channel into the DTE. 5 A piece of computer hardware that translates data between parallel and serial forms. UART is usually an individual (or part of an) integrated circuit used for serial communications over a computer or peripheral device serial port.

JANKY, HORACEK, PEREIRA: UART is connected through optical fibres to the energetics. Communication speed between PC and microcontroller is 9600 bits/s and communication speed microcontroller-energetics is configurable up to 921600bits/s. Software is working in infinite loop interrupted by interrupts generated by messages received either from the PC or energetics. The size for all logical messages ”telegram” is three 9-bit words. Framing of these words is provided through the 9th bit of each word, which defines the beginning of the telegram, asserted (1) on the first 9-bit word (start telegram) and de-asserted (0) on the other 9-bit words. Each word (Table 1) contains an operand, CRC and data value. • Operand - sets current and receives measured current for TF (Toroidal Field), SFPS (Shaping Field Power Supply), EFPS (Equilibrum Field Power Supply), MFPS (Magnitizing Field Power Supply), send, latch, synchronize, request time, request status of energetics devices. These are the first five bits (O0-O4) of the first 8-bit word. • CRC6 calculation is based on the 24 bits in the telegram (excluding the three ninth bits) and zeroing the three bits corresponding to the CRC itself. • Data value - is a value of current (measured or set), time (sent to or received from energetics) or it can be a query for energetics status or received status message from energetics etc. 9th bit 0 msb

8 bits D15...D8

9th bit 0

8 bits D7...D0

9th bit 1

3 bits CRC

5 bits O4...O0 lsb

Table 1.: Example for processing telegram

Figure 5. system schematic

Software for testing energetics devices is controlled by operator who is sending messages from PC to the microcontroller trough serial cable (Figure 5). Sotware on microcontroller calculates CRC, applies 9th bits and sends to the energetics. Energetics do operation and send back the measured value or the response to the PC operator request. This operating sequence has been applied only for testing. In the near future, the operator will mostly be replaced by CODAC (Control Data Acquisition and Communication system) based on real-time Linux. CODAC will calculate values of electric currents needed for plasma position stabilization, shape, vacuum system, gas injection system, etc. It is planned that the operator just starts a discharge and everything will be controlled and monitored by CODAC. 6 Cyclic Redundancy Check is a type of function that takes an input of data stream of any length and produces as output a value of a certain fixed size. A CRC can be used as a checksum to detect alteration of data during the transmission or storage.


Conclusion We have developped the communication protocol for testing energetic devices built by the CKD company and also for communication between energetics and the Compass tokamak operator computer. While testing, the PC is controlled by human operator sending orders. The orders could be time synchronization, setting time, error counting, requesting energetics status, starting and stopping of an operation. Values of currents will be defined for testing by software and not by operator. Human operator will be mostly replaced by CODAC (Control Data acquisition and communication system) for the real-time plasma position and shape control. The coil currents will be calculated by CODAC depending on actual plasma position. The standard response timescales are planned to be around 100 microseconds. Communication protocol will be used for connection and communication between CODAC and energetics.

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