f. sartori 1

Plasma Control System Architecture: lessons learned.

Overview of PCS network and software architectures

May 19, 2010 - 17th Real-Time Conference, Lisbon

F. Sartori 1

G. Ambrosino2 K. Blackler3 M. Cavinato 1 G. De Tommasi2

J. Farthing4 R. Felton4 P.J. Lomas4 A. C. Neto5 G. Raupp 6 G. Saibene1 W. Treutterer 6 L. Zabeo3

and EFDA-JET contributors

1:Fusion for Energy, 2:Assoc. EURATOM-ENEA-CREATE, 3:ITER Organization,4:EURATOM-CCFE Fusion Association,

5:Assoc. EURATOM-IST, 6:Assoc Max-Planck-Institut für Plasmaphysik

A short introduction to Plasmas

Tokamak Plasma = fully ionised gas + nuclei and electrons accelerated in different directions.When the conditions (P,T) are right…

n+

n+n

-

-

-

-

n+

nn+

n+n

+n

We have fusion !

Tokamak: electromagnetic machine

Tokamak contain the Plasma within a torus shaped vacuum vessel doughnut.Surrounding the vessel aretwo sets of magnetic coils.The PF coils create and confine the plasma current.The TF coils create a toroidal field which helps avoiding instabilities.PF and TF coils together with the plasma current generate the spiralling magnetic field that confines the plasma.

Tokamak = Highly Integrated Machine

Tokamak operation requires the coordinated action of several actuators: PF coils, additional heating, gas injectors…

It is very difficult or impossible to pre-calculate correct reference waveform for actuators. The solution is to feedback on the diagnostics information using a Plasma Control System .

Design of diagnostic systems need to account for the dual use of diagnostic information: science and control. They have conflicting requirements: precision and reliability.

Diagnostics of JET, Europe’s largest tokamak

Scope of Plasma Control System

• Stabilisation functions– Stabilise unstable plasma variableEssential for the basic functioning of the machine!

• Regulation functions– Reject disturbances and help programming experimentHelps improving quality of experiments and simplify the programming

• Non Primary - High Level Protection functions– Steer away from machine limits – Minimise wear of the tokamak componentsHelps the experimenter and improves machine availability

• Flexible support to experimental needs– User programmable open/closed loop references– Ability to set up custom feedbacks / protection schemes– Time varying control/protection logic/parametersHelps the experimenter and introduces new experimental possibilities

Example: Vertical StabilisationPlasma with a vertical elliptical shape are unstable vertically.

Without changes in the PF coil currents the plasma position accelerates vertically following an exponential law.

Typical JET plasma growth time is 3ms: For every 3ms of delay the installed stabilisation power needs to be 10x larger. Latency is essential…

JET VS control chain acts in ~300 µs!

Detection = combination of 160 sensors

Actuation = 60MW of electric power

Without VS plasma lasts few ms….

Regulation: Plasma Shape

Plasma shape is function of plasma distribution and external fields (PF).Changes in internal plasma parameters can cause significant variations to the plasma shape.The Shape Controller instructs changes to the PF coils in order to minimise such changes.JET Shape Control : 2ms cycle time.Detection = combination of 200 sensorsActuation = 9 PF coils >200MW power.Without Shape Control, experimenter uses simulation codes to calculate PF current waveforms. He then corrects the waveforms using data from experiments…





• Flexible support to experimental needs– Ability to set up custom feedbacks – Ability to set up custom protection schemesHelps the experimenter and introduces new experimental possibilities

Example: control of plasma pressure β:Additional heating is controlled using a PID and the pressure estimation β.

PCS architecture

How are we going to organise the implementation of the Plasma Control System?Why not a simple architecture where each PCS function is an individual system?But PCS functions share diagnostics, share actuators and in most cases communicate with each other!Why not a single system running all functions?Too complex and un-manageableLet’s look at why

High Level

DiagnosticDiagnostic ControllerController ActuatorActuator



PCS Requirements

PCS function have broadly different technical requirements– Number of channels, cycle time, processing requirements,…

Require different levels of reliability and availabilityImpact on the (re) commissioning procedures

Are gradually installed and commissioned following the needs of an evolving machine– Tokamaks evolve from simple machines with the minimum

number of systems to complex machines able to execute advanced plasma scenarios

Requirements are subject to change especially for the most complex function, those to support advanced operation– Their requirements are the most volatile as subject to evolving

technical and scientific understandingThere is always a limited windows for commissioning PCS

since PCS is the last being tested.

Modular architecture

The only practical way to cope with the conflicting requirements is to break PCS into Modules.

A PCS Module is a distinct system that implements a PCS function or contributes to it.

Modules are commissioned as soon as possible and then kept untouched as long as possible.

Distributed Architecture

Since a large number of Module outputs need to be shared a PCS network becomes necessary.

The network should allow efficient management of PCS, and at the same time satisfy the technical requirements.

PCS Network Solutions

Single or Multiple Star is the ideal topology for PCS network compared to Ring or Bus.

This solution requires high availability switches since they are a single point of failure for the network.

Network solutions should also be as mainstream as possible No single vendor, multiplatform support, long life, backward compatible upgrades….

PCS Network signal topology

PCS requires a point to multi-point signal topology.This can satisfied with Broadcast transmissions. A better choice is Multicast as it allows effective reduction of traffic on branches.Multicast + Centrally Managed routes (non public subscribe) allow controlled introduction of a new PCS Module: until tested isolate output.It also allows seamless replacement of a PCS Module with another

PCS

SWITCH

PCS

PCS

PCS

Latency

The most important performance parameters for the PCS network is the transmission delay

– Control systems requirements impose a bound delay for the overall control chain– Jitter is in fact tolerable– Only deterministic protocols can be used: no->TCP

Diagnostic data is typically sampled synchronously– The network is flooded with information every cycle – Switch queues should be long enough to cope with the flood– Better not rely on time sharing techniques like TDMA (synch net)

• Switches are more reliable than systems

The network bandwidth can be utilised partially 20%The larger the usage the longer the latency….

PCS network traffic

PCS traffic consists of groups of signals (packet) related to a certain diagnostic, actuator or plasma property.Send only information that is usable by many modules!Elaborate the raw diagnostic data at the source.Use dedicated links to transfer high volume of data to Data Elaboration Systems before entering PCS Network.

Existing PCS networks

PCS Module

PCS Modules application architecture

PCS Module application software can always be modelled as a Discrete Time System.It may contain multiple DTSs each operating at different sampling frequency.Or also cascades of DTSs.PCS Module architecture should support application modularity providing the interconnection elements.

ADCPacket 1Packet 2

F1(z)DAC

Packet 3

Re-sampling

F2(z)Packet 4DAC 2

Fa(z) Fb(z)

PCS SW frameworks

A good example of PCS framework is MARTe. For more information you can visit the posters– PFE4 “A survey of recent MARTe based systems” A.C.Neto– PFE13 “Performance comparison of EPICS IOC and MARTe

in a Hard Real Time Control Application”, A. Barbalace.– PFE22 “First steps in the FTU migration towards a

modular and distributed real time architecture based on MARTe and RTnet”, L.Boncagni

– PCM18 “Epics as a MARTe configuration environment”, D.Valcarcel

Conclusions

In large tokamaks the PCS management requirements dominate technical aspects.

• Divide PCS into Modules with the aim of providing the right functions at the right time while minimising commissioning.

• Use PCS network to allow sharing of actuator and resources and allow collaboration of Modules

• Use the network to help manage the evolution of PCS: support isolation, addition and substitution

• Aim at low latency, exchange only the information that is needed with the module that needs it….– Concentrate large local traffic into dedicated links.

• Standardise and modularise Module software– Separate scientific code from interfaces– Make modular scientific code portable to support testing

Try to keep it simple but prepare to manage complexity.

Future of PCS

f. sartori 1

Documents

plasma current

plasmastokamak plasma

plasma position

pf coils

typical jet plasma growth

machine limits

tf coils

tokamak componentshelps