1. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Presented by M. Graham A. Walden, M.Nightingale, I. Monakhov Arcing in RF systems

2. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Introduction Amplifiers –VSWR and optical systems assist with arc detection Transmission Lines –VSWR provides adequate arc protection. Targeted optical arc detection systems may help to locate arcs Antenna strap –Generally well protected by VSWR Areas where arcing can occur

3. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Introduction Areas where arcing can occur Vacuum (Everything between antenna strap and window). –First voltage node produced in this section which is not protected by VSWR detection Screen bars, etc –generally don’t have real time protection however effects are generally seen on camera. Different problem as arcs are induced arcs at lower power

4. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 VSWR Trips Vast majority of arcs are caught by VSWR systems. VSWR or reflected power systems give good protection however their effectiveness is dependant on the standing wave pattern and thus do not provide 100% protection A few arcs are missed due to a maladjusted system There are instances where VSWR changes are so small that they go undetected, for instance –A3 VTL strap on JET A2 antenna –High Power Prototype (HPP) –Modelling of EP antenna. These missed arcs have had significant impact on experimental time, finance and resources as a result of the damage caused.

5. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Undetected Arcing The A3 VTL on JET is a case where arcing should not have occurred –The VSWR was considered to be adequate protection as the field was expected to be too low (voltage node) for arcing to occur.

6. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 EP System Low VSWR arcs seem to occur in vacuum conditions only. There are two distinct areas that are of concern in the EP antenna; VTL T-point and within the vacuum capacitors. If arcing occurs then in-vessel entry for repair of VTL components will be required

7. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Future Systems It is generally assumed that high voltage points will arc Load tolerant systems will hide the effects of arcing making detection harder Despite the fact that arcs are rare, the consequences in future installations will have a much greater impact on operational and economic issues

8. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Detection systems Independent of standing wave pattern Optical –Line of sight or reflected light can be used to provide out of the way protection. Difficulties can be encountered due to plasma light and signal to noise level Acoustic –Acoustic systems are too slow as they are limited by acoustic propagation speeds

9. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Detection systems Independent of standing wave pattern Sub-Harmonic  The Asdex upgrade uses this system but it has run into signal to noise problems with the latest plasmas RF comparative techniques  Sensors required either side of the target area to monitor inconsistencies. These techniques include both the EP antenna proposal from ERM and G. Bosia’s proposal

10. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Data Collection Most of the operational data collection is too slow to be able to adequately assess the arcing mechanism. Future data collection needs to be higher speed with good triggering Very desirable to use high power prototype to reproduce low VSWR arcs Storing raw data is good practice as various data processing/detection modelling can be applied

11. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Conclusion My conclusion is that there is currently no working economic arc detection system that reliably detects low VSWR arcs It is not presently clear what systems under consideration are likely to provide a practical solution It should be recognised that a combination of arc detection techniques may be required

12. 39th Meeting of the Co-ordinating Committee on Fast Wave Heating and Current Drive Arc Detection Meeting - JET - 4th August 2005 Conclusion Important to establish a wide shared database to help identify characteristics imposed by different ICRH systems and plasma machines. Concentration should initially be given to the EP antenna however a general solution is required for the ICRH community as a whole