JOINT RESEARCH DOCTORATE AND EUROPEAN DOCTORAL NETWORK ON FUSION SCIENCE AND
ENGINEERING
Thesis proposal for the 2015-2017 Ph.D. cycle
1) Program where the main part of activity will be realized
RFX-mod
2) Thesis title:
Advanced techniques for error field measurement and correction in fusion devices
3) Thesis supervisor
P. Piovesan
4) Group/s that will be involved in the program
FD
5) Thesis abstract (max 600 characters)
Aim of the Thesis is to develop and compare various techniques to measure and correct magnetic field
errors in fusion devices, to provide useful correction schemes for medium size tokamaks and ITER. This
will be done in RFX-mod by exploiting its flexible set of 192 active coils, and in particular the unique
possibility to use part of them to produce proxy error fields to be then controlled with the remaining
ones. The techniques developed in RFX-mod will contribute to similar experiments in larger tokamaks,
such as ASDEX Upgrade, and will be useful in particular to developed dynamic error field correction by
slow magnetic feedback, a medium-term project in collaboration with the ASDEX Upgrade team.
6) External collaborations (existing or to be started), which might involve the Ph.D. student (if any)
Collaboration on error field correction experiments in ASDEX Upgrade and DIII-D, where the ideas
developed in RFX-mod can be applied. The supervisor is collaborating on proposals on this subject for
the 2014 AUG campaign and on a medium-term project to develop slow magnetic feedback in AUG.
7) Preliminary list of milestones for each of the three years
YEAR 1
Experiments in RFX-mod run both as a tokamak and a RFP to apply and test different approaches for
error field measurement and correction. Techniques to correct both static and slowly evolving error
fields using magnetic feedback will be analyzed. This work will also require implementation of new
real-time algorithms in the MARTe framework. Participation to similar experiments in ASDEX Upgrade.
YEAR 2
Continuation of experiments in RFX-mod and ASDEX Upgrade. Detailed analysis and comparison of the
different techniques with the final aim to assess their relevance for medium size tokamaks and ITER.
This work also aims at elaborating proposals for further error field correction experiments in European
medium size tokamaks, but possibly also in DIII-D.
YEAR 3
The last part of this work will focus on the detection and correction of dynamic error fields arising from
3D eddy currents in RFX-mod and ASDEX Upgrade using slow magnetic feedback. This should become
possible also in ASDEX Upgrade thanks to a new development of their control system to be done in
collaboration with RFX-mod and should benefit from the experience gained in this device.
8) Description of the research project (max 8000 characters)
Magnetic field errors from various sources, e.g. small misalignments of axi-symmetric coils or complex
eddy current patterns induced in the 3D wall structures, are unavoidable in any fusion device. They
can have deleterious consequences for plasma confinement, in particular close to the MHD stability
boundaries, e.g. at high beta or low safety factor, where present and future fusion devices must
operate to maximize their performance. Various approaches have been developed in the last years in
various tokamaks to correct such error fields with externally applied 3D fields, but a systematic and
quantitative comparison between them is still to be done. This would be very important to assess the
relevance of the various approaches for future devices like ITER. RFX-mod is ideally suited to this aim
with its state-of-the-art magnetic feedback system. Moreover the RFX-mod contribution on these
subjects is already well integrated in the European program for next years, in particular as far as
medium size tokamaks experiments are concerned.
The present research project aims at exploiting the flexibility of the RFX-mod 192 saddle coils and of its
control system to develop and compare different approaches for error field measurement and
correction. This will require experimental time and development of new real-time software and has
the final aim to contribute to similar experiments to be conducted in existing medium size tokamaks
equipped with active coils, in particular ASDEX Upgrade, where a collaboration on these subjects is
already ongoing, but not only.
The flexible set of 192 active coils available in RFX-mod offers the unique possibility among all fusion
devices to produce in a controlled way a broad spectrum of proxy magnetic field errors, to be then
corrected with the remaining coils. Not only n=1 harmonics but also n>1 can be produced. This allows
testing quantitatively the efficiency of different error field correction schemes. Error fields are in fact
normally unknown and any correction approach must rely on an indirect estimate of the plasma
response to them, measured from magnetic measurements, effect on confinement, rotation, … RFXmod experiments can check to which degree these indirect measurements reflect the error field by
using known error fields.
The student will develop the real-time software needed to perform these experiments in RFX-mod and
will follow the related experiments and the data analysis. Various correction schemes, such as the socalled compass scan technique, slowly rotating fields, dynamic error field correction, and others will be
compared. The analysis work will also assess the relevance of the various schemes in view of their
application to medium size tokamaks and ITER. Not only the relevance, but also their technical
feasibility will be analyzed.
Dynamic error field correction based on slow magnetic feedback will be also investigated. This is
particularly important to provide useful information to develop slow magnetic feedback for error field
correction in ASDEX Upgrade, a medium-term project now being considered in collaboration with RFXmod. This will allow ASDEX Upgrade to correct error fields and their impact on the plasma, in particular
in high-beta experiments close to or beyond the no-wall ideal beta limit. This work may be also
relevant for other European and international medium-size tokamaks equipped with non axisymmetric coils, such as MAST and DIII-D, to which the collaboration may extend in the next years.