Engineering limits on plasma scenarios

Engineering limits on plasma scenarios I Nunes

Outline • Vacuum vessel • Vacuum vessel structure • Vacuum vessel displacements • Disruption forces • TF coils • TF operation • What limits TF • PF coils • P1 • Other PF coils • Forces acting on the coils and interaction between coils • Role of Engineering Analysis Group (EAG) SL course 22-26 November 2010

Vacuum vessel Vacuum vessel TF coils • Non-circular cross-section • Metal composite welded to be mechanically stable and ultra high vacuum compatible • Double walled • Baking with warm gas (He) • 8 octants (5 sectors/octant, each has 2MVP+1MHP) • Rigid sections to withstand large stresses imposed by external forces • wedge-shaped sections joined together by parallel bellows determine the electric resistance of the metallic vessel (keep induced current as low as possible) • Incorporate openings to vessel interior such as ports, diagnostics, additional heating, etc. SL course 22-26 November 2010

Vacuum Vessel supports VV subject to forces and movements from 4 major sources • It’s own weight • Thermal displacement • Forces between plasma and divertor during normal operations • Reaction to induced magnetic forces from rapid changes in plasma current (e.g. disruptions) • Support systems  used to dampen and arrest the vessel displacement during plasma operations and to allow free movement during transitions in Tv SL course 22-26 November 2010

Vacuum vessel supports Vessel brakes for top and bottom MVP (2 per MVP) • Restrain the upper and lower MVPs against the top and bottom limbs • Adjusted every 30oC when ramping up/down Tv • Locked during plasma operations Fixed vertically MHP restraint operation • resists fast toroidal movements such as sideways movements Fixed horizontally JOI 3.11 SL course 22-26 November 2010

Vacuum Vessel reactions Forces applied to the vacuum vessel during disruptions can be vertical due to Vertical Displacement Events (VDE) and sideways due to Asymmetric VDEs (AVDEs).Although the vacuum vessel is quite robust, there are weak points on the vessel that need to be monitored. Most disruptions will not endanger the vacuum vessel but may jeopardise operations Broken windows, leaks, etc.  STOP OPERATIONS • Welded root of the MVP is strained when the port moves radially (when subjected to a vertical force) • Number of cycles depends most likely on the total strain of the MVPs which is a function of Frad SL course 22-26 November 2010

VV displacements - rolling JOI 1.2 and 1.4  limit < 3.5mm • Due to the vertical force impulse generated by the vertical displacement of the plasma and subsequent current quench • Main vertical ports are prevented from moving vertically  bulk of the vessel appears to rotate around a pivot point (midplane) • Vertical load is reacted mainly by the MVP SL course 22-26 November 2010

VV displacements - sideways • Proportional to the integral of the asymmetry of the vertical current moment (linked to Ip) × BT • Occurs only after Asymmetric Vertical Displacement Event (AVDE) – due to m/n=1/1 kink mode that causes the plasma current to tilt relative to BT creating a net horizontal force • asymmetric toroidal distribution of the vertical force • Seen up to 7mm (3.5MA/2.8T disruption) JOI 1.4 and 1.5  limit < 3.5mm SL course 22-26 November 2010

Vacuum vessel – disruption forces • VERTICAL FORCE • Plasma current • For a pulse with several scenarios, level 1 will chose always the highest force To predict f for a certain configuration the worst case disruption (Fworst) is determined by: • VDEs done at low current (usually at 1.5MA) where the plasma is kicked and the Vertical Stabilisation system (VS) is “switched off”. • For statistics, normally three VDEs are done to calculate f Plasma growth rate: velocity of plasma displacement from a neutral point JOI 1.1, JOI 1.2 and JOI 1.3 • Plasma shape (elongation) and minor radius SL course 22-26 November 2010

Vacuum vessel – probability of leak A model probability based on disruptions that have damaged the vessel or required in-vessel work through years of operation for Fth ≤ F ≤ F1 F: disruption force Fth: force below which the probability of failure is zero (F<300 tonnes) F1: force above which the probability is unity For F=580 tonnes model 1: 1 in 2.6 disruption will cause a leak Model 3: 1 in 1.5 disruptions will cause a leak SL course 22-26 November 2010

Vacuum vessel • Vacuum vessel structure • Vacuum vessel displacements • Disruption forces • TF coils • TF operation • What limits TF • PF coils • P1 • Other PF coils • Forces acting on the coils and interaction between coils • Role of Engineering Analysis Group (EAG) SL course 22-26 November 2010

TF coils Water or coolant Copper conductor Epoxy/glass insulation • 32 D-shaped coils – each made of 24 turns • Initial design value to operate up to 3.45T (R=2.9m) but built flexibility makes operation up to 4T possible (constrained) • “D” shaped  only tensile stresses in the conductor, no bending stresses • Each conductor has two parallel cooling channels  all turns are cooled in parallel SL course 22-26 November 2010

TF coils – stress • Inner tail • Stress due to ITF alone and occurs in every plasma pulse and every dry run • No current flow  thermal gradient between tail and next current carrying conductor • Hoop stress pulling the last turn is transferred to the tail section and reacted in tension and in shear  high shear stresses at the tip of the tail Outer tail Inner tail 1/g: TF coil-set L/R time tH: EM stress at ITFref=78kA tT: Thermal stress at I2tref=80×109A2s SL course 22-26 November 2010

TF coils – life consumption • Operation at high TF (>3.45T) • Limited in I2t due to thermal loads • Life consumption is defined as “equivalent pulses” and depends on the shear stress at the coils tails • “equivalent pulse”  ITF=78kA (4T) and I2t = 112×109 A2s has a shear stress of 25MPa G: defines the limit of the TF operational space JOI 3.1 and JOI 5.2 (scenario at high TF) JOI 1.6 and JOI 1.7 (disruptions at high TF) JOI 3.1 SL course 22-26 November 2010

Vacuum vessel • Vacuum vessel structure • Vacuum vessel displacements • Disruption forces • TF coils • TF operation • What limits TF • PF coils • PF coils • P1 • Forces acting on the coils and interaction between coils • Role of Engineering Analysis Group (EAG) SL course 22-26 November 2010

PF coils • Same design concept as the TF coils • Changes to the magnetic configuration can be performed. Most common: • Shaping circuit turns (P2+P3) modified only to predefined values due to mechanical (forces induced by P4) and thermal stresses (I2t) and effect on TF out-of plane forces • D1 polarity JOI 3.14 and JOI 3.3 SL course 22-26 November 2010

P1 coil • P1 made of 10 coils stacked at the centre of the machine. • P1 carries the magnetising current for the JET magnetic circuit • central core normally saturated • high current in the coils  coils are stressed in tension • TF applies compression, which can partially relieve the hoop stresses • PFX circuit – 6 central pancakes SL course 22-26 November 2010

Interaction between TF and PF coils • Each toroidal coil is subjected to a bursting force (~BT2) reacted by the P1 coils • TF coils exert a compressive force on the P1 central coils helping to compress the tensile stresses on P1 due to EM hoop and thermal expansion High TF allows higher IP1 SL course 22-26 November 2010

Forces acting on the TF coils • Each toroidal coil is subjected to • In-plane forces: due to ITF • Out-of-plane forces:due to the field normal to the TF coils caused by the plasma and the PF coils(ITF×Bpol) • The total force applied to the TF coils is a torsional force that increases with Ip and depends on the type of scenario JOI 3.5 SL course 22-26 November 2010

Forces acting on the TF coils • This force is reacted by a mechanical structure  outer part of the coil lies in grooves in the outer shell and the force is transmitted to the mechanical structure JOI 1.7 and 3.5 • However, the ring and collar teeth protrude from the main structure and are highly stressed • Collar tooth: because of the peak shear in the coil (420kN) • Ring tooth: because of its bolting assembly (560kN) • Load is calculated using the TF transverse fluxes loops • Collar tooth: 10.4 kAWb • Ring tooth: 15.6 kAWb SL course 22-26 November 2010

Interface with ATO and EAG • The EiC is responsible for the safety and integrity of personnel and machine. • The EiC responds to the Torus ATO (Authorisation to Operate) Holder and the Chief Engineer. • The EAG (Engineering Analysis Group) supports the Chief Engineer. Chief Engineer ATO Holder Engineering Analysis Group Scientific Coordinator Session Leader RTC Duty Officer Diagnostic Coordinator Engineer in Charge Control Room

Role of the EAG • The EAG can be called in for help in developing plasma scenarios within the engineering limits of JET. • In order to optimise the use of a relatively scarce resource, queries should be limited to matters related to engineering, protection and procedures. However, if in doubt about the pertinence of the question, it is better to ask while planning the session (or earlier) instead of waiting till close to the pulse execution. If a question has already been asked repeating the answer takes little effort. If the question is not an EAG competence, most likely we know who to send you to talk to… so do not be afraid of asking. • Get EAG involved when you plan: • non-ordinary shapes (almost-double X-point, slim plasmas, …) • non-ordinary heat loads (long pulses, limiter configurations, …) • Gold Form related (high TF, use of SC, modifications to PTN, …) Close and timely interaction is appreciated.

Engineering limits on plasma scenarios