Advancing local helicity injection for non-solenoidal tokamak startup

Self Cite

The PEGASUS-III experiment is a solenoid-free, low aspect ratio spherical tokamak that will serve as a dedicated U.S. platform for comparative nonsolenoidal tokamak plasma startup studies. Approximately 175 megavolt-ampere (MVA) of reconfigured and expanded programmable power systems, 7 MJ of new stored energy, and new digital control and protection systems for the facility are being commissioned to support PEGASUS-III upgrades. These include: increased toroidal field (0.15-0.6 T); new divertor and poloidal field coils; increased pulse length; local and coaxial helicity injectors for solenoid-free plasma initiation; radio frequency (RF) systems for heating and current drive; and a diagnostic neutral beam (DNB). A new realtime digital control system implements 16 proportional-integral differential (PID) feedback controllers with 25 kHz loop rates to control the electromagnets and helicity injectors. The poloidal field coils, helicity injector arc currents, and toroidal field are driven by 36 3.6 MVA (4 kA, 900 V) insulated-gate bipolar transistor (IGBT) buck converters. Helicity injector bias voltage and current will be provided by a set of four 10.8 MVA multilevel buck converters (MLBCs). Each is comprised of an 1800 V integrated gate-commutated thyristor (IGCT) stage and a ±900 V IGBT stage in series, providing controllable I inj ≤ 4 kA at V inj ≤ 2.7 kV. A field programmable gate array (FPGA)-based digital fault protection system multiplexes controller commands to individual power semiconductors in these supplies, monitors their operational status, and executes shutdown sequences within 10 µs of fault detection. An 80 kV, 4 A zero-voltage-switching (ZVS) resonant converter with <1% output ripple is under development for the DNB and is being evaluated as a topology to drive RF sources.

“…Representative loads for two-and four-quadrant bridges are electromagnets and helicity injector arcs; one quadrant IGCT bridges were previously employed for LHI bias [6].…”

Section: Modular Buck Convertersmentioning

confidence: 99%

Section: Helicity Injector Multi-level Buck Convertermentioning

confidence: 99%

See 1 more Smart Citation

Digital Control and Power Systems for the Pegasus-III Experiment

Bongard

Borchardt

Diem

et al. 2022

Self Cite

“…The upgraded device will have a new mission: to compare, contrast, and combine several of the leading concepts for nonsolenoidal startup. These include coaxial helicity injection (CHI) [4], local helicity injection (LHI) [5], radio frequency (RF) [6], and external poloidal field (PF) induction startup [7]. The goal is to develop a validated physics and technology basis for mega amp class startup on NSTX-U and beyond.…”

Section: Introductionmentioning

confidence: 99%

A Coaxial Helicity Injection System for Nonsolenoidal Startup Studies on the PEGASUS-III Experiment

Reusch

Raman

Bongard

et al. 2022

Self Cite

Initiating current without using magnetic induction from a central solenoid is a critical scientific and technical challenge facing the spherical tokamak (ST). One such technique that has shown promise on several devices is coaxial helicity injection (CHI). In CHI, a dc voltage applied to large area coaxial electrodes injects current and helicity into the vacuum vessel for plasma startup and plasma current (I p ) sustainment. Major outstanding issues for CHI include eliminating the need for a vacuum vessel break; the scaling of I p with injector and/or flux footprint shape and separation; mitigating plasma material interaction (PMI) and minimizing impurity injection; and the degree of axisymmetry required to achieve high I p . Thus, a first of its kind, CHI system is being installed on PEGASUS-III. It utilizes two coaxial, segmented, floating electrodes located entirely within the vacuum vessel in the upper divertor region. The design enables I p scaling studies as the electrode shape, coupled with a new 480 kA/288 kA/244 kA-turn divertor coil triplet, and allows for variation of the flux footprint shape and location simply through manipulation of coil currents. Together, they are projected to allow over 50 mWb connected flux and I p > 300 kA. The segmented electrodes facilitate simple changes to their shape, position, and plasma-facing material. This flexibility may be critical for mitigating PMI or impurity sourcing from the electrodes. Independent current feeds to each segment enable tests of the impact of axisymmetric drive on I p .

“…The PEGASUS program developed the physics basis for local helicity injection (LHI) startup and identified the important parameters that define the limits of this technique [4]. PEGASUS-III will focus on evaluating multiple solenoid-free startup techniques on a single platform including: LHI, coaxial helicity injection (CHI) [5], and RF-electron Bernstein wave (EBW)/electron cyclotron (EC)-startup techniques [6].…”

mentioning

confidence: 99%

The New PEGASUS-III Experiment

Sontag

Fonck

Borchardt

et al. 2022

Self Cite

Developing attractive means of initiating current without using magnetic induction from a central solenoid is a critical scientific and technical challenge facing the spherical tokamak (ST). The PEGASUS program has focused on developing the physics basis and predictive models for nonsolenoidal tokamak startup using local helicity injection (LHI) and has demonstrated startup to ∼0.2 MA in a low-field, near-unity aspect ratio ( A) ST. The PEGASUS facility is being upgraded into a solenoid-free ST called PEGASUS-III. Major features include: increased toroidal field (TF) to 0.6 T for up to 100 ms, improved shape control, and retaining the low-aspect ratio geometry of A ∼ 1.2. This TF directly supports the new mission to expand the breadth of solenoid-free research on the facility with multiple reactor-relevant techniques. The implemented TF upgrade is comprised of a new center rod, outer TF coil system, torque plate assemblies, and TF interconnects between the center rod and outer conductors. The center rod is comprised of 24 watercooled, insulated wedge conductors inside a new inner vacuum wall. There is no ohmic solenoid, which allows additional TF conductor for the 48-kA/turn TF current. The outer TF conductors are 12 pairs of air-cooled, reinforced Al plate conductors. Pairwise crossed conductor links connect the outer C-plates to the central rod conductor to eliminate the need for a toroidal compensation wind-back coil. Torque plate assemblies on top and bottom mechanically secure both the outer C-conductor and the interconnecting links, counteract torsional magnetic loads during a pulse, and provide compliance for vertical displacement. The assembly accommodates magnetic and thermal forces, limiting the axial excursion of the central assembly to <1 mm.