Insert Chart, Photo or Image Technology to Produce Rotation in Reactor Systems by N.W. Eidietis General Atomics Presented to FESAC TEC Rockville, MD May 31, 2017 Hemsworth New J. Phys. 2017 Insert Chart, Photo or Image Raman Fusion Eng. Design 2008

Fusion reactors will lack the beneficial rotation of contemporary devices Rotation & shear can provide many benefits Improved confinement Avoidance of locked modes, stabilization of tearing modes, RWM, ballooning mode etc… ITER & reactors will rotate slowly due to relatively small neutral beam (NBI) torque Maintaining Q in reactor prioritizes minimizing input power DIII-D  ITER: NBI power increases 2-3X, mass increases > 40X Momentum input efficiency (P/E, or ϵ) falls with increasing beam energy ∝ 𝑚/𝐸 Fusion reactors would benefit greatly from development of efficient toroidal momentum sources Solomon NF 53 2013 Buttery PoP 15 2008

Low energy + high mass key to increasing ϵ Low energy NBI (LNBI) Two technologies show promise for efficiently injecting angular momentum into reactor Low energy + high mass key to increasing ϵ Low energy NBI (LNBI) Ubiquitous technology in fusion Penetration limited by ionization Steady state Compact toroid injection (CTI) Studied on small tokamaks + military Penetrates until kinetic pressure = magnetic pressure (𝜌 𝑉 2 = 𝐵 2 / 𝜇 0 ) Pulsed Hemsworth New J. Phys. 2017 Onchi Fusion Eng. Design 2017

ITER NBI: 33MW, 1 MeV negative ion beam Low energy NBI: Low energy NBI case study: Double ITER injected torque with < 10% reduction in Q ITER NBI: 33MW, 1 MeV negative ion beam Low energy NBI: Power = 3 MW  Momentum injection efficiency 𝜖 𝐿𝑁𝐵𝐼 =10 𝜖 𝐼𝑇𝐸𝑅 Accelerating voltage: 𝑉 𝐿𝑁𝐵𝐼 𝑉 𝐼𝑇𝐸𝑅 = 𝜖 𝐼𝑇𝐸𝑅 𝜖 𝐿𝑁𝐵𝐼 2  V LNBI = 10𝑘𝑉 Current: I LNBI = 3𝑀𝑊 10 𝑘𝑉 =300𝐴 Perveance (𝑰/ 𝑽 𝟑 𝟐 ) is key parameter quantifying space charge limit for electrostatic accelerator ITER NBI: 4𝑥 10 −8 𝐴/ 𝑉 3 2 (negative ion) DIII-D NBI: 3𝑥 10 −6 𝐴/ 𝑉 3 2 (positive ion) LNBI: 3𝑥 10 −4 𝐴/ 𝑉 3 2 (positive ion) Note: Molecular D2 LNBI would increase 𝜖 by 2 Tech Development: 100x perveance increase

Technological developments required for LNBI Hemsworth New J. Phys. 2017 Goal: Increase perveance 100X Method 1: Increase area of ion source & accelerator grid Conceptually simple, but space at premium in toroidal geometry Limited by port size & focusing optics Method 2: Increase beam brightness Minimizes size Requires ion source development (increase A/m2) Increase aperture density in accelerator grid Cooling & beamlet focusing challenging ITER NBI Accelerator grid Cooling channel Aperture

Uncertainties exist in effectiveness of LNBI momentum transfer to plasma Low energy particles will not penetrate beyond reactor pedestal Will momentum efficiently transfer momentum to plasma medge? How significant will LNBI ion losses at edge be? Will edge momentum effectively transport to core region? Pedestal Te LNBI stops here LNBI Plasma minor radius

Mature technology basis exists for LNBI development Large pool of expertise exists for rapid development of high perveance LNBI system Method 1: Increase source & accelerator area Could be brought to TRL5 in 2-3 years without large technical leaps, but subsequent retrofit into existing NBI may prove difficult due to size Method 2: Increase source & accelerator brightness May require longer (5 years) to TRL5, but subsequent retrofit for TRL6 demonstration in existing mid-sized tokamak much easier

ITER Scale CTI Case Study (see Raman FED 2008) Tech Development: CW operation CTI Specifications Power: 5.5 MW Repetition rate: 20 Hz Injection velocity: 500 km/s Injected mass/ kinetic energy per shot: 2.2 mg D2 / 275 kJ Parameters demonstrated, single pulse [Degnan Phys. Fluids B (1993)] CTI 33 MW ITER D2 NBI Momentum injection rate (N) 22 6.5

Technological development required for CTI CW power supplies Must expand from single-pulse to ~ 20Hz CW Initial efforts already underway [Onchi FED 2014] Low-erosion electrodes Electrode coating technology must prevent release of metallic impurities from formation & acceleration electrodes Ensure electrode integrity over millions of pulses before servicing Raman FED 2008

Uncertainties exist for CTI development Basic physics & compatibility to be verified: Will CTI deposition physics hold up when aCTI < aplasma ? Is metallic erosion consistent with high performance ops? Comparison of CT & tokamak sizes (Raman FED 2008)

Maturity of CTI technology for fusion application Decades of experiments, but smaller expert population than NBI CQ Power Supplies: 1-2 years to TRL4 Erosion control: 2-3 year to TRL4 (in parallel with above) Combined: ~2 years TRL5 offline testing to verify impurities in CW After TRL5, ~ 1 year to install on existing mid-sized tokamak for TRL6 Repetitive CT formation circuit [Onchi FED 2014]

High efficiency angular momentum sources would be great benefit to fusion reactor Rotation plays beneficial role in tokamak stability & transport Heating NBI, ⍺, and RF provide minimal torque in reactor-class devices Two viable avenues proposed for angular momentum sources: Low-energy NBI: Mature steady-state technology, but space-charge limits performance & edge coupling physics uncertain Compact toroid injection: Flexible & efficient deposition, but CW technology & erosion control immature Both technologies are feasible for efficiently rotating reactor plasmas