1. 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

2. 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
Buttery PoP

3. 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

4. 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 (negative ion)
DIII-D NBI: 3𝑥 10 −6 𝐴/ 𝑉 (positive ion)
LNBI: 3𝑥 10 −4 𝐴/ 𝑉 (positive ion)
Note: Molecular D2 LNBI would increase 𝜖 by 2
Tech Development:
100x perveance increase

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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)

11. 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]

12. 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