Thread by @bananelectrons

#ScientificWednesday

Radiative pulsed L-mode operation in ARC-class reactors - fresh one from @CFS_energy and @MIT_Fusion

Enhanced confinement & internal transport barriers create large pressure gradients providing significant bootstrap current fractions. High confinement time allows minimization of plasma current improving stability while also reducing external current drive requirements needed

Recently, high temperature superconductor (HTS) technology has dramatically increased achievable on-axis magnetic field in reactor designs. Since fusion power density scales, this technological advancement provides opportunities to improve self-consistent reactor scenarios

AT scenarios, combined with the stronger on-axis magnetic fields available using HTS have been shown to significantly decrease the minimum device major radius needed in a steady state reactor

As size is one of the largest drivers increasing levelized cost of electricity, high B scenarios are economically attractive

Proposed pulsed L-mode operation scenario with largely radiative heat exhaust by impurity line radiation in the plasma edge, denoted as “RPL-mode,” in an ARC-class high-field compact tokamak (R ∼ 3 m to 4 m, a ∼ 1 m, B ∼ 10 T)

The RPL-mode facilitates power density maximization while removing the need for an advanced divertor by dramatically decreasing the power exhausted to the scrape-off layer (SOL). This allows us to produce some of the highest power densities of any tokamak reactor concept

However, energy density in high-field compact devices is limited by the available heat exhaust solutions, and advanced divertors are required to minimize damage to plasma-facing components (PFC)

Introducing radiative heat exhaust allows to push past divertor-related energy density limits and maximize fusion power density. Unlike heat exhaust through the divertor, where only a small physical area is exposed to heat loading, ...

...radiative heat exhaust uniformly distributes the heat load over the plasma viewing PFC increasing the exposed area and substantially reducing the peak energy fluences

Previously, L-mode scenarios were not reactor-relevant due to lack of sufficient confinement, but high B from HTS magnets enables Ip and n maximization, substantially increasing the achievable fusion power with L-mode

Radiative heat exhaust was examined ex-perimentally in a number of devices. Highly radiative L-mode tokamak plasmas have been demonstrated in: ISX-B, ASDEX, TEX-TOR94, DIII-D, Alcator C-Mod, TFTR, ASDEX-U, and COMPASS
@d3dfusion @TokCOMPASS

To begin RPL-mode scoping authors performed a 0-D power balance calculation using a slightly modified version of the Plasma Operational Contours (POPCONs) technique

One surprising result: despite only slightly enhanced L-mode confinement, plasma has large ignition region and thermally stable operating point is achievable. This results from high operational densities enabled by combination of high-field, compact size, and pulsed operation

Another attractive feature of RPL-mode operation is strong electron-ion temperature coupling & an exceptionally small, < 5 MW, Cordey pass indicating ARC tokamaks in RPL-mode could use rudimentary RF systems

Using integrated modeling authors have confirmed that thermally stable, near-ignited, RPL-mode can be obtained. Integrated modeling procedure is a major advance over previous ARC studies, which used POPCONs with imposed profiles not self-consistent core transport simulations

Additional analysis of RPL-modes using ’physics informed’ POPCONs indicated the they will scale favorably to negative triangularity. Increased confinement from the reduction of turbulent transport lowers Ip, reducing Vloopand extending the reactor’s pulse time

Once these constraints are addressed, optimization of plasma shaping and size with a focus on negative triangularity to reduce turbulent transport is a high-yield next step. This could be done using an optimization

After transport optimization, startup modeling, field-coil-set optimization, and engineering design of an RPL-mode ARC reactor capable of accommodating optimized plasma shape could be seriously attempted

Startup & current ramp modeling with more realistic coil-set & saw-tooth model will be required for accurate flux consumption estimation. It may be necessary to iterate between plasma shaping optimization, startup, and coil-set modeling...

...as coil positioning and currents as well as shaping can be closely linked to startup f l ux consumption through the external and plasma inductance

When accurate flux consumption calculations and a solenoid engineering design are obtained, one could estimate pulse time, determine component lifetimes under cyclic loads, and assess economic viability

Disruptions are an especially serious concern in RPL-mode scenarios. The high plasma current densities and large radiated power fractions can trigger instabilities and radiative collapse