Shell Pellet Research Articles

Novel rapid shutdown strategies for runaway electron suppression in DIII-D

New rapid shutdown strategies have been recently tested in the DIII-D tokamak to mitigate runaway electrons (REs). Disruptions in ITER are predicted to generate multi-MeV REs that could damage the machine. The RE population in large tokamaks is expected to be dominated by avalanche amplification which can be mitigated at high density levels by collisional drag. Particle injection schemes for collisional suppression of RE have been developed and tested in ITER-relevant scenarios: massive gas injection, shattered pellet injection (SPI) and shell pellet injection. The results show an improved penetration of particles injected with the SPI. Another strategy has been developed to harmlessly deconfine REs by applying a non-axisymmetric magnetic perturbation to worsen their confinement. This technique appeared to deconfine seed RE before the avalanche process could amplify the RE beam. The last method tested was to use the plasma position control system on the RE beam to prevent it from contacting the wall. This proved effective in preventing high current RE beam from touching the wall and providing more time to mitigate an existing RE beam but a successful ‘soft landing’ (without fast final losses) of the RE has not been observed yet.

Experiments in DIII-D toward achieving rapid shutdown with runaway electron suppression

Experiments have been performed in the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] toward understanding runaway electron formation and amplification during rapid discharge shutdown, as well as toward achieving complete collisional suppression of these runaway electrons via massive delivery of impurities. Runaway acceleration and amplification appear to be well explained using the zero-dimensional (0D) current quench toroidal electric field. 0D or even one-dimensional modeling using a Dreicer seed term, however, appears to be too small to explain the initial runaway seed formation. Up to 15% of the line-average electron density required for complete runaway suppression has been achieved in the middle of the current quench using optimized massive gas injection with multiple small gas valves firing simultaneously. The novel rapid shutdown techniques of massive shattered pellet injection and shell pellet injection have been demonstrated for the first time. Experiments using external magnetic perturbations to deconfine runaways have shown promising preliminary results.

Inertial fusion research over the past 30 years

Laser fusion research has been pursued intently over the past 30 years. The experiments using the GEKKO XII laser demonstrated a high-temperature compression of DT fuel of up to 10 keV, a neutron yield of 10 13 and high-density compression of CDT hollow shell pellet reached 1000 g/cm 3. These data are the first fruitful attainment of implosion fusion research in the world. These results gave the society a strong confidence in achieving ignition and burn in the near future. The US National Ignition Facility and French MJ Laser Program have started in this direction. A new method for ignition is also proposed to use a petawatt femtosecond pulse laser which can deliver the energetic electrons into the compressed fuel core to ignite the target. The energy deposition mechanism is a very interesting topic for study. The aim of ICF is to develop the civil energy for the next century. The level of ICF research is not at all inferior to MCF. The international collaboration, which was not so prosperous in the past compared to the MCF, has been highly recommended. No other alternate energy source holds as bright a promise as fusion and none has ever presented such formidable scientific and engineering challenges.

LASER FUSION—High density compression experiment and ignition program with GEKKO XII

High density compression of main fuel and stable formation of hot spark at the center of imploded core have been investigated to obtain the scaling and the requirements for fusion ignition and high gain. For this purpose, the experimental data of the high density compression up to 600 times solid density with hollow shell pellet have been analyzed and compared with simulations.

Status and Plans for GEKKO XII and Japanese Laser Fusion Program

Recent achievements in the pellet implosion include the generation of 1013 neutrons per shot and 10 keV plasma with stagnation free mode compression, and the high density compression up to 600 times solid density with plastic shell pellet implosion. Based on the present data base, we are now confident in achieving ignition and high gain using sufficient driver energy. The reactor design study and the development of the reactor driver have become important in order to demonstrate the feasibility of the fusion energy by ICF.

Symmetry in inertial confinement fusion implosion

A high density implosion of a hollow shell pellet by a GEKKO XII glass laser has been carefully diagnosed to clarify the relation between the achievable density and possible nonuniformities. The density achieved, 600 times solid density, has been well interpreted with the experimental conditions of pellet and laser intensity uniformity.

Acceleration of shell pellet targets subjected to laser irradiation

An analysis of many experiments is used to propose for the first time a scaling procedure for laser acceleration of shell pellet targets in a wide range of the radiation flux densities. It follows from the scaling procedure that the hydrodynamic transfer coefficient cannot exceed 20%.