Developing the science and technology for the Material Plasma Exposure eXperiment

Abstract

Linear plasma generators are cost effective facilities to simulate divertor plasma conditions of present and future fusion reactors. They are used to address important R&D gaps in the science of plasma material interactions and towards viable plasma facing components for fusion reactors. Next generation plasma generators have to be able to access the plasma conditions expected on the divertor targets in ITER and future devices. The steady-state linear plasma device MPEX will address this regime with electron temperatures of 1 -10 eV and electron densities of 10 21 -10 20 m -3 . The resulting heat fluxes are about 10 MW/m 2 . MPEX is designed to deliver those plasma conditions with a novel Radio Frequency plasma source able to produce high density plasmas and heat electron and ions separately with Electron Bernstein Wave (EBW) heating and Ion Cyclotron Resonance Heating (ICRH) with a total installed power of 800 kW. The linear device Proto-MPEX, forerunner of MPEX consisting of 12 water-cooled copper coils, is operational since May 2014. Its helicon antenna (100 kW, 13.56 MHz) and EC heating systems (200 kW, 28 GHz) have been commissioned. The operational space was expanded in the last year considerably. 12 MW/m 2 was delivered on target. Furthermore electron temperatures of about 20 eV have been achieved in combined helicon and ECH/EBW heating schemes at low electron densities. Overdense heating with Electron Bernstein Waves was achieved at low heating powers. The operational space of the density production by the helicon antenna was pushed up to 8 x 10 19 m -3 at high magnetic fields of ~1.0 T at the target. Proto-MPEX has been prepared to allow for first material sample exposures, albeit for short pulse duration. The experimental results from Proto-MPEX will be used for code validation to enable predictions of the source and heating performance for MPEX. MPEX, in its last phase, will be capable to expose neutron-irradiated samples. In this concept, targets will be irradiated in ORNL's High Flux Isotope Reactor and then subsequently exposed to fusion reactor relevant plasmas in MPEX. The current state of the MPEX pre-conceptual design and unique technologies already developed, including the concept of handling irradiated samples, are presented.

Coupling of RF antennas to large volume helicon plasma

“…Large volume helicon plasma has been attracting growing interest in various fields, including large scale semiconductor processing [1], electrodeless plasma propulsion with high power input [2,3,4,5], and emerging plasma-material interaction under fusion conditions recently [6,7,8,9]. Large volume allows high power capacity and big heat flux, besides large cross sectional area.…”

“…Except the novel design of spiral antenna to excite helicon plasma from an axial end and with diameter of 75 cm [10,11], most helicon sources employ cylindrical RF (Radio Frequency) antennas wrapping plasma with diameter less than 40 cm. For extremely high power applications such as MPEX (Material Plasma Exposure eXperiment) [6,7,8], the usually involved quartz tube is even removed. Although half helix, Boswell, Nagoya III and loop antennas are widely used for small helicon sources, it is still not yet clear that which type of RF antenna can best couple helicon plasma of large size.…”

Considerations for in situ, real time measurement of plasma-material interactions using Digital Holographic imaging

“…ORNL has several devices which can produce plasmas with a range of heat fluxes. The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) has demonstrated plasma heat flux of ∼ 5 MW/m 2 onto material targets in pulsed operation [6]. Additionally, the ElectroThermal Arc (ET-Arc) source produces ELM-like transient plasma heat flux of 1-2 GW/m 2 [7].…”

“…Proto-MPEX utilizes an RF helicon antenna to produce high density, low temperature, light-ion (H, D, He) plasma in a cylindrical magnetic geometry, and is also being considered as a platform for in situ DH measurements [3,6]. The magnetic field is established from t = 0 to ∼ 9 s, with a "flat top" duration of constant field from t = 2 to 6 s. In the middle of this steady field, the plasma discharge is initiated at t =∼ 4 s, for typical durations of 0.5 to 1.0 s. During this discharge duration, auxilliary electron and/or ion heating can be applied to the plasma column (with durations of 50 to 300 ms).…”

Helicon normal modes in Proto-MPEX

“…Ever since Boswell published on the ionization efficiency of inductively coupling RF waves to the natural oscillations in a plasma column [1,2], helicon plasma sources have gained interest in many applications. A recent application of helicon sources has been as plasma sources for fusion-relevant plasmamaterial interactions (PMI) investigation [3][4][5][6][7][8]. However, in order for helicon sources to be relevant to PMI investigation, they must be able to produce high-density plasmas with light ion fuels (H 2 , D 2 , He).…”