Depth into surface

Material S

urface

Depth into surface

Abstract

Boundary science in magnetic fusion devices is severely hindered by a dearth of in-situ diagnosis of the surfaces of Plasma Facing Components (PFC). The customary laboratory surface diagnostic, nuclear scattering using MeV-range ions, is being adapted to the Alcator C-Mod tokamak. The design features toroidally and poloidally resolved measurements of PFC surface element and isotope compositions on a shot-to-shot basis with ~1 cm spatial resolution and sub-micron depth resolution. Several novel design features are described. An RF quadrupole accelerator supplies a high-current 0.93 MeV deuteron ion beam for injection into C-Mod between discharges. The ion beam is steered to a large fraction of the PFC surfaces by applying tokamak toroidal and poloidal fields. The deuterons have high Q, large nuclear-reaction cross-sections with most low-Z isotopes; the resulting high-energy gammas and neutrons are then detected for quantitative analysis of the surface compositions. Numerical modeling of these processes towards PFC measurements of interest are described, including boron film depth, deuterium fuel retention, isotope tracing for transport studies and PFC net erosion.

Basic principles of in-situ IBA for a tokamak

beamline

γn Detectors

Toroidal field provides

poloidal steering

Vertical field provides

toroidal steering :

vbeam R

F

B

vbeam R

FBz

D+

(1) Radio Frequency Quadrupole (RFQ) linear accelerator injects 0.9 MeV D+ beam into the vacuum vessel through a radial port

(2) Tokamak magnetic fields provide steering via the Lorentz force:

(3) D+ induce high Q nuclear reactions with low Z isotopes in PFC surfaces producing ~MeV neutrons and gammas

(4) In-vessel detection and energy spectroscopy provides a veritable cornucopia of PFC information

PSI science is severely hindered by the lack of comprehensive in-situ diagnostics; new PSI diagnostics are required

The ideal PFC surface diagnostic would provide measurements:● in­situ without vacuum break ● on a shot­to­shot frequency for time resolution● of large areas of PFC surfaces (poloidally and toroidally resolved)● of elemental/isotope discrimination to depths of ~10 microns

● Existing in­situ PSI surface diagnostics (QMB, colorimetry) are severely limited in deployment and unable to meet all requirements.

● Ion beam analysis (IBA) is the “gold standard”, but it is ex­situ and intrinsically yields “archaeological” information.

Optimally, the installation position maximizes geometric access to the PFC surfaces while minimizing the required magnetic fields for beam steering.

The simulation tracks D+ through C-Mod's magnetic fields and geometry from potential RFQ installation positions.

After “unwrapping” the first wall PFC “skin”, the D+ hits on the first wall can be plotted as a function of magnetic field to determine the PFC surface coverage as a function of magnetic field.

ACRONYM is a 3D Monte Carlo particle transport-in-matter simulation that functions as a complete synthetic diagnostic

A 3D RFQ beam dynamics simulation has been designed to optimize the installation position of the RFQ accelerator

Simulation results show that an angled, midplane D+ beam injections results in excellent toroidal and poloidal coverage of the first wall for acceptable values of magnetic field

POOR COVERAGE GOOD COVERAGE

Vacuum Vessel

Moderator Station (w/ Fission

Chamber #5)

Concrete Igloo

252Cf Neutron Source

Top View of Alcator C-

Mod

Visualization of Alcator C-Mod geometry in ACRONYM

Experimental setup for Neutron Diagnostic System

and ACRONYM

Benchmarking Result (counts/source neutron) Neutron calibration experiment : 4.13 x 10-8 ACRONYM simulation: 4.06 x 10-8

M a t e r i a l S u r f a c e

Primary neutron spectrum from

retained deuterium is up-shifted in energy above

expected noise

● ACRONYM (Alcator C-Mod RFQ Official Neutron Yield Model) is a highly realistic particle transport-in-matter simulation for Alcator C-Mod built using the Geant4 toolkit1:

● RFQ D+ beam● C-Mod magnetic fields● geometry from C-Mod Solid Edge models● full models of neutron and gamma detectors● parallel architecture for scalable processing

ACRONYM has been successfully benchmarked

● ACRONYM has been successfully benchmarked against the Alcator C-Mod Neutron Diagnostic System calibration experiments2:

● a known Cf-252 neutron source is placed inside the vacuum vessel● neutrons are transported through the geometry● thermal neutron fission detectors record incident hits● the calibration result is (detector counts) per (Cf252 source neutron)

ACRONYM generates fast neutron detector responses for any given deuterium retention profiles in a PFC

ACRONYM generates inorganic scintillating detector responses forany given amounts of low-Z impurities in a PFC

● 1 MeV D+ enter PFC material and lose kinetic energy ● D+ react with retained deuterium via: ● Neutron birth energy is 2.53MeV + F(ED+, angle of neutron emission)

● Forward scattered neutron are significantly up-shifted in kinetic energy

DD n 2.53MeV3 He 0.74MeV Q=3.27 MeV

Unfolding the Alcator C-Mod first wall “skin” onto a 2 dimensional grid

REFERENCES:(1) S. Agostinelli, J. Allison, et al. “Geant4 – a simulation toolkit”. NIM A 506 (2003) 250.(2) C.L. Fiore and R.L. Biovin. “Performance of the neutron diagnostic system for Alcator C-Mod”. Rev. Sci. Instr. 66 (1995) 945.

The amounts of low-Z isotopes present in the PFC material can be measured by examining the deuteron-induced gamma emission from the generic reaction:

Because the gammas are emitted via nuclear de-excitation, the gamma energies are characteristic of the reactant isotope, allowing low-Z isotope discrimination.

Example of a simulated NaI(Tl) detector ideal energy spectrum (left) and realistic pulse height spectrum (right) for the thick

target yields of several low-Z isotopes of interest

XZA

H12

Y *Z1A1

n01

YZ1A1

E *