Experimental facility HiPER 4a (operation in bunch mode for scientific viability) [3]

Neutron Flux γ-rays Dose

Thermal annealing

Silica Final Lenses under HIPER Laser Fusion Reactor Operation Conditions

REFERENCES:

[1] G. Zimmerman et al., Journal of the Optical Society of America 68 (1975) 549.

[2] J. Perkins. Internal report (http://www.docstoc.com/docs/54806737/Temperature-

Response-and-Ion-Deposition-in-the-1-mm)

[3] Proc. of SPIE Vol. 7916 79160S

[4] Marshal et al., Journal of Non-Crystalline Solids 212 (1997) 59-73.

The authors thank the Spanish Ministry of Science and Innovation for economical

support via the ACI-PROMOCIA program 2009.

Finite element solver ASTER CODE isused to estimate temperature, stressesand deformation in final lenses.

Ions must be mitigated to avoidmelting. X-rays are tolerable, theylead to maximum compressionstresses of 0.65 MPa, correspondingto maximum temperature increasebelow 15 K in one shot. Neutrons andγ-rays lead to negligible thermo-mechanical effects in bunch mode.

ODC and E’ defects increase witheach pulse due to neutron andγ-ray irradiation, producing laser

absorption at λ=350 nm.

Even at RT absorptionremains below 5% in thelifetime of HiPER 4a (<6000shots). Anyway thermalannealing (see inset)restores lens transparency.

Demo Power Plant HiPER 4b (operation in continuous mode for commercial demonstration)

Geometry Thermo-mechanical response Absorption by colour centres [4]

Introduction

We have studied the thermo-mechanical response and atomisticdegradation of final lenses in HiPER project. Final silica lensesare squares of 75 × 75 cm2 with a thickness of 5 cm. There aretwo scenarios where lenses are located at 8 m from the centre:•HiPER 4a, bunches of 100 shots (maximum 5 DT shots <48 MJ at≈0.1 Hz). No blanket in chamber geometry.•HiPER 4b, continuous mode with shots ≈50 MJ at 10 Hz togenerate 0.5 GW. Liquid metal blanket in chamber design.

Irradiation conditions From the spectra of a 48 MJ shockignition target from LASNEX [1,2], wehave calculated based on our reactordesigns realistic: neutron and γ-raydoses with MCNPX; ion energydeposition with SRIM; and X-rayenergy deposition with appropriateabsorption coefficients. Ion and X-raydeposit most of their energy densityin the first 10 μm.

The reactor chamber is a sphericalsteel chamber of 5 m inner radius with10 cm thickness and 1 mm W armor.48 laser beam lines are used forsymmetrical illumination of directtargets. Final silica transmission lensesare located at 8 m from the chambercentre. Final lens dimensions are 75 ×75 × 5 cm3. Radiation shielding areshowed in the figure above.

Absorption by colour centresThermo-mechanical responseGeometry

Conclusions•Silica lenses withstand irradiation conditions of HiPER 4a. Colour centre generation leads to higher absorption, however, the lens lifetime exceeds HiPER 4a operation time.•Thermal gradients appear inside the lenses under HiPER 4b conditions with detrimental effects such as aberrations, focal length change and operation complexity.

In continuous mode the role of neutrons and γ-rays are ofparamount importance. 3D finite element calculations show that asteady state situation is reached very fast (≈25000 shots equivalent to≈40 min at 10 Hz).

It is remarkable theappearance of a thermalgradient along both the axialand radial directions of thelens. The consequence is avarying refractive index. Inaddition note, that side facingthe target reaches highertemperature than the backside due to direct exposureto X-rays. As in the previouscase ions must be mitigated.The associated stress do notinduce deleterious effects inany case.

For the power plant concept wehave developed a chamber of 6.5m inner radius provided with ametal liquid blanket (thickness1m). Final silica transmissionlenses are located at 8 m from thechamber centre, as in theprevious case. Final lensdimensions are 75 × 75 × 5 cm3.Realistic radiation fluxes havebeen calculated making use ofthis geometry.

In steady state the lens temperatureis higher that 800 K, therefore defectannealing is very efficient resulting inlow optical absorption.

Summary•The reactor start up procedure isconditioned by change in refractiveindex until steady state is reached (focallength may vary up to 10 cm).•The temperature gradient leads todifferences in refractive index inside thelens possible origin of aberrations.

D. Garoz, A. Rivera, R. Juarez*, J. Alvarez , R. Gonzalez-Arrabal, J. M. Perlado Instituto Fusión Nuclear (DENIM) - Universidad Politécnica Madrid

*Departamento de Ingeniería Energética - UNED