1. Experimental and Numerical Evaluation of Different Fluidization Parameters for Successful Target Layering Kurt J. Boehm 1,2 N. B. Alexander 2, A. Bozek 2, D. Frey 2, D. Goodin 2, A. R. Raffray 1 ( 1 UCSD, 2 General Atomics) Comparison between Numerical modeling and the room temperature loop experiments Sputter Coating -- Preparation of Studies to Analyze Surface Roughening During Fluidization Problems and Challenges Soft pellet- pellet collisions lead to static charges Pellets stick to the pan and ultimately lead to a non-uniform layer Possible surface damage due to rolling over the dish surface and/or soft particle collisions Thickness of the sputtered layer can only be estimated. Gas stream Funnel Frit Swirling Frit Cone Frit Angled holes Particle Movement Expected particle paths using different frits Gas stream Wedge Frit Fluidized Bed Analysis – Optimizing Fluidization Parameters Replicable Fluidized Bed Section Capsules Frit Closed loop plumbing circuit Polonium Strip The Optimal Frit Design Provides: - Spin rate and circulation rate higher than a certain threshold value for uniform layering (about 5Hz). -(The spin rate is the more important performance criterion) - A certain randomness of the orientation of the spin (hard to quantify) The performance of different frits during fluidization was analyzed to determine the “best design” for the given criteria. Analysis has been done by post experimental processing of high speed videos (250-500 frames per second) Frit Performances Surface damage during bouncing pan experiments Presented in “High Z coatings for IFE applications” by Abbas Nikroo et alt., HAPL Review Meeting, November 13-14, 2001, Pleasanton, CA Electro-magnetic shaker High Z target Bounce Pan Previous set up: bouncing pan Proposed design: spinning dish High Z target Rotating Dish Targets roll up on one edge and down though the middle Wyko surface data taken from a shell coated in the rotating dish set up. The surface roughness before the coating process was unknown. Numerical Simulation using MFIX MFIX (Multiphase Flow with Interphase eXchanges) is a general-purpose computer code developed at the National Energy Technology Laboratory (NETL) for describing the hydrodynamics, heat transfer and chemical reactions in fluids-solids systems. MFIX has two significantly different models of particle movement: Model A - Particle movement averaged  computes the average velocity for all particles within one fluid cell Model B - Discrete Element Simulation (DES)  each particle motion tracked individually Tracking random individual paths using DES This run simulates a fluidized bed at 2 BE. The simulated time was 1s, particle diameter 4mm, the diameter of the fluidized bed was 15 cm. We want to modify MFIX to calculate: - time averaged circulation and spin rates of the pellets - relation between spin and circulation rates at different locations in the bed  in experiments only pellets next to the wall can be seen - Explore different bed sizes and fluidization parameters to predict optimized layering - Model the layering process using the heat transfer capabilities - Explore the effects of non-uniform mass density in target (unlayered condition) The following modifications will be necessary: - A suitable fluid - particle model for needs to be found. - Tracking of the rotational motion and angular position of each target - A collision model for unlayered targets (asymmetric mass distribution in capsule) Observations: - Flow in the lab shows a preference to circulate across the whole width of the bed (at BE > 2.0) - Pellets may behave differently near tube wall than in center of the bed, although - Analysis of top and side view indicate - similar behavior of the particle motion across the bed - some degree of randomness in the spin orientation - All bed designs show a spin rate above the threshold of 5Hz. - The circulation rate shows a maximum for all frit designs between 2 and 4 BE. - In the swirling bed, the particles roll along the inside of the tube causing a steep and steady increase in spin rate - The cone and the funnel are expected to provide best randomness of rotation Schematic example of the observed flow using the cone frit Air flow Preferred motion of the pellet Screenshot of a side and a top view video with a regular straight frit A white foam ball makes it easier to track a target The circulation rate vs. bed expansion for the different fluidized bed configurations The spin rate vs. bed expansion for the different fluidized bed configurations Using cell averaged model (A) in symmetric cylindrical coordinates Compared to room temperature experiment under same conditions At low flow speed: - Strong agreement for the bubbling flow pattern - Bubbling frequency can be predicted quite well At higher flow speeds: -3-D effect (circulation pattern) dominant in experiment  3D package of MFIX needed Different particle void fractions in the fluid cells over time Video of the room temperature fluidized bed Bubbles can wander off center The sputter coater set up with a spinning dish: different dish sizes, angles of incline and the rotational speeds can be chosen. Goal: Produce a uniform DT Layer on the Inside of the Capsules while Maintaining a Smooth Outer Surface T = 0.00 s T = 0.04 s T = 0.08 s T = 0.12 s T = 0.00 s T = 0.04 s T = 0.08 s T = 0.12 s Nominal Value of 2 BE All frits above “critical value” of 5 Hz Fluidized bed test stand at General Atomics laboratory Avoides: Hard particle- particle collisions to preserve smooth surface Development of a new method to produce a large number of smooth and uniformly coated shells Current techniques require further work since: - Number of targets produced needs to be increased to 100-1000 - Surface roughening during target coating needs to be eliminated - Reduction of surface damage expected, since only soft collision will occur - Dish can be loaded with larger number of shells 10  m 50  m Ablation of the gold layer was concluded to be a result of collisions