{"id":838,"date":"2017-03-22T19:02:39","date_gmt":"2017-03-22T19:02:39","guid":{"rendered":"http:\/\/www.btitargetry.com\/?page_id=838"},"modified":"2017-03-28T13:45:03","modified_gmt":"2017-03-28T13:45:03","slug":"visualization-target-wttc15","status":"publish","type":"page","link":"https:\/\/www.btitargetry.com\/?page_id=838","title":{"rendered":"Visualization Target (WTTC15)"},"content":{"rendered":"

Visual Observation of Boiling in Batch-Style Water Targets<\/strong><\/p>\n

J. Peeplesa,1<\/sup>, M. Stokelya<\/sup>, M. Poormana<\/sup>, M. Magerlb<\/sup>, B. Wielanda<\/sup><\/p>\n

a<\/sup><\/em>BTI Targetry LLC, 1939 Evans Rd. Cary, NC, USA<\/em><\/p>\n

b<\/sup><\/em>IBA Molecular, 801 Forestwood Dr. Romeoville, IL, USA<\/em><\/p>\n

Introduction<\/strong><\/p>\n

Previously, a top pressurized, batch style, 3.15 mL total volume water target with transparent viewing windows was operated on an IBA 18\/9 cyclotron at 18 MeV proton energy and beam power up to 1.1 kW.\u00a0 Video recordings documented bubble formation and transport, and blue light from de-excitation of water molecules produced images of proton beam stopping geometry including location of the Bragg peak.<\/p>\n

Visualization Targets<\/strong><\/p>\n

The same methodology was applied to three additional visualization targets to examine the effect of target geometry on observed boiling phenomena.\u00a0 All three targets featured a wider chamber to allow for higher beam transmission and an increased chamber height, consistent with current trends in high power targets.\u00a0 One target featured a reduced chamber depth, and another had a ramp in the back of the chamber to reduce fill volume.\u00a0 Target pressure was limited to a maximum of 14 bar (200 psi) due to the larger diameter beam window.<\/p>\n

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Experimental Results<\/strong><\/p>\n

The three targets were each tested at 7 bar (100 psi) and 14 bar (200 psi) for several fill volumes.\u00a0 For each experimental condition, the beam current was slowly increased to determine the values corresponding to the onset of boiling and to the onset of beam penetration.\u00a0 Each target was tested at a fill volume of roughly 1 mm above the top of the beam strike area.\u00a0 If beam penetration ultimately occurred due to beam passing through the overpressure bubble, the target was tested again at a slightly higher fill volume.<\/p>\n

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For all of the visualization targets, a stable natural convection current was visible at all power levels, even before the onset of boiling. For higher beam currents, bubbles form in the Bragg peak region and near the surface of the beam window. Bubbles formed in these regions rapidly travel upwards due to buoyancy forces.\u00a0 As a result, average void fraction in the target increases with height. The increase in void fraction leads to a reduction in density and a corresponding increase in the proton range, which is exaggerated at higher beam currents.\u00a0 Unlike the original visualization target, disruption of the helium bubble was not observed for the new targets, most likely due to the increased chamber height.<\/p>\n

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Observed Thermal Limit<\/strong><\/p>\n

Two thermal limits were observed which result in some beam penetration in the top region of the beam.\u00a0 For lower fill volumes, steam accumulation in the helium overpressure bubble causes the bubble to expand into the beam region.\u00a0 The lower density of the bubble is insufficient to stop the beam.\u00a0 For higher fill volumes, proton interactions in the water lead to boiling, and excessive voiding occurs when bubbles produced in the beam region cannot rise quickly enough out of the path of the beam.\u00a0 The second condition corresponds to a higher total beam power.\u00a0 The averaged power density for each of the three visualization targets at the observed thermal limit was the same, within measurement error.\u00a0 At 14 bar (200 psi), the thermal limits were observed at roughly 300 W\/mL, and at 7 bar (100 psi), the thermal limits were observed at roughly 270 W\/mL.<\/p>\n

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Sapphire Viewing Window Failure<\/strong><\/p>\n

The same sapphire viewing windows were used in all four visualization targets, including six days of testing in 2012 and three days of testing in 2014.\u00a0 After several hours of testing with the ramp target, the front sapphire viewing window cracked, resulting in a loss of inventory.\u00a0 As a result, the shallow target was tested using an aluminum disk in place of the back viewing window.\u00a0 Although the backlight condition could not be achieved, natural circulation effects, boiling onset, particle range as a function of beam power and chamber height, and onset of beam penetration were still visible with a single viewing window.<\/p>\n

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