Sunday, April 12, 2020

Diffusion Cloud Chamber (Part 3)

All cable interconnects for this project were salvaged from old PC power supplies. The only exceptions are the Teflon insulated corona-free HV cable (rated at 15kV) and all top-side wires (for heater and light bars) which are silver-plated Teflon insulated stranded wire. All rocker switches are rated for 20A but the only critical circuit in terms of max current is the Power B switch for the 3rd TEC Stage where the current is 18A.

This is the underside of the completed Cloud Chamber. 
Removing residual heat left after the liquid cooling stage helps a bit by lowering the temperature by another 2-3°C. To accomplish this, a system of fans was designed - two small fans blow air horizontally across the fins of the heatsink, while another exhaust fan picks up the hot air on the other side of the heatsink and blows it out in the open thru the back of the enclosure. This system works for the tight space we had to work with. The screws keeping together the Cooling Unit "sandwich" are used to support the brackets for the exhaust fan and the thin plastic air guides using a set of stand-offs.

The intake pair of fans and the blue exhaust fan. Sheets of thin transparent plastic form air channels to contain and direct the stream of air into the fins of the heatsink.
An aluminum L-bracket secures the two stainless steel T- splitter for the coolant - the bottom one on the picture is the inlet for the cold water and the top one is the warm water outlet. The splitters also act as adapters converting the 3/8" ID water supply tubing to 2 x 1/4" ID silicone tubing connected to the copper water cooling blocks.

The front panel's Industrial Digital Thermocouple Thermometer (Lascar DTM-995B) is on the right-hand side. DTM-995B supports T, J and K-type thermocouples, it has a switchable backlighting, °C/°F display units and switchable Current/Min/Max display (a little button, installed next to the display, not seen in the pictures, scrolls through the 3 readings). I installed a Cryogenic-optimized Type T thermocouple and the thermometer was calibrated for this probe before the installation at the Cooling Unit. K-type thermocouple which is more common can also be used but it is not as accurate at low temperatures. There are two adjustments on the back of the thermometer for calibration - Offset to set the 0°C value and Calibration for 100°C

Due to the close proximity between the thermocouple probe cables / thermometer and the HV lines leading to the Discharge switch, additional insulation (Neoprene foam sheet) was added and zip-tied to the HV cables. Any arcing there can destroy the digital thermometer or the power supply.
I took apart an identical switch to the one used for the HV/Discharge circuit and made sure that the distance between the contacts is at least 2 mm in the opposite throw of "Discharge" when the switch is in HV ON position - this should be enough to prevent HV leakage / arcing thru the Discharge circuit inside the switch as Air breakdown Voltage is generally 3kV/mm.
I was toying with the idea of using a HV vacuum relay instead of the switch but it was going to be just over-engineering on my part for only 4kV.

Thin, transparent plastic sheets, salvaged from plastic packaging are used to create a "scoop" for the heatsink exhaust hot air, just above the exhaust fan. This guide directs the hot air coming out of the heatsink fins directly into the blades of the fan. The exhaust scoop is visible on the right, partially covering the 3 switches. The sheet separating the air intake and the air exhaust can be seen on the left/center of the picture and it is secured by the screws on the heatsink.

In this picture, on the right side, one can see the two power resistors (50 Ohm (2 x 100 Ohm /3W in parallel) and switch used to control the power levels to the heater. Heater power is switchable between 1W on LOW (running)  and 1.4W on HIGH (start-up).
The 3 terminal strips are used to distribute different PS voltages to various circuits of the Cloud Chamber - this makes it really easy to change the power configuration of the TEC modules.

Proper tangential lighting above the Active Zone is absolutely essential for good observation of the vapor trails left behind the charged particles. We used 2 opposing light elements - each light bar contains 9 evenly-spaced Super-Bright White LEDs. The LEDs are mounted very low, just above the Active Zone, illuminating the supersaturated alcohol vapors at a steep angle. Standard 5mm LEDs with a built-in lens work best providing sharp bright light beams compared to large diffuse light LEDs.
The 18 LEDs are electrically divided into 6 groups and each group of 3 LEDs in parallel has its own 150 Ohm current limiting resistor. There is a simple Brightness control facilitated by a potentiometer and DPDT light switch with OFF, HIGH and LOW (adjustable) beam settings. LOW brightness includes a 1 kOhm trimmer-potentiometer in the circuit, allowing for a variable brightness level. This can help when doing long exposure still photography or videography of the Active Zone of the Cloud Chamber.
Two PCB strips with width of 1cm are used to form each Light bar. The strips are attached to each other at 90 deg using heavy gauge solid bus wire. The PCB design is fairly simple and can be etched easily at home or the copper can be mechanically removed with a Dremel tool to form the pads and traces.
Visible on the picture is the closed-cell foam "Vapor Fence" intended to contain the supersaturated alcohol vapors above the cold plate so they don't spill over the entire surface of the volume.
Since the vapor trails are whitish, everything inside the Cloud Chamber should be colored black for maximum contrast and visibility of the white trails.
The Active Zone is covered with Black Self-Adhesive Aluminum foil - this type of foil tape is ideal for the purpose as it has a wide temperature range adhesive, it has alcohol-resistant paint and it is heat conductive. This kind of tape is normally used in Broadcast studios by the Lighting Technicians to modify / mask studio spot lights and in comes in 2" width.
Also visible here is the outer Neoprene foam seal for the chamber's cover.
The air inside the chamber must be perfectly still so the vapor trails are not disturbed by air turbulence when formed. This seal prevents any air from coming in or out when the volume cover is in place, eliminating any draft and possible air turbulence.
All seams on the bottom and around the Active Zone are sealed with Black RTV Silicone sealant to prevent alcohol from seeping thru and infiltrating into the Cooling Unit.

This is the finished top side of the Cloud Chamber setup without the volume cover.
The top surface of the chamber's enclosure is covered with a fairly thin (~ 2 mm) black rubber sheet (We picked up this in the flooring section of Home Depot). The rubber sheet was glued using non-foaming Gorilla Glue to the top of the plywood. Gluing this rubber sheet was a bit tricky though - I had to make a jig to clamp and provide even downward pressure on the entire surface while the glue was curing to avoid any wrinkles or air pockets but the result turned out to be excellent.
Aluminum L-shaped profiles are used as a trim for the side edges of the enclosure.
The rubber surface around the chamber's volume provides a nice, durable, non-slippery, alcohol-resistant work area which can be used for additional experimental equipment, specimens, photographic equipment, etc.

The Heater element of the Alcohol Evaporator Unit. The Cloud Chamber relies on a steep temperature differential / gradient between the top part and bottom part of the chamber's volume. Raising the temperature of the top side helps with the alcohol evaporation. We constructed the Heater element from 6 x 68 Ohm (0.25W) resistors organized in two parallel strings - each string has 3 evenly spaced resistors in series. The total resistance of the heater is approximately 100 ohms, dissipating 1.44W at High setting. At Low setting, the heater runs thru an additional 50 Ohm resistance and the heater element dissipates 0.64W.
The heater is powered with 12V from the main power supply. The resistor strings are connected to Teflon insulated wires and placed inside heat-shrink tubing, . A small pair of barrel connectors (M/F) is used for the electrical connection, making the heater replaceable / interchangeable in case we decide to experiment with different heaters / power levels.
The power supply line for the Heater comes thru the top surface, goes into one of the Delrin rods thru a hole on the side and exits from the top opening, terminated by the heater jack.

The completed Alcohol Evaporator Unit. The Heater element is inserted between 4 sheets of black felt squares. The felt squares serve as "alcohol reservoir" and are thoroughly soaked (but not dripping!!!) with Isopropyl Alcohol before the Cloud Chamber is closed and started. Felt material is ideal for releasing the alcohol vapors as it provides a very large surface area and even evaporation. Different number of felt sheets can be used for different run times / amount of alcohol. Heat is absorbed by the top and bottom sheets and alcohol vapors seep thru the mesh supporting them above the Active Zone of the Cloud Chamber. With 2 felt squares the Cloud Chamber can run for about 1.5 hours before it requires removal of the alcohol condensate from the bottom plate. This can be done with a Squeeze-bottle, syringe or just soaked with one of the felt squares and then recycled.

The High-Voltage Positive Supply line comes thru the top surface and connects to the Ion Scrubber mesh on top. The wire is inserted in a black heat-shrink tubing and spliced to a thinner and more flexible Teflon insulated wire, which allows for the metal frame holding the mesh to be adjusted up or down as needed.
While the Cloud Chamber will operate without an Ion Scrubber (IOS), the IOS greatly improves the performance of the Cloud Chamber and IMHO it is a "must-have" feature. 
The electric field cleans the air from dust contaminants when the Chamber is first started and vastly improves the visibility and the definition of the vapor trails by quickly removing (scrubbing) the ions. With IOS turned off, tracks are much thicker, linger for a longer time, less defined and much lower in number. When IOS is turned ON the effect is rather dramatic - the number of visible tracks jumps many times, tracks are very crisp and well defined and the chamber quickly resets itself, clearing old trails.
Total of 4 cables come from the bottom compartment to the top surface - 2 x for the two Light Elements, the High-Voltage IOS supply line and the Heater lines. The rubber top surface provides a very nice and tight seal around each cable as it tries to self-close.

The transparent acrylic Cloud Chamber cover. It is a 6" x 6" x 6" inches (inside) hollow, 5-sided cube and provides enough vertical height for a good temperature gradient and excellent viewing angles. We picked up the acrylic cover on eBay, sold as Trophy / Memorabilia cover and it works for the purpose as a temporary solution during testing but it will be replaced with a custom-made glass cover.
This cover contains and seals the volume of the chamber and it is best if it is made out of glass - we are planning on using 3/16" glass when the cover is replaced (just the standard "fish-tank" type construction and silicone adhesive). Acrylic plastic doesn't react well to saturated alcohol vapors and low temperature - eventually it becomes brittle, cracks and flakes.
 I designed a special "Neoprene" foam seal for the bottom edge of the volume cover. This seal is air-tight and completely seals off the volume of the Chamber when the cover is inserted. It is made of 2 different thicknesses Closed-Cell Rubber Foam material - the thin, bottom layer seals the under-edge of the cover - it has an inside lip on which the cover "steps on", while the thicker outside foam rim seals the cover on the outside wall. The seal is glued with black RTV Silicone to the top surface and provides also mechanical stability of the cover as it is a tight-fit and locks-in the cover in place.

Update: Finally, I built a proper volume cover to replace the acrylic one . It is made of 3/16" thickness glass with an inside dimensions of 6" x 6" x 6". The glass sides are glued together with clear RTV silicone sealant (just as if you are building a box or a fish tank)  and then the edges were reinforced and protected by gluing over each of the edges an aluminum angle (L-profile) 1/2" x 1/2" x 1/16".
The top edge trim was made of the same material in sections of 6" with 1/2" cutout on one of the sides to cover each corner's top side.
I built the cover in 3 steps - first is to glue the glass, after the silicone completely cured, I removed all the excess from the seams (inside and outside) and then glued with the same sealant, the corner aluminum L-trim. Working with silicone gets very messy and you have only a limited time before it starts setting so doing it in 3 steps affords more control over the build process (but the cleanup stage can be time-consuming and I recommend using masking tape to minimize cleanup).
The bottom 1/2" glass of the side edges was left without the protective trim in order to ensure a tight side seal when the cover is inserted into the custom gasket.
The bottom edges were trued by polishing the cube edge over a very flat, machined surface with sandpaper until the edges became smooth and straight on the bottom for a really good seal.

No comments: