Patent Description:
Sodium-cooled nuclear reactors may use sodium ionization detectors to diagnose system performance in removing alkali metal vapors and/or detecting leaks in safety systems. The sodium ionization detectors produce an electrical signal proportional to a concentration of sodium vapor in an inert gas. As such, quantitative measurement of sodium vapor concentrations requires calibration of sodium ionization detectors so that the relation between the signal and the concentration is known.

In order to calibrate the sodium ionization detectors, heated inert gas is passed through a sodium containing vaporizer. The sodium is heated, and the resulting vapor is carried in the inert gas through a detector which produces ion current between a filament and a collector. Then the vapor is trapped downstream of the detector and its contents are elementally analyzed to determine a total amount of sodium trapped over the calibration run. The vaporizer must produce a steady vapor output so that the time averaged vapor concentration in the inert gas is known with low variance, and the corresponding average detector signal can be measured. To get a series of data points for a calibration curve, the vaporizer temperature is incremented to produce more or less signal (e.g., ion current), and a new run is performed.

It is desirable that the vapor source produce a stable vapor output over the calibration run and over a range of sodium vapor concentration levels. This is because a particular concentration in the gas, which corresponds to a mass of sodium collected and averaged over the duration of the calibration run, will be correlated to a single average signal value at the sodium ionization detector. It can be difficult, however, to generate a stable vapor flow for both high and low sodium vapor concentration levels. Sodium can form an oxide film/layer over its molten surface that inhibits vaporization of the sodium into the inert gas. While increasing the temperature of the sodium slowly dissolves the sodium oxide (e.g., over many hours), during the dissolving process an unstable vapor output is generated. However, significantly increasing the temperature of the molten sodium (e.g., <NUM>°-<NUM>° C) can rapidly dissolve the oxide layer and yield a stable vapor output. But, this stability is only generated for a high sodium vapor concentration flow and low sodium vapor concentration flows remain unstable. This is because a high vapor output concentration is always produced with the high temperatures, and this output is difficult to dilute using a higher flow rate of inert gas since the detector must be calibrated at a set (e.g., constant) flow rate for certain diagnostic operations. Accordingly, improved vaporizers are desired. <CIT> discloses an ion source of a type used on ion implanters which includes a crucible having a hollow interior and a hole for providing fluid communication between the interior of and exterior to the crucible. <CIT> discloses an arrangement for installing a source into a gas deposition reactor. <CIT> discloses a unibody, monolithic, one-piece negative draft crucible for a MBE diffusion cell.

Vaporizers and vaporization methods are described herein for holding a charge of molten material at sufficiently high temperatures to rapidly dissolve oxides and produce a stable vaporization output. This vapor output can be mixed with a flow of heated inert gas for use in calibration of ionization detectors. The molten material is held in a movable crucible within a guide tube that allows for a surface area of the molten material exposed to the inert gas to be adjustable and vary the concentration of the vapor within the inert gas. As such, a stable vaporization output is generated for a wide range of vapor concentrations (e.g., both higher and lower concentration levels).

These and various other features as well as advantages which characterize the vaporizers and methods described herein will be apparent from a reading of the following detailed description and a review of the associated drawings. Additional features are set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the technology. The benefits and features of the technology will be realized and attained by the structure in the written description and claims hereof, as well as the appended drawings.

It is to be understood that both the foregoing introduction and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

The following drawing figures, which form a part of this application, are illustrative of described technology and are not meant to limit the scope of the technology as claimed in any manner, which scope shall be based on the claims appended hereto.

This disclosure describes a vaporizer and methods for creating a variable concentration stream of vaporized material. The vaporizer includes a crucible that holds a charge of molten material. The crucible is slidably disposed at least partially within an inner chamber/guide tube and is configured to selectively extend and retract therefrom. An outer chamber surrounds both the crucible and the inner chamber, and receives a flow of heated inert gas. The inert gas passes over the exposed surface area of molten material and mixes with the vapor. The mixture of inert gas and vaporized material can be used for calibration of ionization detectors (e.g., sodium ionization detectors in sodium fast reactor plants), physical vapor deposition processes, or any other process/method as required or desired.

When the molten material is retracted into the inner chamber, the inner chamber shields the molten material from the inert gas flowing within the outer chamber. As such, the surface area of the molten material within the outer chamber can be selectively adjusted so as to control the vapor output into the inert gas, and thus, the concentration thereof. By being able to control the vapor output of the molten material into the inert gas, the vaporizer can heat the molten material to temperatures that are suitable for producing a stable vaporization output over a range of concentrations between a high (e.g., greater amounts of molten material surface area) and a low (e.g., smaller amounts of molten material surface area) vapor concentration level.

<FIG> is a schematic view of an exemplary vaporizer system <NUM>. The vaporizer system <NUM> includes a vaporizer <NUM> that is configured to hold a charge of molten material for vaporization and allow heated inert gas to pass over the charge to mix with the vapor. A gas supplier <NUM> is coupled in flow communication to the vaporizer <NUM>. The gas supplier <NUM> generates a heated flow <NUM> of inert gas, such as, but not limited to, argon or nitrogen, for collecting at least a portion of the vapor generated within the vaporizer <NUM>. After the vapor mixes with the heated inert gas, the mixture <NUM> is carried downstream from the vaporizer <NUM> into a calibration system <NUM>. The calibration system <NUM> includes a detector <NUM> that is configured to produce an ion current between an anode and a cathode and record the current signal. Additionally, the calibration system <NUM> includes a gas scrubber/trap <NUM> that is configured to collect and analyze the vapor within the mixture <NUM>. The calibration system <NUM> can then generate a calibration curve based on the measured data points and over a plurality of calibration runs (e.g., a steady flow with differing vapor concentration levels in the mixture <NUM>).

The vaporizer <NUM> includes a gas manifold <NUM> that couples to the gas supplier <NUM> and receives the heated flow of inert gas <NUM>. The gas manifold <NUM> is coupled to an outer chamber <NUM> that receives the heated inert gas and also holds the charge of molten material therein. A clamshell heater <NUM> at least partially encloses the outer chamber <NUM> and enables the molten material to be heated as required or desired. The heater <NUM> is illustrated as transparent in <FIG> so as to be able to view the components within. Downstream of the outer chamber <NUM> is a sintered metal filter <NUM> that is configured to remove entrained droplets or aerosols in the vapor mixture <NUM> before entering the calibration system <NUM>. The filter <NUM> can be disposed at least partially within the heater <NUM>. A gas thermocouple <NUM> is coupled to the gas manifold <NUM> so as to measure the temperature of the flow of heated inert gas <NUM>. The thermocouple <NUM> can be coupled in communication with the calibration system <NUM> so as to provide temperature data of the heated gas for the calibration process. Additionally, a molten material thermocouple <NUM> is coupled to the charge of molten material so as to measure the temperature of the molten material. The thermocouple <NUM> can be coupled in communication with the calibration system <NUM> so as to provide temperature data of the molten material for the calibration process. For example, the thermocouples <NUM>, <NUM> are used so that the temperature of the gas can be substantially matched to the temperature of the molten material and aerosol formation is reduced or prevented.

In the example, the heater <NUM> is a clamshell heater that surrounds the outer chamber <NUM> and the filter <NUM> but does not surround the gas manifold <NUM>. This configuration allows the heater <NUM> to heat the molten material to the required or desired temperature and keep the filter <NUM> at a similar temperature to reduce or prevent generation of aerosols that are detrimental to the calibration of the detector, and to produce a steady vapor output. In other examples, the heater <NUM> can include any other heater that enables the vaporizer <NUM> to function as described herein. For example, the heater <NUM> may extend so as to partially surround other components, such as, the gas manifold <NUM>. This configuration can allow the heater <NUM> to maintain uniform molten material and other vaporizer component temperatures to reduce or prevent generation of aerosols and reduce the need for the filter <NUM>.

Additionally or alternatively, the heater <NUM> may comprise a plurality of heaters so that the vaporizer <NUM> can have one or more heating zones. In an aspect, the heater <NUM> can have a first heater (e.g., the clamshell heater) that surrounds the outer chamber <NUM> and a second heater (not shown) that is configured to heat the gas manifold <NUM>. In this example, the second heater can heat the molten material that is disposed within the gas manifold <NUM> to a temperature that is about equal to that of the first heater <NUM>. In other examples, the second heater can heat the molten material to a temperature that is above, or lower than, that of the first heater <NUM>. By at least partially heating the gas manifold <NUM>, the molten material that is disposed therein will be prevented from cooling. This allows for the mechanical adjustment of the vaporizer <NUM> to adjust vapor output. In other examples, by heating the molten material in different heating zones, vapor output can be adjusted thermally, in addition to, or alternatively from, the mechanical adjustment.

The vaporizer <NUM> also includes a movement mechanism <NUM> that is configured to control the position of the charge of molten material within the outer chamber <NUM>. The movement mechanism <NUM> is coupled to an actuator <NUM> that is configured to generate linear movement M. The actuator <NUM> can include any type of system or device that generates linear movement as described herein. For example, an electronic motor, a solenoid, manual movement, etc. The movement mechanism <NUM> enables for the surface area of the molten material to be selectively controlled and induce a steady flow of the mixture <NUM> with differing vapor concentration levels as described herein.

<FIG> is an exploded perspective view of the vaporizer <NUM> of the vaporizer system <NUM> (shown in <FIG>). As described above in reference to <FIG>, the vaporizer <NUM> includes the gas manifold <NUM>, the outer chamber <NUM>, the heater <NUM>, the filter <NUM>, the thermocouples <NUM>, <NUM>, and the movement mechanism <NUM>. Additionally, the vaporizer <NUM> includes an inner chamber <NUM> that is disposed at least partially within the outer chamber <NUM> and spaced apart therefrom. In the example, both the outer chamber <NUM> and the inner chamber <NUM> are substantially tube-shaped components, and thus, the outer tube <NUM> has a diameter that is greater than a diameter of the inner tube <NUM> and both tubes are substantially concentric with one another.

An elongate boat <NUM> is disposed at least partially within the inner chamber <NUM> and is configured to hold the charge of molten material and allow the heated inert gas to pass above the surface of the molten material. In the example, the boat <NUM> is an open container, and in some examples, can be a crucible as required or desired. In the example, the heater <NUM> at least partially surrounds the boat <NUM> so as to induce vaporization of the molten material. The boat <NUM> is coupled to the movement mechanism <NUM> so that the boat <NUM> can extend and retract relative to the inner chamber <NUM> while still being housed within the outer chamber <NUM>. The inner chamber <NUM> acts as a guide tube and cover for the boat <NUM> in the vaporizer <NUM>.

In the example, the gas manifold <NUM> can be a four-way connector such as a union cross fitting. One opening can include a reducer fitting <NUM> so that the manifold <NUM> can be coupled in flow communication with the gas supplier <NUM> (shown in <FIG>). The opposite opening can also include a reducer fitting <NUM> that is configured to support the gas thermocouple <NUM> and enable the thermocouple <NUM> to measure the temperature of the heated inert gas that flows though the manifold <NUM>. One end of the outer chamber <NUM> can be coupled to another opening <NUM> so that the flow of heated gas can be channeled into the outer chamber <NUM>. The opposite opening can include a fitting <NUM> that is used to support the inner chamber <NUM> that extends through the manifold <NUM>. For example, the fitting <NUM> can be a bore-through fitting that allows the inner chamber <NUM> to slide through. Additionally, the movement mechanism <NUM> includes a reducing union fitting <NUM> that connects to a nut on the back end of the inner chamber <NUM> and swages to the movement mechanism <NUM>. The fitting <NUM> can be used to allow the boat <NUM> to be completely removed from within the inner chamber <NUM>. It should be appreciated that other orientations and layouts of the components of the manifold <NUM> can be used as required or desired.

<FIG> is a cross-sectional view of the vaporizer <NUM> in an extended position. Certain components are described above, and thus, are not necessarily described further. Additionally, the heater <NUM> (shown in <FIG> and <FIG>) is not illustrated for clarity. The outer chamber <NUM> has an inlet <NUM> and an opposite outlet <NUM> with a longitudinal axis <NUM> defined therebetween. The inlet <NUM> is coupled to the opening <NUM> of the gas manifold <NUM> so that the heated inert gas can flow therethrough.

The inner chamber <NUM> has a first end <NUM> and an opposite second end <NUM>, and extends along the longitudinal axis <NUM>. The first end <NUM> of the inner chamber <NUM> is supported at one or more locations proximate the fitting <NUM> that is opposite of the opening <NUM> of the gas manifold <NUM> and which receives the outer chamber <NUM>. As such, the inner chamber <NUM> cantilevers through the gas manifold <NUM> via the fitting <NUM> and extends partially into the outer chamber <NUM> to generate the cantilevered second end <NUM>. Additionally in the example, the reducer fitting <NUM> is positioned adjacent to the fitting <NUM> and at the first end <NUM> of the inner chamber <NUM>. The second end <NUM> of the inner chamber <NUM> is disposed within the outer chamber <NUM> and closer to the inlet <NUM>. The reducer fitting <NUM> forms a seal to the interior of the vaporizer <NUM> and allows the movement mechanism <NUM> to extend within the inner chamber <NUM> and couple to the elongated boat <NUM>. For example, the reducer fitting <NUM> includes a Teflon ferrule that allows the movement mechanism <NUM> to slide relative thereto. The fitting <NUM> also forms a seal around the inner chamber <NUM> at the gas manifold <NUM> so that the inner chamber <NUM> can extend out therefrom.

The elongate boat <NUM> is slidably disposed within the inner chamber <NUM> and proximate the second end <NUM>. The boat <NUM> has a proximal end <NUM> that couples to the movement mechanism <NUM> and an opposite distal end <NUM> that is configured to extend and retract relative to the second end <NUM> of the inner chamber <NUM> and along the longitudinal axis <NUM>. As illustrated in <FIG>, the boat <NUM> is in its fully extended position. In the extended position, the boat <NUM> is still disposed completely within the outer chamber <NUM> and the distal end <NUM> extends a distance D from the second end <NUM> of the inner chamber <NUM>. While a fully extended position of the distal end <NUM> is illustrated in <FIG>, it should be appreciated that the distal end <NUM> can extend to any position that is less than the fully extended position via the movement mechanism <NUM> as required or desired.

In operation, the flow of heated inert gas <NUM> is channeled into the outer chamber <NUM> from the gas manifold <NUM> and towards the outlet <NUM>. More specifically, the inert gas <NUM> is channeled into the annular space between the inner chamber <NUM> and the outer chamber <NUM> at the inlet <NUM> of the outer chamber <NUM>. The inert gas <NUM> can then travel through this annular space until the second end <NUM> of the inner chamber <NUM>, when the gas can pass over the boat <NUM> and the molten material disposed therein. The flow of inert gas mixes with the vapor emitted from the molten material to generate the mixture <NUM> that is expelled from the outlet <NUM> of the outer chamber <NUM> and used for calibration measurements.

In the example, the distance D that the boat <NUM> extends from the inner chamber <NUM> defines the surface area of the molten material that produces vapor which mixes with the inert gas as it passes over. As such, the surface area of the molten material is easily adjustable within the vaporizer <NUM> so that the vapor concentration within the emitted mixture <NUM> is easily controllable. Furthermore, this movement of the boat <NUM> allows for the heater to maintain a temperature of the molten material that produces stable vaporization (e.g., reducing oxide layers from forming on the material), while still enabling the vapor concentration in the mixture <NUM> to be controllable as required or desired.

<FIG> is a cross-sectional view of the vaporizer <NUM> in a retracted position. Certain components are described above, and thus, are not necessarily described further. Additionally, the heater <NUM> (shown in <FIG> and <FIG>) is not illustrated for clarity. In the fully retracted position, the flow of heated inert gas <NUM> can still be channeled through the vaporizer <NUM> as described above, however, the boat <NUM> is retracted so that the distal end <NUM> is positioned adjacent to the second end <NUM> of the inner chamber <NUM> and the molten material is fully disposed within the inner chamber <NUM>. This position of the boat <NUM> shields the molten material from the heated inert gas <NUM> so that the inert gas no longer directly passes over the molten material and the vapor is reduced or prevented from mixing with the gas.

It should be appreciated that while a fully extended (<FIG>) and a fully retracted (<FIG>) position of the boat <NUM> is illustrated and described. The boat <NUM> can be positioned in any intermediate position as required or desired. The intermediate positions allow for the surface area of the molten material directly in contact with the flow of heated inert gas <NUM> to be adjusted and control of the vapor output that mixes with the inert gas because the gas does not directly pass over the molten material that is retracted within the inner chamber <NUM>.

The movement mechanism <NUM> can used to position the boat <NUM> relative to the inner chamber <NUM>. The movement mechanism <NUM> can include a linkage <NUM> that extends through the first end <NUM> of the inner chamber <NUM> via the reducer fitting <NUM>. One end of the linkage <NUM> couples to the proximal end <NUM> of the boat <NUM> so as to drive movement of the boat <NUM> to a desired or required distance D (shown in <FIG>). The other end of the linkage <NUM> is coupled to a coupler <NUM> so that linkage <NUM> can be attached to the actuator <NUM> (shown in <FIG>) that drives linear movement M along the longitudinal axis <NUM>. As such, the position of the movement mechanism <NUM> determines the position of the boat <NUM>. The coupler <NUM> also allows for attachment of the molten material thermocouple <NUM>. In the example, the linkage <NUM> is an elongated tube concentric with both the chambers <NUM>, <NUM> so that the junction of the thermocouple <NUM> can extend to the boat <NUM>.

<FIG> is a perspective view of the elongate boat <NUM> of the vaporizer <NUM> (shown in <FIG>). The inner chamber <NUM> is illustrated as transparent in <FIG> to show the boat <NUM> partially disposed therein. As illustrated in <FIG>, the boat <NUM> is in a partially extended (or partially retracted) position, where the distal end <NUM> of the boat <NUM> is at a distance D from the second end <NUM> of the inner chamber <NUM>. As such, a portion of the molten material disposed within the boat <NUM> is shielded within the inner chamber <NUM>.

In the example, the boat <NUM> is substantially semicircular in cross-sectional shape so that it can hold the molten material within the vaporizer and form a surface area that is exposed to the heated inert gas. The second end <NUM> of the inner chamber <NUM> is partially enclosed with an endcap <NUM> so as to further shield the molten material that is within the inner chamber <NUM> from the flow of heated inert gas. While the structure of the inner chamber <NUM> and the boat <NUM> restricts vapor from within the inner chamber <NUM> from mixing with the inert gas outside of the inner chamber <NUM>, the endcap <NUM> does not fully seal the second end <NUM> of the inner chamber <NUM> and at least a portion of the inert gas and/or the vapor can pass through the second end <NUM>. In the example, the endcap <NUM> can be a semicircle shape, and as such, the endcap <NUM> can also be used to prevent the boat <NUM> from rotating about the longitudinal axis (e.g., the sidewalls of the boat <NUM> engaging with the endcap <NUM>) during operation. Additionally, the junction of the thermocouple <NUM> extends through the proximate end <NUM> of the boat <NUM> so as to be able to measure the molten material contained within. The thermocouple <NUM> is disposed at the bottom of the boat <NUM> and during operation can covered by the molten material. As such, a sheathed thermocouple <NUM> can be used so that the thermocouple wires and junction are protected from the liquid metal.

Additionally, the inner chamber <NUM> has at least one aperture <NUM> defined therein. The aperture <NUM> is defined between the ends of the inner chamber <NUM>. In an example, the aperture <NUM> is located on the inner chamber <NUM> so that the aperture <NUM> is positioned proximate the fitting <NUM> on the gas manifold <NUM> (shown in <FIG>) when the vaporizer is assembled. This position enables for a portion of the heated inert gas to flow through the inner chamber <NUM> and expelled from the second end <NUM>. The gas flow through the inner chamber <NUM> is less than the gas flow through the outer chamber, and is used to reduce or prevent the vapor from the molten material condensing at the first end <NUM> of the inner chamber <NUM> (shown in <FIG>). As the first end of the inner chamber <NUM> may not be disposed within the heater <NUM> (shown in <FIG>). In some examples, when the boat <NUM> is in its fully retracted position (shown in <FIG>), at least a portion of the boat <NUM> may cover the aperture <NUM> so as to reduce the flow of heated inert gas through the inner chamber <NUM>.

In the example, because the boat <NUM> retracts towards the gas manifold and at least partially out of the heater, the temperature of the molten material may decrease when the boat <NUM> is in a retracted position. This temperature change of the molten material may increase aerosol formation, however, the calibration curve can still be determined and any aerosols that are produced can be removed at the filter. In other examples, the heater may be configured to maintain the temperature of molten material at any position of the boat <NUM> (e.g., when the heater also surrounds at least a portion of the gas manifold) such that the vapor output is controlled at a constant temperature and aerosol formation is reduced or prevented.

During operation of the vaporizer, the position of the boat <NUM> relative to the inner chamber <NUM> defines the surface area of the molten material exposed to the flow of heated inert gas, and thus, the vapor output of the vaporizer. As such, reducing the surface area of the molten material lowers the vapor output of the vaporizer, while increasing the surface area of the molten material raises the vapor output of the vaporizer. In the examples described herein, it is the boat <NUM> that moves relative to the inner chamber <NUM>. In other examples, the boat <NUM> can remain stationary while the inner chamber <NUM> can move relative to the boat <NUM> and adjust the surface area of the molten material exposed to the flow of heated inert gas.

The vaporizer described herein can be used to vaporize any type molten material as required or desired. In an aspect, the molten material may be an alkali metal, such as sodium. In other examples, other alkali metals, such as cesium, or a mixture of sodium and cesium that can be placed into the vaporizer. In another aspect, potassium or rubidium may be vaporized.

<FIG> is a flowchart illustrating an exemplary method <NUM> for vaporizing a molten metal. The method <NUM> begins with a charge of molten metal held within a crucible being heated to generate a vapor output (operation <NUM>). The crucible being disposed at least partially within an inner chamber. Additionally, heated inert gas is flowed into an inlet of an outer chamber (operation <NUM>). The inlet chamber being at least partially disposed within the outer chamber. The flow of heated inert gas is then channeled between the inner chamber and the outer chamber (operation <NUM>). Downstream of the inner chamber, the flow of heated inert gas is passed over a surface area of the molten metal contained within the crucible (operation <NUM>). Upon the heated inert gas passing over the surface area of the molten metal, at least a portion of the vapor output mixes with the heated inert gas and is carried further downstream. The crucible can be selectively moved relative to the inner chamber (operation <NUM>) so as to adjust the surface area of the molten metal exposed to the flow of heated inert gas and change the vapor output of the molten metal.

In some examples, the method <NUM> may further include filtering the mixture of vapor output and heated inert gas (operation <NUM>) to remove entrained droplets or aerosols. The heating operation (operation <NUM>) may further include heading the molten metal such that oxides within the molten metal are dissolved (operation <NUM>). Also, the molten metal and the inert gas can be heated to approximately an equal temperature so as to reduce aerosol formation. In an aspect, the molten metal in the method <NUM> may be sodium. The method <NUM> can further include calibrating a sodium ionization detector via the vapor output (operation <NUM>).

In an aspect, prior to vaporizing the molten metal, the crucible may be actively wetted. Otherwise the molten metal may have a tendency to bead up and stay off of non-wetted sections such that the molten metal does not spread evenly within the crucible. To wet the crucible, a strip of metal (e.g., sodium) may be placed within the crucible and heated. The molten metal can then be mechanically agitated so that the crucible is wetted at all locations. In some examples, the heat generated for this initial wetting may be from the clamshell heater. In other examples, a separate localized heater may be used for the initial wetting, and then the crucible is placed into the vaporizer.

It is to be understood that this disclosure is not limited to the particular structures, process steps, or materials disclosed herein, but is extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular examples only and is not intended to be limiting. It must be noted that, as used in this specification, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

It will be clear that the systems and methods described herein are well adapted to attain the ends and advantages mentioned as well as those inherent therein. Those skilled in the art will recognize that the methods and systems within this specification may be implemented in many manners and as such is not to be limited by the foregoing exemplified examples and examples. In this regard, any number of the features of the different examples described herein may be combined into one single example and alternate examples having fewer than or more than all of the features herein described are possible.

Claim 1:
A vaporizer (<NUM>) comprising:
an outer chamber (<NUM>) having an inlet (<NUM>) and an outlet (<NUM>), wherein a longitudinal axis (<NUM>) is defined between the inlet and the outlet;
an inner chamber (<NUM>) disposed at least partially within the outer chamber, wherein the inner chamber has an end (<NUM>) disposed proximate the inlet of the outer chamber;
an elongate boat (<NUM>) disposed at least partially within the inner chamber, wherein the elongate boat has a distal end (<NUM>) that is configured to extend and retract relative to the end of the inner chamber and along the longitudinal axis between at least an extended position and a fully retracted position, wherein the elongate boat is configured, when in the fully retracted position, to allow a portion of heated gas to pass into the inner chamber, and wherein the elongate boat is prevented from rotating about the longitudinal axis; and
a heater (<NUM>) at least partially surrounding the elongate boat.