Patent Publication Number: US-11388932-B2

Title: Aerosol-generating device with flat inductor coil

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage application of PCT/EP2018/071707, filed on Aug. 9, 2018, which is based upon and claims the benefit of priority from European patent application no. 17185599.2, filed Aug. 9, 2017, the entire contents of each of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an aerosol-generating device having a flat spiral inductor coil and a susceptor element. The present invention also relates to an aerosol-generating system comprising the aerosol-generating device and an aerosol-generating article. 
     DESCRIPTION OF THE RELATED ART 
     A number of electrically-operated aerosol-generating systems in which an aerosol-generating device having an electric heater is used to heat an aerosol-forming substrate, such as a tobacco plug, have been proposed in the art. One aim of such aerosol-generating systems is to reduce known harmful smoke constituents of the type produced by the combustion and pyrolytic degradation of tobacco in conventional cigarettes. Typically, the aerosol-generating substrate is provided as part of an aerosol-generating article which is inserted into a chamber or cavity in the aerosol-generating device. In some known systems, to heat the aerosol-forming substrate to a temperature at which it is capable of releasing volatile components that can form an aerosol, a resistive heating element such as a heating blade is inserted into or around the aerosol-forming substrate when the article is received in the aerosol-generating device. In other aerosol-generating systems, an inductive heater is used rather than a resistive heating element. The inductive heater typically comprises an inductor forming part of the aerosol-generating device and an electrically conductive susceptor element fixed within the aerosol-generating device and arranged such that it is in thermal proximity to the aerosol-forming substrate. During use, the inductor generates an alternating magnetic field to generate eddy currents and hysteresis losses in the susceptor element, causing the susceptor element to heat up, thereby heating the aerosol-forming substrate. 
     Inductive heating systems require two components, that is, an inductor and a susceptor element. This may add complexity to the manufacture and assembly of the aerosol-generating device and may increase the size of the aerosol-generating device when compared to devices comprising a resistive heater. 
     It would be desirable to provide an aerosol-generating device comprising an inductive heating system that mitigates or overcomes these problems with known systems. 
     SUMMARY 
     According to a first aspect of the present invention there is provided an aerosol-generating device comprising a housing defining a chamber having an open end for insertion of an aerosol-generating article into the chamber and a closed end opposite the open end. The aerosol-generating device also comprises a flat spiral inductor coil disposed at the closed end of the chamber and a susceptor element positioned within the chamber at the closed end. The aerosol-generating device also comprises a power supply and a controller connected to the flat spiral inductor coil and configured to provide an alternating electric current to the flat spiral inductor coil such that, in use, the flat spiral inductor coil generates an alternating magnetic field to inductively heat the susceptor element and thereby heat at least a portion of an aerosol-generating article received within the chamber. 
     According to a second aspect of the present invention there is provided an aerosol-generating system. The aerosol-generating system comprises an aerosol-generating device according to the first aspect of the present invention, in accordance with any of the embodiments described herein. The aerosol-generating system also comprises an aerosol-generating article having an aerosol-forming substrate and configured for use with the aerosol-generating device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is further described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  shows a perspective view of an aerosol-generating system according to an embodiment of the present invention; 
         FIG. 2A  shows a cross-sectional view of the aerosol-generating system of  FIG. 1  with the aerosol-generating article inserted into the aerosol-generating device; 
         FIG. 2B  shows a cross-sectional view of an alternative arrangement of the aerosol-generating device; 
         FIG. 3  shows a perspective view of the inductor coil of the aerosol-generating device of  FIG. 2A ; 
         FIG. 4  shows a perspective view of the induction assembly of the aerosol-generating device of  FIG. 2A ; 
         FIG. 5  shows a perspective view of an alternative induction assembly for the aerosol-generating device of  FIG. 2A ; 
         FIG. 6  shows a perspective view of a further alternative induction assembly for the aerosol-generating device of  FIG. 2A ; 
         FIG. 7  shows a perspective view of a still further alternative induction assembly for the aerosol-generating device of  FIG. 2A ; 
         FIG. 8  shows a perspective view of a still further alternative induction assembly for the aerosol-generating device of  FIG. 2A ; 
         FIG. 9  shows a perspective view of a still further alternative induction assembly for the aerosol-generating device of  FIG. 2A ; 
         FIG. 10  shows a perspective view of a still further alternative induction assembly for the aerosol-generating device of  FIG. 2A ; 
         FIG. 11  shows a perspective view of a still further alternative induction assembly for the aerosol-generating device of  FIG. 2A ; 
         FIG. 12  shows a perspective view of a still further alternative induction assembly for the aerosol-generating device of  FIG. 2A ; and 
         FIG. 13  shows a cross-sectional view of a still further alternative induction assembly for the aerosol-generating device of  FIG. 2A . 
     
    
    
     DETAILED DESCRIPTION 
     As used herein a “flat spiral inductor coil” means a coil that is generally planar coil wherein the axis of winding of the coil is normal to the surface in which the coil lies. Preferably, the flat spiral coil is planar in the sense that it lies in a flat Euclidean plane. 
     As used herein, the term “longitudinal” is used to describe the direction along the main axis of the aerosol-generating device, or of an aerosol-generating article, and the term ‘transverse’ is used to describe the direction perpendicular to the longitudinal direction. When referring to the chamber, the term ‘longitudinal’ refers to the direction in which an aerosol-generating article is inserted into the chamber and the term ‘transverse’ refers to a direction perpendicular to the direction in which an aerosol-generating article is inserted into the chamber. 
     As used herein, the term “width” refers to the major dimension in a transverse direction of a component of the aerosol-generating device, or of an aerosol-generating article, at a particular location along its length. The term “thickness” refers to the dimension of a component of the aerosol-generating device, or of an aerosol-generating article, in a transverse direction perpendicular to the width. 
     As used herein, the term “aerosol-forming substrate” relates to a substrate capable of releasing volatile compounds that can form an aerosol. Such volatile compounds may be released by heating the aerosol-forming substrate. An aerosol-forming substrate is part of an aerosol-generating article. 
     As used herein, the term “aerosol-generating article” refers to an article comprising an aerosol-forming substrate that is capable of releasing volatile compounds that can form an aerosol. For example, an aerosol-generating article may be an article that generates an aerosol that is directly inhalable by the user drawing or puffing on a mouthpiece at a proximal or user-end of the system. An aerosol-generating article may be disposable. An article comprising an aerosol-forming substrate comprising tobacco is referred to as a tobacco stick. 
     As used herein, the term “aerosol-generating device” refers to a device that interacts with an aerosol-generating article to generate an aerosol. 
     As used herein, the term “aerosol-generating system” refers to the combination of an aerosol-generating article, as further described and illustrated herein, with an aerosol-generating device, as further described and illustrated herein. In an aerosol-generating system, the aerosol-generating article and the aerosol-generating device cooperate to generate a respirable aerosol. 
     As used herein, the term “elongate” refers to a component having a length which is greater than both its width and thickness, for example twice as great. 
     As used herein, a “susceptor element” means an electrically conductive element that heats up when subjected to a changing magnetic field. This may be the result of eddy currents induced in the susceptor element, hysteresis losses, or both eddy currents and hysteresis losses. The susceptor element is located in thermal contact or close thermal proximity with the aerosol-forming substrate of an aerosol-generating article received in the chamber of the aerosol-generating device. In this manner, the aerosol-forming substrate is heated by the susceptor element during use such that an aerosol is formed. 
     Advantageously, providing an inductor coil and a susceptor element as parts of the aerosol-generating device makes it possible to construct an aerosol-generating article that is simple, inexpensive and robust. Aerosol-generating articles are typically disposable and produced in much larger numbers that the aerosol-generating devices with which they operate. Accordingly, reducing the cost of the articles, even if it requires a more expensive device, can lead to significant cost savings for both manufacturers and consumers. 
     Advantageously, the use of inductive heating rather than resistive heating may provide improved energy conversion because of power losses associated with a resistive heater, in particular losses due to contact resistance at connections between the resistive heater and the power supply. 
     Advantageously, the use of a flat spiral inductor coil allows for the design of a compact aerosol-generating device, with a simple design that is robust and inexpensive to manufacture. The use of a flat spiral coil also allows for a simple interface between the device and a cartridge, allowing for a simple and inexpensive cartridge design. 
     As used herein, the terms “upstream” and “downstream” refer to the general direction of airflow. That is, generally, air flows from an upstream end to a downstream end. The airflow may be through the aerosol-generating device or a portion of the aerosol-generating device. The airflow may be through an aerosol-generating article or a portion of an aerosol-generating article. The closed end of the chamber may be an upstream end. The open end of the chamber may be a downstream end. 
     Preferably, the housing defines a longitudinal axis extending between the closed end and the open end of the chamber, and wherein the flat spiral inductor coil lies within a plane that is orthogonal to the longitudinal axis. Advantageously, this may further simplify the manufacture and assembly of one or both of the inductor coil and the aerosol-generating device. 
     The housing may define a chamber end wall forming the closed end of the chamber, wherein the flat spiral inductor coil is disposed on the chamber end wall. Advantageously, the chamber end wall supports the flat spiral inductor coil in a desired position and orientation with respect to the chamber. Advantageously, positioning the flat spiral inductor coil on the chamber end wall may facilitate positioning of the inductor coil close to the susceptor element. 
     Preferably, the chamber end wall comprises a first surface defining the closed end of the chamber and an opposed second surface. 
     The flat spiral inductor coil may be disposed on the first surface of the end wall within the chamber. Advantageously, this arrangement may minimise or eliminate spacing between the flat spiral inductor coil and the susceptor element. Advantageously, this may maximise heating of the susceptor element during use. 
     The flat spiral inductor coil may be disposed on the second surface of the end wall. Advantageously, this arrangement positions the flat spiral inductor coil outside of the chamber. That is, the chamber end wall is positioned between the flat spiral inductor coil and the chamber. Advantageously, this may eliminate exposure of the flat spiral inductor coil to aerosol generated within the chamber. Advantageously, this eliminates one or both of deposits forming on the inductor coil and corrosion of the inductor coil. 
     Preferably, the housing defines a cavity in which the power supply, the controller and the flat spiral inductor coil are positioned, wherein the second surface of the end wall defines a first end of the cavity. Advantageously, this arrangement facilitates electrical connection of the inductor coil with the power supply and the controller. Advantageously, this arrangement may simplify the manufacture and assembly of the aerosol-generating device. The controller, the power supply and the inductor coil may be electrically connected to each other and inserted into the housing as an electronics package. 
     Preferably, the susceptor element comprises a planar portion disposed within the chamber at the closed end, wherein the planar portion extends within a plane parallel with the flat spiral inductor coil. Advantageously, this arrangement optimises heating of the susceptor element by the flat spiral inductor coil. 
     As used, herein, the terms “parallel” and “substantially parallel” mean within plus or minus 10 degrees, preferably within plus or minus 5 degrees. 
     The planar portion may have any suitable shape. The planar portion may have the same shape as the cross-sectional shape of the chamber. The planar portion may have the same shape as the overall shape of the flat spiral inductor coil. The planar portion may have a substantially circular shape. 
     The susceptor element may comprise at least one airflow aperture extending through the planar portion between a first side of the planar portion and a second side of the planar portion. The at least one airflow aperture may comprise a plurality of airflow apertures. Preferably, the plurality of airflow apertures are evenly spaced from each other. Preferably, the plurality of airflow apertures are distributed symmetrically across the planar portion. 
     Advantageously, providing at least one airflow aperture extending through the planar portion may facilitate heating of air flowing through the aerosol-generating device. For example, air entering the airflow device may be heated by the planar portion prior to flowing across or through an aerosol-generating article received within the chamber. Preferably, the aerosol-generating device comprises at least one airflow inlet extending through the housing, wherein the at least one airflow inlet is in fluid communication with the first side of the planar portion. Preferably, the first side of the planar portion is positioned nearer to the closed end of the chamber than the second side of the planar portion. Preferably the first side of the planar portion faces the closed end of the chamber. Preferably, the second side of the planar portion is positioned adjacent to or in contact with an aerosol-generating article when the aerosol-generating article is received within the chamber. Preferably, the second side of the planar portion faces the open end of the chamber. 
     The planar portion may be disc shaped. Each of the one or more airflow apertures extending through the planar portion may be a hole extending through the disc shaped planar portion. 
     The planar portion may comprise a central hub and a plurality of fins extending radially outward from the central hub. The central hub and the plurality of fins extend in a plane parallel with the flat spiral inductor coil. Each space between adjacent fins may form one of the one or more airflow apertures extending through the planar portion. 
     In any of the embodiments described herein in which the susceptor element comprises a planar portion, the susceptor element may comprise at least one elongate portion extending from the planar portion and into the chamber. Advantageously, the at least one elongate portion may facilitate heat transfer from the planar portion to an aerosol-generating article received within the chamber. 
     Preferably, the at least one elongate portion is configured to pierce an aerosol-generating article when an aerosol-generating article is inserted into the chamber. Advantageously, piercing an aerosol-generating article may position the at least one elongate portion inside an aerosol-forming substrate of the aerosol-generating article. Advantageously, this may facilitate the transfer of heat from the planar portion to the aerosol-forming substrate. 
     The at least one elongate portion may comprise a plurality of elongate portions extending from the planar portion. Advantageously, this may further facilitate the transfer of heat from the planar portion to an aerosol-generating article. Advantageously, a plurality of elongate portions may facilitate more even heating of an aerosol-generating article. 
     Preferably, the plurality of elongate portions are substantially parallel to each other. Advantageously, this facilitates insertion of the plurality of elongate portions into an aerosol-generating article when the aerosol-generating article is inserted into the chamber. 
     Preferably, the at least one elongate portion is orthogonal to the planar portion. Preferably, the at least one elongate portion is parallel with the longitudinal axis defined by the housing, as previously described herein. Advantageously, this facilitates insertion of the at least one elongate portion into an aerosol-generating article when the aerosol-generating article is inserted into the chamber. 
     The at least one elongate portion extends into the chamber from the planar portion, preferably orthogonally with respect to the planar portion. Therefore, inductive heating of the at least one planar portion by the flat spiral inductor coil may be minimal. The primary mechanism for heating the at least one elongate portion may be conductive heat transfer from the planar portion when the planar portion is heated by the flat spiral inductor coil. 
     In any of the embodiments described herein in which the susceptor element comprises a planar portion, the susceptor element may comprise a sleeve portion extending from a periphery of the planar portion, wherein the sleeve portion is disposed around at least a portion of the chamber for receiving at least a portion of an aerosol-generating article within the sleeve portion. Advantageously, the sleeve portion may facilitate heat transfer from the planar portion to an aerosol-generating article received within the chamber. 
     The sleeve portion extends from the planar portion and is disposed around at least a portion of the chamber. Therefore, inductive heating of the sleeve portion by the flat spiral inductor coil may be minimal. The primary mechanism for heating the sleeve portion may be conductive heat transfer from the planar portion when the planar portion is heated by the flat spiral inductor coil. 
     In any of the embodiments described herein, the flat spiral inductor coil may be a first flat spiral inductor coil and the aerosol-generating device may further comprise a second flat spiral inductor coil. In embodiments in which the susceptor element comprises a planar portion, preferably the first flat spiral inductor coil, the second flat spiral inductor coil and the planar portion of the susceptor element are parallel with each other. Preferably, the planar portion is positioned between the first flat spiral inductor coil and the second flat spiral inductor coil. Advantageously, providing a first and second flat spiral inductor coils may increase the inductive heating of the susceptor element. 
     The planar portion of the susceptor element may be a first planar portion and the susceptor element may further comprise a second planar portion separate from the first planar portion. Preferably, the second planar portion extends within a plane parallel with the second flat spiral inductor coil, wherein the second flat spiral inductor coil is positioned between the first planar portion and the second planar portion. Advantageously, providing first and second inductor coils and first and second planar portions may increase the heating of an aerosol-generating article received within the chamber. Advantageously, providing multiple inductor coils may increase heating without requiring an increase in the electrical current supplied to a single inductor coil. Advantageously, this may facilitate the use of smaller inductor coils, which may facilitate a more compact arrangement. 
     The aerosol-generating device may comprise more than two flat spiral inductor coils. The susceptor element may comprise more than two planar portions, each separate from each other. In embodiments in which the aerosol-generating device comprises multiple flat spiral inductor coils and multiple planar portions, preferably the flat spiral inductors and the planar portions are arranged in an alternating pattern. That is, preferably no two flat spiral inductor coils are positioned adjacent to each other and preferably no two planar portions are positioned adjacent to each other. Preferably, the flat spiral inductor coils and the planar portions are arranged in an alternating manner along the longitudinal axis of the housing. Preferably, the flat spiral inductor coils and the planar portions are all substantially parallel to each other. Preferably, each of the flat spiral inductor coils and each of the planar portions is substantially orthogonal to the longitudinal axis of the housing. 
     In any of the embodiments described herein, the aerosol-generating device may further comprise an additional inductor coil disposed around at least a portion of the chamber, wherein the power supply and the controller are connected to the additional inductor coil and configured to provide an alternating electric current to the additional inductor coil. Preferably, the additional inductor coil is a helically wound inductor coil. 
     Advantageously, the additional inductor coil may provide additional inductive heating of the susceptor element. The additional inductor coil may be particularly advantageous in embodiments in which the susceptor element comprises one or both of at least one elongate portion and a sleeve portion. Advantageously, the additional inductor coil may provide inductive heating of one or both of the at least one elongate portion and the sleeve portion. As described herein, without an additional inductor coil the primary heating mechanism for the at least one elongate portion and the sleeve portion may be conductive heat transfer from the planar portion. Advantageously, in embodiments comprising an additional inductor coil, the primary heating mechanism for the at least one elongate portion and the sleeve portion may be inductive heating by the additional inductor coil. 
     In any of the embodiments described herein, the susceptor element may be formed from any material that can be inductively heated to a temperature sufficient to aerosolise an aerosol-forming substrate. Suitable materials for the susceptor element include graphite, molybdenum, silicon carbide, stainless steels, niobium, and aluminium. Preferred susceptor elements comprise a metal or carbon. Preferably, the susceptor element comprises or consists of a ferromagnetic material, for example, ferritic iron, a ferromagnetic alloy, such as ferromagnetic steel or stainless steel, ferromagnetic particles, and ferrite. A suitable susceptor element may be, or comprise, aluminium. The susceptor element preferably comprises more than about 5 percent, preferably more than about 20 percent, more preferably more than about 50 percent or more than 90 percent of ferromagnetic or paramagnetic materials. Preferred susceptor elements may be heated to a temperature in excess of about 250 degrees Celsius. 
     The susceptor element may comprise a non-metallic core with a metal layer disposed on the non-metallic core. For example, the susceptor element may comprise one or more metallic tracks formed on an outer surface of a ceramic core or substrate. 
     The susceptor element may have a protective external layer, for example a protective ceramic layer or protective glass layer. The protective external layer may encapsulate the susceptor element. The susceptor element may comprise a protective coating formed by a glass, a ceramic, or an inert metal, formed over a core of susceptor material. 
     The susceptor element may have any suitable cross-section. For example, the susceptor element may have a square, oval, rectangular, triangular, pentagonal, hexagonal, or similar cross-sectional shape. The susceptor element may have a planar or flat cross-sectional shape, particularly in embodiments in which the susceptor element comprises only one or more planar portions. 
     The susceptor element may be solid, hollow, or porous. Preferably, the susceptor element is solid. 
     In embodiments in which the susceptor element comprises one or more planar portions, preferably each planar portion has a thickness of from about 10 microns to about 200 microns, more preferably from about 15 microns to about 100 microns, most preferably from about 12 microns to about 25 microns. The thickness of each planar portion is measured in a direction between the closed end and the open end of the chamber. In embodiments in which each planar portion comprises a first side and a second side, the thickness of each planar portion is measured between the first side and the second side. Preferably, each planar portion has a width or a diameter of between about 3 millimetres and about 12 millimetres, more preferably between about 4 millimetres and about 10 millimetres, more preferably between about 5 millimetres and about 8 millimetres. The width or diameter of each planar portion is orthogonal to its thickness. 
     In embodiments in which the susceptor element comprises one or more elongate portions, preferably each elongate portion is in the form of a pin, rod, blade, or plate. Preferably, each elongate portion has a length of between about 5 millimetres and about 15 millimetres, for example between about 6 millimetres and about 12 millimetres, or between about 8 millimetres and about 10 millimetres. Each elongate portion preferably has a width of between about 1 millimetre and about 8 millimetres, more preferably from about 3 millimetres to about 5 millimetres. Each elongate portion may have a thickness of from about 0.01 millimetres to about 2 millimetres. If each elongate portion has a constant cross-section, for example a circular cross-section, it has a preferable width or diameter of between about 1 millimetre and about 5 millimetres. 
     In embodiments in which the susceptor element comprises a sleeve portion, preferably the sleeve portion has a length of between about 5 millimetres and about 15 millimetres, for example between about 6 millimetres and about 12 millimetres, or between about 8 millimetres and about 10 millimetres. The sleeve portion may have a thickness of from about 0.01 millimetres to about 2 millimetres. 
     In embodiments in which the susceptor element comprises a planar portion and at least one of an elongate portion and a sleeve portion, the planar portion, the elongate portion and the sleeve portion may be formed from the same material. The planar portion and at least one of the at least one elongate portion and the sleeve portion may be integrally formed as a unitary part. 
     At least two of the planar portion, the at least one elongate portion and the sleeve portion may be formed from different materials. At least two of the planar portion, the at least one elongate portion and the sleeve portion may be separately formed and connected to each other. At least two of the planar portion, the at least one elongate portion and the sleeve portion may be connected to each other by at least one of an interference fit, a weld and an adhesive. 
     Preferably, the aerosol-generating device is portable. The aerosol-generating device may have a size comparable to a conventional cigar or cigarette. The aerosol-generating device may have a total length between approximately 30 millimetres and approximately 150 millimetres. The aerosol-generating device may have an external diameter between approximately 5 millimetres and approximately 30 millimetres. 
     The aerosol-generating device housing may be elongate. The housing may comprise any suitable material or combination of materials. Examples of suitable materials include metals, alloys, plastics or composite materials containing one or more of those materials, or thermoplastics that are suitable for food or pharmaceutical applications, for example polypropylene, polyetheretherketone (PEEK) and polyethylene. Preferably, the material is light and non-brittle. 
     The housing may comprise a mouthpiece. The mouthpiece may comprise at least one air inlet and at least one air outlet. The mouthpiece may comprise more than one air inlet. One or more of the air inlets may reduce the temperature of the aerosol before it is delivered to a user and may reduce the concentration of the aerosol before it is delivered to a user. 
     Alternatively, the mouthpiece may be provided as part of an aerosol-generating article. 
     As used herein, the term “mouthpiece” refers to a portion of an aerosol-generating device that is placed into a user&#39;s mouth in order to directly inhale an aerosol generated by the aerosol-generating device from an aerosol-generating article received in the chamber of the housing. 
     The aerosol-generating device may include a user interface to activate the device, for example a button to initiate heating of the device or display to indicate a state of the device or of the aerosol-forming substrate. 
     The aerosol-generating device comprises a power supply. The power supply may be a battery, such as a rechargeable lithium ion battery. Alternatively, the power supply may be another form of charge storage device such as a capacitor. The power supply may require recharging. The power supply may have a capacity that allows for the storage of enough energy for one or more uses of the device. For example, the power supply may have sufficient capacity to allow for the continuous generation of aerosol for a period of around six minutes, corresponding to the typical time taken to smoke a conventional cigarette, or for a period that is a multiple of six minutes. In another example, the power supply may have sufficient capacity to allow for a predetermined number of puffs or discrete activations. 
     The power supply may be a DC power supply. In one embodiment, the power supply is a DC power supply having a DC supply voltage in the range of about 2.5 Volts to about 4.5 Volts and a DC supply current in the range of about 1 Amp to about 10 Amps (corresponding to a DC power supply in the range of about 2.5 Watts to about 45 Watts). 
     The power supply may be configured to operate at high frequency. As used herein, the term “high frequency oscillating current” means an oscillating current having a frequency of between about 500 kilohertz and about 30 megahertz. The high frequency oscillating current may have a frequency of from about 1 megahertz to about 30 megahertz, preferably from about 1 megahertz to about 10 megahertz and more preferably from about 5 megahertz to about 8 megahertz. 
     The aerosol-generating device comprises a controller connected to each inductor coil and the power supply. The controller is configured to control the supply of power to each inductor coil from the power supply. The controller may comprise a microprocessor, which may be a programmable microprocessor, a microcontroller, or an application specific integrated chip (ASIC) or other electronic circuitry capable of providing control. The controller may comprise further electronic components. The controller may be configured to regulate a supply of current to the inductor coil. Current may be supplied to the inductor coil continuously following activation of the aerosol-generating device or may be supplied intermittently, such as on a puff by puff basis. The controller may advantageously comprise DC/AC inverter, which may comprise a Class-D or Class-E power amplifier. 
     The aerosol-forming substrate may comprise nicotine. The nicotine-containing aerosol-forming substrate may be a nicotine salt matrix. The aerosol-forming substrate may comprise plant-based material. The aerosol-forming substrate may comprise tobacco. The aerosol-forming substrate may comprise a tobacco-containing material including volatile tobacco flavour compounds which are released from the aerosol-forming substrate upon heating. Alternatively, the aerosol-forming substrate may comprise a non-tobacco material. The aerosol-forming substrate may comprise homogenised plant-based material. The aerosol-forming substrate may comprise homogenised tobacco material. Homogenised tobacco material may be formed by agglomerating particulate tobacco. In a particularly preferred embodiment, the aerosol-forming substrate comprises a gathered crimped sheet of homogenised tobacco material. As used herein, the term ‘crimped sheet’ denotes a sheet having a plurality of substantially parallel ridges or corrugations. 
     The aerosol-forming substrate may comprise at least one aerosol-former. An aerosol-former is any suitable known compound or mixture of compounds that, in use, facilitates formation of a dense and stable aerosol and that is substantially resistant to thermal degradation at the temperature of operation of the system. Suitable aerosol-formers are well known in the art and include, but are not limited to: polyhydric alcohols, such as triethylene glycol, 1,3-butanediol and glycerine; esters of polyhydric alcohols, such as glycerol mono-, di- or triacetate; and aliphatic esters of mono-, di- or polycarboxylic acids, such as dimethyl dodecanedioate and dimethyl tetradecanedioate. Preferred aerosol formers are polyhydric alcohols or mixtures thereof, such as triethylene glycol, 1,3-butanediol. Preferably, the aerosol former is glycerine. Where present, the homogenised tobacco material may have an aerosol-former content of equal to or greater than 5 percent by weight on a dry weight basis, and preferably from about 5 percent to about 30 percent by weight on a dry weight basis. The aerosol-forming substrate may comprise other additives and ingredients, such as flavourants. 
     In any of the above embodiments, the aerosol-generating article and the chamber of the aerosol-generating device may be arranged such that the article is partially received within the chamber of the aerosol-generating device. The chamber of the aerosol-generating device and the aerosol-generating article may be arranged such that the article is entirely received within the chamber of the aerosol-generating device. 
     The aerosol-generating article may be substantially cylindrical in shape. The aerosol-generating article may be substantially elongate. The aerosol-generating article may have a length and a circumference substantially perpendicular to the length. The aerosol-forming substrate may be provided as an aerosol-forming segment containing an aerosol-forming substrate. The aerosol-forming segment may be substantially cylindrical in shape. The aerosol-forming segment may be substantially elongate. The aerosol-forming segment may also have a length and a circumference substantially perpendicular to the length. 
     The aerosol-generating article may have a total length between approximately 30 millimetres and approximately 100 millimetres. In one embodiment, the aerosol-generating article has a total length of approximately 45 millimetres. The aerosol-generating article may have an external diameter between approximately 5 millimetres and approximately 12 millimetres. In one embodiment, the aerosol-generating article may have an external diameter of approximately 7.2 millimetres. 
     The aerosol-forming substrate may be provided as an aerosol-forming segment having a length of between about 7 millimetres and about 15 millimetres. In one embodiment, the aerosol-forming segment may have a length of approximately 10 millimetres. Alternatively, the aerosol-forming segment may have a length of approximately 12 millimetres. 
     The aerosol-generating segment preferably has an external diameter that is approximately equal to the external diameter of the aerosol-generating article. The external diameter of the aerosol-forming segment may be between approximately 5 millimetres and approximately 12 millimetres. In one embodiment, the aerosol-forming segment may have an external diameter of approximately 7.2 millimetres. 
     The aerosol-generating article may comprise a filter plug. The filter plug may be located at a downstream end of the aerosol-generating article. The filter plug may be a cellulose acetate filter plug. The filter plug is approximately 7 millimetres in length in one embodiment, but may have a length of between approximately 5 millimetres to approximately 10 millimetres. 
     The aerosol-generating article may comprise an outer paper wrapper. Further, the aerosol-generating article may comprise a separation between the aerosol-forming substrate and the filter plug. The separation may be approximately 18 millimetres, but may be in the range of approximately 5 millimetres to approximately 25 millimetres. 
       FIGS. 1 and 2  show an aerosol-generating system  10  in accordance with an embodiment of the present invention. The aerosol-generating system  10  comprises an aerosol-generating device  12  and an aerosol-generating article  14 . 
     The aerosol-generating device  12  comprises a housing  16  defining a chamber  18  for receiving a portion of the aerosol-generating article  14 . In  FIG. 1  a portion of the housing  16  defining the chamber  18  is shown as semi-transparent to illustrate components of the aerosol-generating device  12  disposed within the chamber  18 . However, it will be understood that the portion of the housing  16  defining the chamber  18  may comprise an opaque material. The chamber  18  comprises an open end  20  through which the aerosol-generating article  14  is inserted into the chamber  18  and a closed end  22  opposite the open end  20 . 
     The aerosol-generating device  12  also comprises an induction assembly  23  disposed at the closed end  22  of the chamber  18 , the induction assembly  23  comprising a susceptor element  24  and an inductor coil  26 . As shown more clearly in  FIG. 3 , the inductor coil  26  is a flat spiral inductor coil. As shown in  FIG. 4 , the susceptor element  24  comprises a planar portion  27  overlying the flat spiral inductor coil  26  and an elongate portion  29  extending from the planar portion  27 . 
     The aerosol-generating device  12  also comprises a controller  30  and a power supply  32  connected to the flat spiral inductor coil  26 . The controller  30  is configured to provide an alternating electric current from the power supply  32  to the flat spiral inductor coil  26  to generate an alternating magnetic field, which inductively heats the planar portion  27  of the susceptor element  24 . The primary heating mechanism of the elongate portion  29  of the susceptor element  24  is conductive heat transfer from the planar portion  27 . 
       FIG. 2B  shows an aerosol-generating device  12 ′ having an alternative arrangement of the flat spiral inductor coil  26 . In the arrangement shown in  FIG. 2B , a chamber end wall  25  defined by the housing  18  comprises a first surface defining the closed end  22  of the chamber  18  and an opposed second surface. The opposed second surface of the chamber end wall  25  defines a first end of a cavity  19  in which the controller  30  and the power supply  32  are disposed. The flat spiral inductor coil  26  is also disposed in the cavity  19  and is disposed on the opposed second surface of the chamber end wall  25 . 
     The aerosol-generating article  14  comprises an aerosol-forming substrate  34  in the form of a tobacco plug, a hollow acetate tube  36 , a polymeric filter  38 , a mouthpiece  40  and an outer wrapper  42 . During use, a portion of the aerosol-generating article  14  is inserted into the chamber  18  and elongate portion  29  of the susceptor element  24  is inserted into the aerosol-forming substrate  34 . The controller  30  provides the alternating electric current to the flat spiral inductor coil  26  to inductively heat the susceptor  24 , which heats the aerosol-forming substrate  34  to generate an aerosol. The aerosol-generating device  12  comprises an air inlet  44  extending through the housing  16  and providing fluid communication between the exterior of the aerosol-generating device  12  and the chamber  18  adjacent the closed end  22 . During use, a user draws on the mouthpiece  40  of the aerosol-generating article  14  to draw an airflow into the chamber  18  via the air inlet  44 . The airflow then flows into the aerosol-forming substrate  34  at which point the aerosol is entrained in the airflow. The airflow and aerosol then flow through the hollow acetate tube  36 , the polymeric filter  38  and a mouthpiece  40  for delivery to the user. 
       FIGS. 5 to 13  show several induction assemblies having alternative configurations that may be used with the aerosol-generating device  12  described with reference to  FIGS. 1 and 2 . The use and operation of the aerosol-generating device comprising any of the induction assemblies shown in  FIGS. 5 to 13  is substantially the same as described with reference to the aerosol-generating device  12  of  FIGS. 1 and 2 . In the following description of the alternative inductions assemblies, like reference numerals are used to designate like parts. 
       FIG. 5  shows an induction assembly  123  that is substantially the same as the induction assembly  23  shown in  FIG. 4  and comprises the same susceptor element  24  and flat spiral inductor coil  26 . The induction assembly  123  additionally comprises a helical inductor coil  131  extending around the elongate portion  29  of the susceptor element  24 . When assembled into the aerosol-generating device  12 , the helical inductor coil  131  is disposed around the chamber  18  so that the aerosol-generating article  14  is received within the helical inductor coil  131  when the aerosol-generating article  14  is inserted into the chamber  18 . During use, the controller  30  provides an alternating electric current to the flat spiral inductor coil  26  and the helical inductor coil  131  to inductively heat the planar portion  27  and the elongate portion  29  of the susceptor element  24  respectively. 
       FIG. 6  shows an induction assembly  223  that is substantially the same as the induction assembly  23  shown in  FIG. 4  and comprises the same susceptor element  24  and flat spiral inductor coil  26 . The flat spiral inductor coil  26  is a first flat spiral inductor coil  26  and the induction assembly  223  additionally comprises a second flat spiral inductor coil  233 . The first and second flat spiral inductor coils  26 ,  233  are arranged so that the planar portion  27  of the susceptor element  24  is positioned between the first and second flat spiral inductor coils  26 ,  233 . During use, the controller  30  provides an alternating electric current to the first and second flat spiral inductor coils  26 ,  233  to inductively heat the planar portion  27  of the susceptor element  24 . Providing two inductor coils increases the inductive heating of the planar portion  27  when compared to the induction assembly  23  shown in  FIG. 4 . 
       FIG. 7  shows an induction assembly  323  that is a combination of the induction assemblies  123 ,  223  shown in  FIGS. 5 and 6 . That is, the induction assembly  323  comprises first and second flat spiral inductor coils  26 ,  233  and a helical inductor coil  131 . 
       FIG. 8  shows an induction assembly  423  in which the susceptor element  424  comprises only a planar portion  427 . The planar portion  427  comprises a plurality of airflow apertures  435  extending through the planar portion  427 . During use, airflow from the air inlet  44  flows through the plurality of airflow apertures  435  to preheat the airflow before it enters the aerosol-forming substrate  34  of the aerosol-generating article  14 . 
       FIG. 9  shows an induction assembly  523  that is substantially the same as the induction assembly  423  shown in  FIG. 8  and comprises the same susceptor element  424  and flat spiral inductor coil  26 . The flat spiral inductor coil  26  is a first flat spiral inductor coil  26  and the induction assembly  523  additionally comprises a second flat spiral inductor coil  233 . The first and second flat spiral inductor coils  26 ,  233  are arranged so that the planar portion  427  of the susceptor element  424  is positioned between the first and second flat spiral inductor coils  26 ,  233 . As described with reference to  FIG. 6 , providing two flat spiral inductor coils increases the inductive heating of the planar portion  427  of the susceptor element  424 . 
       FIG. 10  shows an induction assembly  623  that is similar to the induction assembly  523  shown in  FIG. 9 . The planar portion  427  is a first planar portion  427  and the susceptor element  624  further comprises a second planar portion  627  identical to the first planar portion  427 . The second flat inductor coil  233  is positioned between the first and second planar portions  427 ,  627 . The induction assembly  623  also comprises a third flat spiral inductor coil  633  overlying the second planar portion  627 . The addition of a second planar portion  627  and a third flat spiral inductor coil  633  increase heating of the aerosol-forming substrate  34  during use. 
       FIG. 11  shows an induction assembly  723  that is similar to the induction assembly  423  shown in  FIG. 8  and comprises the flat spiral inductor coil  26  and a susceptor element  724  comprising only a planar portion  727 . The planar portion  727  comprises a central hub  737  and a plurality of fins  739  extending radially outward from the central hub  737 . The spaces between adjacent fins  739  form a plurality of airflow apertures  735  extending through the planar portion  727 . During use, airflow from the air inlet  44  flows through the plurality of airflow apertures  735  to preheat the airflow before it enters the aerosol-forming substrate  34  of the aerosol-generating article  14 . 
       FIG. 12  shows an induction assembly  823  that is substantially the same as the induction assembly  723  shown in  FIG. 11  and comprises the same susceptor element  724  and flat spiral inductor coil  26 . The flat spiral inductor coil  26  is a first flat spiral inductor coil  26  and the induction assembly  823  additionally comprises a second flat spiral inductor coil  833 . The first and second flat spiral inductor coils  26 ,  833  are arranged so that the planar portion  727  of the susceptor element  724  is positioned between the first and second flat spiral inductor coils  26 ,  833 . As described with reference to  FIG. 6 , providing two flat spiral inductor coils increases the inductive heating of the planar portion  727  of the susceptor element  724 . 
       FIG. 13  shows an induction assembly  923  that is substantially the same as the induction assembly  123  shown in  FIG. 5 . The susceptor element  924  of the induction assembly  923  additionally comprises sleeve portion  941  extending from the planar portion  27  and disposed within the helical inductor coil  131 . During use, the controller  30  provides an alternating electric current to the flat spiral inductor coil  26  to inductively heat the planar portion  27  and provides an alternating electric current to the helical inductor coil  131  to inductively heat the elongate portion  29  and the sleeve portion  941 .