Patent Publication Number: US-10783920-B2

Title: Tape drive with gas jet impinging on tape

Description:
BACKGROUND 
     The present disclosure relates in general to the field of methods of operating magnetic tapes and tape drive systems. In particular, it is directed to systems and methods relying on a gas flow (e.g., air) that impinges on the tape to locally urge it against a tape head transducer area. Related tape drives are also disclosed. 
     Various data storage media or recording media such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks or cards are known, which allow for storage and retrieval of data. In magnetic media, data are typically stored as magnetic transitions, i.e., they are magnetically recorded in the magnetic layer of the media. The data stored is usually arranged in data tracks. A typical magnetic storage medium, such as a magnetic tape, usually includes several data tracks. Data tracks may be written and read individually, or sets of data tracks may be written and read in parallel. Transducer (read/write) heads are positioned relative to the data tracks to read/write data along the tracks, at the level of or under the tape-bearing surface of the tape head, in which are embedded magnetic read and write transducers. 
     To write and read at high areal densities as used by modern tape systems, the magnetic tape has to come in close proximity to the read/write elements on the tape head. To sustain areal density improvement, modern tape systems feature ever decreasing spacing between the head and the magnetic layer-coated side of the tape as the tape is streamed by the head. Research efforts are accordingly spent to achieve viable solutions to reduce the distance between the tape and the head, as reducing this gap is what allows for increasing the areal density. The current technology typically requires a tape-head spacing of several tens of nanometers. 
     Historically, tapes used to be simply run wrapped over a curved-surface head, causing a layer of stable, compressed air to appear between the two surfaces when streaming the tape, and giving rise to moderate Couette flow shear stress (friction). The thickness of this air bearing is however relatively large, limiting system performance (the air bearing prevents the tape to come in close contact with the head), and is strongly dependent on the tape velocity. 
     To decrease the spacing and make it less dependent on velocity, various techniques have been developed, which typically rely on engineering the tape head to modify the air flow and local pressure field in the vicinity of the tape-head interface. Such techniques result in an underpressure occurring in the space between the tape and the head relative to the ambient, resulting in a pressure exercised on the section of tape overlapping the head, which pushes this section towards the head surface. 
     According to the main method in use today, this underpressure is obtained thanks to tape heads with skiving (i.e., sharp) edges, to scrape (or skive) off the air. That is, the underpressure is obtained by skiving the air boundary layer off the tape, which is run at a positive wrap angle over a sharp leading edge of the head, as illustrated in  FIG. 1 . This results in a low-pressure region directly after the skiving edge, such that the tape is pushed into intimate contact with the tape head, due to the higher air pressure on the opposite side of the tape. An advantage of this solution is that the tape-head spacing is relatively small and stable over a wide range of tape speeds. A disadvantage is the friction (which limits performance) and wear (which limits the operable lifetime of both tape and head) that arise due to the direct contact and the high pressure with which the tape is pushed into contact with the head. Furthermore, the tape is in contact over the whole width (as measure along axis z in  FIG. 1 ) of the head, again causing undesirable friction. In extreme cases, friction can even cause the tape drive motors to stall and tape breakage. 
     To prevent excessive friction, the tape can be intentionally made rough, i.e. with sporadic bumps on the tape surface so that only a fraction of the tape surface is in actual contact with the tape bearing surface of the head. Effectively, these bumps increase the tape-head spacing. To increase the linear density, one may seek to reduce the tape-head spacing by using a smoother tape. However, using smoother media again results in an increased friction that can degrade the recording and read back performance of the tape drive. 
     SUMMARY 
     According to a first aspect, the present invention is embodied as a method of operating a magnetic tape within a tape drive. The tape drive comprises a tape head, which shows a tape-bearing surface meant to face a front side of a magnetic tape, in operation. The tape-bearing surface comprises a transducer area. This area includes at least one transducer, which is a read or write element configured to read or write to the magnetic tape, respectively. The present method in some embodiments includes driving the tape (along a longitudinal direction of circulation thereof above the tape-bearing surface) and concomitantly ejecting a gas flow toward the transducer area. The gas flow ejected impinges on the back side of the driven tape (i.e., opposite to the front side of the tape), so as to locally urge the front side of the tape against the transducer area and thereby read or write to the tape via said at least one transducer. 
     The present approach makes it possible to force the flexible tape into contact with the tape head only where it is necessary, i.e., at the locations of the magnetic transducers. Thus, no sharp skiving edges or otherwise engineered bearing surfaces are necessary. This can, in turn, be exploited to limit friction and wear of both the tape and the tape head. 
     In embodiments, the method further comprises positioning the tape for it to form both a lead wrap angle α and a trail wrap angle β with respect to a plane subtended by the transducer area while driving the tape and ejecting the gas flow. The tape is positioned and the gas flow ejected so as for one or each of the lead wrap angle α and the trail wrap angle β to be between +2.0±0.05 and −4.0±0.05 degrees (or in some embodiments between −0.5±0.05 and −1.5±0.05 degrees). Note, all angle values given in this document are assumed to be defined to an accuracy of ±0.05 degrees. 
     In some embodiments, the tape is positioned and the gas flow ejected so as for the lead wrap angle α to be negative. Optimizing the wrap angle(s) makes it possible to optimize the contact surface, as present inventors have concluded from various experiments and simulations. 
     In some embodiments, a flow rate of the gas flow ejected is dynamically adapted according to a characteristic of a contact between the tape and the tape head, a tension of the tape, and/or a speed of the tape as the latter is being driven. This way, an adequate distance may be maintained between the tape and the head, notwithstanding the fact that speed and tension of the tape may typically vary, in operation. Slightly negative wrap angles may, in general, in some embodiments be sought, in order to prevent undesired sticking of the tape. Also, negative wrap angles make it possible to minimize the contact surface between the tape and the transducer area, given the gas flow otherwise applied. 
     In embodiments, the gas flow is ejected so as for a cross-sectional profile of the ejected gas flow to have a form factor corresponding to a form factor of the transducer area, as measured parallel to a plane subtended by the transducer area. This way, the contact surface can be optimally adjusted to the transducer area. 
     The gas flow may for instance be ejected perpendicularly to the tape. In some embodiments, the gas flow is ejected at an oblique angle with respect to the tape, so as to transfer some of a momentum of the ejected gas flow to the tape. This, as one may realize, makes it possible to counteract friction, at least in some extent, and further helps in entraining the tape. 
     According to another aspect, the invention is embodied as a tape drive. Consistently with the above method, the tape drive comprises a tape head, a drive mechanism, and a gas ejection mechanism. The tape head has a tape-bearing surface that is meant to face a front side of a magnetic tape, in operation. The tape-bearing surface comprises a transducer area, comprising at least one transducer that is a read or write element configured to read or write to the magnetic tape, respectively. The drive mechanism is adapted to drive the tape along a longitudinal direction of circulation thereof above the tape-bearing surface. The gas ejection mechanism is configured to eject a gas flow toward the transducer area, for the gas flow to impinge on the back side (opposite to the front side) of the tape as the latter is being driven, so as to locally urge the front side of the tape against the transducer area and thereby read or write to the tape via said at least one transducer, in operation. The gas ejection mechanism is in some embodiments configured as an air knife. 
     In embodiments, the gas ejection mechanism comprises a nozzle having an aperture for ejecting the gas flow, wherein said aperture advantageously has a form factor corresponding to a form factor of the transducer area. The aperture of the nozzle has an area that is in some embodiments not larger than the transducer area, to optimize the contact surface. 
     The nozzle may for instance be arranged so as to in some embodiments eject the gas flow perpendicularly to the tape. In some embodiments, however, the nozzle is arranged so as to eject the gas flow at an oblique angle with respect to the tape, so as to transfer some of a momentum of the ejected gas flow to the tape. 
     The tape head in some embodiments comprises non-skiving edges. For example, the tape-bearing surface of the tape head may have a convex profile along said longitudinal direction of circulation. In that case, the transducer area is located closer to an apex (e.g., right at the apex) of the convex profile than to outer ends thereof along that same direction. 
     In some embodiments, the tape drive further comprises a tape positioning mechanism adapted to position the tape for it to form both a lead wrap angle α and a trail wrap angle β with respect to a plane subtended by the transducer area while driving the tape and ejecting the gas flow. As a result, the tape typically forms a U-shaped profile above the tape-bearing surface, in operation. 
     In embodiments, the gas ejection mechanism is configured to eject the gas flow so as for one or each of the lead wrap angle α and the trail wrap angle β to be between +2.0 and −4.0 degrees (and in some embodiments between −0.5 and −1.5 degrees). 
     In some embodiments, the gas ejection mechanism is further configured to dynamically adapt a flow rate of the gas flow ejected according to one or more of: a characteristic of a contact between the tape and the tape head; a tension of the tape; and a speed of the tape as the latter is being driven by the drive mechanism, in operation. 
     Methods and apparatuses (or systems) embodying the present invention will now be described, by way of non-limiting examples, and in reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, and which together with the detailed description below are incorporated in and form part of the present specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present disclosure, in which: 
         FIG. 1  is a 2-dimensional cross-sectional view of a tape head with skiving edges, above which a tape is driven, so as to reach an intimate contact with the tape-bearing surface of the head, according to countless, typical prior art solutions; 
         FIG. 2  is a 2-dimensional cross-sectional view of a tape drive, which involves an air knife, according to embodiments. The tape is shown to be retracted from the tape-bearing surface and the air knife is off; 
         FIG. 3A  is a 2-dimensional cross-sectional view of the tape drive of  FIG. 2 , illustrating the tape drive in operation, i.e., after turning the air knife on, so as to force a portion of the tape in local contact with the transducer area of the head, as in embodiments. Some of the components of  FIG. 2  were omitted for concision; 
         FIG. 3B  illustrates a variant to  FIGS. 2 and 3A , where the nozzle of the air knife is slanted with respect to the normal of the tape-bearing surface, as in embodiments; 
         FIG. 4  is a 3-dimensional view of a variant to the tape drive of  FIGS. 2 and 3   a,  involving a convex tape-bearing surface, as in embodiments; 
         FIGS. 5A-5C  depicts possible variants to the geometry of the tape head; 
         FIG. 6  is a plot representing the normal (reaction) and friction forces according to the lead/trail wrap angles formed between the tape and the tape head, as obtained according to experiments and measurements performed by the Inventors; 
         FIG. 7  is a plot representing the tape geometry as the tape is being locally forced into local contact with the head by the air jet, for different values of imposed (symmetric) wrap angles, as obtained by finite element simulation methods; 
         FIG. 8  is a flowchart illustrating high-level steps of a method of operating a tape in a tape drive, according to embodiments. 
     
    
    
     The accompanying drawings show simplified representations of devices or parts thereof, as involved in embodiments. Technical features depicted in the drawings are not to scale, for the sake of depiction and understanding. Similar or functionally similar elements in the figures have been allocated the same numeral references, unless otherwise indicated. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     As noted in the background section, current solutions routinely rely on skiving edges or otherwise engineered tape-bearing surfaces, which result in urging the tape against the whole width of the head and causes undesirable friction. However, and as it may be realized, it is only necessary to have the tape in local contact with the head at the location of the read/write transducers, to ensure optimal performance. The present inventors have accordingly devised a simple solution, which relies on a gas flow impinging on the tape, to force the latter in local contact with the transducer area of the head. 
     The following description is structured as follows. First, general embodiments and high-level variants are described (sect. 1). The next section addresses more specific embodiments, technical implementation details, as well as simulations and experiments (sect. 2). 
     1. General Embodiments and High-level Variants 
     In reference to  FIGS. 2, 3 and 8 , an aspect of the invention is first described, which concerns a method S 10 -S 50  of operating a magnetic tape  10  within a tape drive  1 ,  2 ,  3 . 
     The tape drive  1 - 3  notably comprises a tape head  20 ,  20   a - 20   d.  The tape head  20  is generally configured for reading and/or writing to a magnetic tape  10 . As usual, this tape head shows a tape-bearing surface  22 ,  22   a - 22   d  (i.e., the surface meant to face the front side FS of the magnetic tape  10 , in operation) and the tape-bearing surface comprises a transducer area  24 . Depending on the implementation, the area  24  may comprise one or more transducers  25 . Typically, several transducers are involved, to read or write tracks in parallel. The head  1  may for instance comprise an array of transducers  25  arranged along the transverse (e.g., cross-track) direction x. In that case, the area  24  will typically have a form factor and extend transversally along the cross-track direction x, i.e., transversally to axis z but parallel to the bearing plane (x, z). As seen in  FIG. 3A or 3B , the travel direction (also called along-track direction or direction of circulation) of the tape is in some embodiments parallel to the transducer area  24  and, thus, parallel to axis z at the level of the area  24 . 
     Each transducer  25  is a read or a write element, i.e., an element that is configured to read or write to the magnetic tape  10 , respectively, as known per se. 
     In some embodiments, the present method includes ejecting S 20   b  a gas flow (e.g., air) while driving S 20   a  the tape  10  above the tape-bearing surface  22 . That is, steps S 20   a  and S 20   b  are at least partly concomitant, as suggested by the flowchart of  FIG. 8 . 
     The gas flow is ejected S 20   b  in the direction of the transducer area  24 . Yet, owing to the presence of the tape  10 , the gas flow impinges on the backside BS of the tape  10  (i.e., opposite to the front side FS thereof), so as to locally urge the front side FS of the tape  10  against the transducer area  24 , in operation. That is, the gas flow bends the tape, which roughly has a U-shaped profile above the tape-bearing surface  22 . The lowermost arc of the tape comes in local contact with the transducer area  24 , which makes it possible to read or write S 30  to the tape  10  via the transducer(s)  25 . 
     Note, the tape head  20  may possibly comprise several arrays of transducers, arranged in distinct areas (not shown). There, a single gas flow or several gas flows (forming distinct vertical flows, distributed along axis z) may be involved, to locally urge the tape  10  against the various transducer areas. Whether to use a single gas flow or distinct flows depends on the dimensions of and gaps between such areas and the dimensions and other characteristics of the ejected gas flows. The ejected gas will typically be air, though other gas such as inert gases may be used in specific contexts (e.g., in clean rooms, or for experimental purposes). 
     The gas flow (pressure, flow profile) need be adapted to the problem at hand, which can be achieved by trial and error. Now, beyond the pressure and flow geometry required to achieve an optimal tape profile, the ejected gas may further be engineered to exhibit specific physical properties (e.g., in terms of temperature, in order to optimize the bending of the tape and/or minimize damages caused to the tape). In addition, the ejected gas may be slanted, as discussed latter in detail. 
     As per the above method, the tape is locally urged against the area  24 , meaning that a local (stationary) area of the tape is locally forced into closer contact with the tape-bearing surface  22 ,  22   a - 22   d  of the tape head, compared with outer areas of the tape. Said local area compares to a moving window, which is moving with respect to the tape (inversely with the latter as the tape travels above the transducer area) but fixed with respect to the tape head. The local contact area is also characterized by a spacing, or closeness of contact between the tape and the tape head, and a width (measured along axis z) over which this spacing may be considered constant, provided the tape profile locally exhibits a flat bottom. 
     The present approach makes it possible to force the flexible tape into contact with the tape head  20  only where it is necessary i.e., at the locations of the embedded magnetic transducers  25 . Thus, no sharp skiving edges or otherwise engineered structures on the surface  22  are necessary. The tape-bearing surface may thus be made smoother. In turn, no high contact pressure is anymore needed at the edges or other structures (as sometimes provided on or in the surface  22 ), which limits friction (thus improving performance) and wear (thus improving the operable lifetime) of both the tape  10  and head  20 ,  20   a - 20   d.    
     The position and geometry of the tape  10  may otherwise be optimized, as in embodiments discussed now. For instance, referring to  FIGS. 2, 3 and 8 , the tape  10  may be adequately positioned S 10  for it to form specific wrap angles with respect to the tape-bearing surface  22 , when operating the tape  10  (while driving S 20   a  the tape  10  and ejecting S 20   b  the gas flow). That is, given an initial position of the tape  10  (e.g., retracted from the surface  22 , as in  FIG. 2 ), the gas flow may be adjusted to achieve optimal contact. Conversely, for a given gas flow, the position of the tape  10  may be adjusted S 10  to optimize contact. Still, both the initial position of the tape and the gas flow may be jointly optimized. 
     As explicitly shown in  FIGS. 2, 3 , the wrap angles include both a lead wrap angle α and a trail wrap angle β. Such angles are formed by the tape path with respect to the plane delimiting the bearing surface  22 , i.e., the plane subtended by the area  24 , parallel to (x, z). Note, this plane corresponds the top surface  22  of the head if the latter is flat (as in  FIGS. 2, 3 and 5 ). More precisely, the lead wrap angle α is defined between said delimiting plane and the path the incoming tape  10  takes about the bearing surface  22 , owing to the initial positioning S 10  of the tape and the ejected gas S 20   b.  The trail wrap angle β is similarly defined on the opposite side of the vertical plane (x, y), see  FIG. 3 . A zero-degree value of the angles α and β is defined, with the gas ejection mechanism  30 ,  35 ,  37  being off and the plane of tensioned tape being moved gradually from off-contact towards the head, up to a point where the onset of a normal reaction force on the head becomes measurable. Assuming the angle rotation direction convention for tape drives, positive lead and trail wrap angles α, β ensue when the tape is pushed further into contact with the head, as in  FIG. 1 , while negative wrap angles α, β arise when the tape is retracted from the zero-degree position, as in  FIGS. 2, 3 . The lead wrap angle α and trail wrap angle β are commonly referred to (altogether) as wrap angles in the literature. 
     A number of factors will influence the geometry of the tape about the transducer area  24 , including, e.g., the bending stiffness of the tape, the momentum and tension of the tape as it is driven S 20   a,  the pressure of the compressed air bearing on the head side FS of the tape (which depends on the geometry of the tape head), and the pressure exerted S 20   b  by the gas flow impinging on the backside BS of the tape  10 . All such parameters can be taken into account into simulations, to find optimal parameters for the initial positioning S 10  of the tape and/or the gas flow. Thus, for given tape operating parameters of speed and tension S 20   a,  steps S 10  and S 20   b  may be carried out so as to obtain desired wrap angles and, in turn, optimize the contact between the tape and the head. In particular, the tape may initially be positioned S 10  and the gas flow ejected S 20   b  in such a manner that one or each of the lead angle α and the trail angle β be between +2.0 and −4.0 degrees (using the usual angle rotation direction convention for tape drives, as discussed below). Note, the angles α, β formed in operation (i.e., as per implementation of steps S 10 , S 20   a  and S 20   b ) will typically have same signs (be it in a symmetric or slanted configuration, as in  FIGS. 3A and 3B , respectively). 
     Imposing wrap angles in the above range makes it possible to optimize the contact surface, as present inventors have concluded from various experiments and simulations. This is discussed in more detail in sect. 2, in reference to  FIGS. 6 and 7 . More generally, the angles α, β may typically be between +5.0 and −10.0 degrees. Some embodiments, however, have angles α, β between +2.0 and −4.0 degrees or, even better, between −0.5 and −1.5 degrees (for example close to 1.25 degrees), in order to further optimize the contact for width and spacing, as further discussed in section 2. 
     The usual angle rotation direction convention for tapes is assumed throughout this document. The tape convention differs from the counterclockwise convention used in trigonometry, in that the lead and trail angles are measured counterclockwise and clockwise, respectively, so as to confer the same sign for a symmetric (left-right) profile of the tape, i.e., if the profile of the tape is symmetric with respect to the vertical median plane parallel to (x, y). Thus, the trail angle has the same sign as the lead angle if the tape is always on the same side of the plane delimiting the bearing surface (parallel to (x, z)) and, in particular, when the profile is left-right symmetric. For instance, in the example of  FIGS. 3 and 7 , both the angle α and β are assumed to be negative in the angle rotation direction convention for tape drives, whereas β would be negative if measured counterclockwise. 
     The positioning means  40 ,  40   a  are, in the example of  FIG. 3 , assumed to be positioned so as for the tape  10  to be initially retracted from the top surface  22  of the head (see  FIG. 2 ), while the gas flow impinging S 20   b  on the tape yields negative wrap angles (again, assuming the angle rotation direction convention for tape drives). Slightly negative wrap angles α, β in some embodiments may, in general, be used, in order to locally confer a U-shape profile to the tape. This not only makes it possible to optimize the contact surface between the tape and the transducer area  24  but also lowers the risk of undesired sticking of the tape to the head (e.g., due to condensation). 
     In variants, the present approach may also be implemented using slightly positive wrap angles, contrary to depictions in  FIGS. 2-4 . Similarly, slightly asymmetric profile can be contemplated. This, however, in some embodiment may be not preferred, inasmuch as a positive wrap angle could cause additional friction and wear, even though the tape head can be shaped convex (as in  FIG. 4 or 5A ) to mitigate this. 
     Referring to  FIG. 3A, 3B , the gas flow is in some embodiments ejected S 20   b  so as for the ejected gas to have a cross-sectional profile with a form factor corresponding to the form factor of the transducer area  24 . Such form factors are appreciated in the plane subtended by the transducer area  24 . E.g., as noted earlier, the transducer area  24  may be elongated and extend along the cross-track axis x (especially if it comprises an array of transducer linearly arranged along that direction) and thus exhibit a form factor in the plane (x, z). Therefore, the exhausted fluid flow may be shaped accordingly, see  FIGS. 3 and 4 , so as to optimize the contact area. This is most simply achieved using a suitably shaped nozzle  30 . In variants, several nozzles (not shown) may be relied on, which are arranged linearly and side-by-side along axis x. Note, the latter case should be distinguished from the particular case where an array of nozzles (distributed along axis z) are used for the tape to locally contact distinct arrays of transducers (not shown). Thus, one may more generally have a 1-dimensional or 2-dimensional array of nozzles. Most simple, however, is to rely on a single nozzle for each array of transducers, where each nozzle has a rectangular exhaust shape, as in air knife systems. Yet, the dimension and pressure of exhausted gas need be adapted to the problem at hand, as noted earlier. 
     In embodiments as depicted in  FIG. 3A or 7 , the gas flow is in some embodiments ejected perpendicularly to the tape  10 . In variants, however, the gas flow may be in some embodiments ejected at an oblique angle with respect to the tape  10 , so as to transfer some of a momentum of the ejected gas flow to the tape  10 . This scenario is illustrated in  FIG. 3B , where the nozzle  30  is slanted by ˜10 degrees with respect to the vertical axis y. That is, the angle of impingement of the air jet deviates slightly from the normal axis, such that some of the momentum of the air jet is imparted to the tape  10  in its direction of travel. This, as one may realize, makes it possible to counteract friction (from Couette flow shear stress arising in the air bearing between the tape and the head), at least in some extent, and further helps in entraining S 20   a  of the tape. 
     Referring now to  FIG. 8 , the flow rate of the gas flow ejected S 20   b  may possibly be dynamically adapted S 50 , according S 40  to the contact spacing between head  20  and tape  10  (or other operational characteristics impacting the contact, such as the tape speed and/or tension) as the tape is being streamed over the head. This may notably be achieved by monitoring S 40  the contact spacing. This is useful as the tape speed and tension typically vary, in operation, in ways that can affect the spacing. As the skilled person may appreciate, many methods are known which allow the contact spacing to be monitored S 40 , such as, e.g., measuring the amplitude of the magnetic readback signal. 
     In all cases, a pressure source  37  (e.g., a gas bottle or compressor) will typically be relied on, together with a pressure valve  35 , combined with a sensor or gauge, so as to be able to control the pressure of exhausted gas. Note, such components are not necessarily integrated in the tape drive but can instead be coupled thereto. For example, the tape drive  1 - 3  may, a minima, comprise a nozzle  30  or any other similar gas ejection device, comprising a port (e.g., a tubing port), to which a tube, and then a valve and a pressure source can be connected. 
     Referring to  FIGS. 2-5 , another aspect of the invention is now described, which concerns a tape drive  1 ,  2 ,  3  suited for implementing methods such as described above. Aspects of such tape drives have already been implicitly described in reference to the present methods. They are only briefly described in the following. 
     In some embodiments, the tape drive  1 - 3  comprises a tape head  20 ,  20   a - 20   d,  a drive mechanism, and a gas ejection mechanism  30 ,  35 ,  37 . 
     In embodiments, aspects of the tape head have already been described: the head has a tape-bearing surface  22 ,  22   a - 22   d  with a transducer area  24 , meant to face a front side FS of the magnetic tape  10 , while the opposite back side BS faces the gas outlet  30  of the gas ejection mechanism  30 ,  35 ,  37 , in operation. 
     The drive mechanism  40  is adapted to drive the tape  10  along its travel direction (parallel to axis z at the level of the area  24 ). Tape drive mechanisms are known per se; they include reels (not shown) to which tape drive motors (not shown) are coupled, and rollers  40 ,  40   a  which guide the tape&#39;s streaming path, in particular over the head. 
     The gas ejection mechanism  30 ,  35 ,  37  need be configured to eject a gas flow toward the transducer area  24 , for the gas flow to impinge on the back-side BS of the tape  10  as the latter is being entrained S 20   a,  so as to locally urge the front side FS of the tape  10  against the transducer area  24 , as described earlier. This, in turn, makes it possible to read and/or write to the tape  10 , while minimizing the contact area to the sole useful area, in operation. 
     In embodiments such as depicted in  FIGS. 2-4 , the gas ejection mechanism  30 ,  35 ,  37  comprises a profiled nozzle  30 ,  30   d.  That is, the nozzle  30  has an exhaust aperture with a form factor corresponding to the form factor of the transducer area  24 , as discussed earlier. The nozzle&#39;s aperture may for instance be rectangular, just like a corresponding transducer area  24 , as assumed in the accompanying drawings. 
     The area of the aperture of the nozzle in some embodiments may preferably not be larger than the transducer area  24 . This way, a well delimited gas flow profile (e.g., laterally centered, low-divergence air jet) is ejected, which impinges on the backside of the tape  10 , over an area that in some embodiments preferably does not substantially exceed the area  24 , so as to effectively restrict the contact to the sole area  24 . Note, the dimensions of the area  24  are exaggerated in the accompanying drawings. In reality, this area  24  in some embodiments corresponds to the span of the magnetic transducers. 
     In  FIG. 3A , the nozzle is arranged so as to in some embodiments eject the gas flow perpendicularly to the tape  10 : the aperture area and the area  24  are measured in parallel planes, i.e., parallel to the plane subtended by the transducer area  24  and parallel to (x, z). In  FIG. 3B , where the nozzle  30  is slanted, the aperture of the nozzle is measured in the exhaust plane (normal to the deviation axis). Yet, its form factor still in some embodiments corresponds to that of the area  24 . 
     As said, the gas ejection mechanism  30 ,  35 ,  37  is in some embodiments configured as an air knife. As the present approach does not require skiving edges (or other structures provided in or on the surface  22 ) to obtain a close contact, the tape head may be in some embodiments free of skiving edges and, more generally, made smooth on top (no structures, cavities, etc., are provided on top), which simplifies the fabrication of the head. Thus, the present tape heads in some embodiments may comprise non-skiving edges, which may either be rounded or beveled, as illustrated in  FIGS. 4-5 . Some embodiment may have non-skiving edges on both the leading and trailing side, as illustrated in  FIGS. 4 and 5A . Still, an asymmetric tape profile may be required in some cases, such that an asymmetric edge configuration may be contemplated, as assumed in  FIGS. 5B and 5C . 
     In embodiments such as depicted in  FIG. 4 , the tape-bearing surface  22   d  of the head  20   d  of the tape drive  3  has a convex profile along the tape travel direction z. I.e., the profile of the tape-bearing surface  22   d  appears convex from the viewpoint of the tape  10  or the nozzle  30   d.  In such a case, the transducer area  24  shall typically be located at the apex of this convex profile or, more precisely, the main (longitudinal) axis of extension of the area  24 , which longitudinal axis is parallel to axis x, passes through the line joining apices of the cross-sectional convex profiles of the surface  22   d,  as formed in planes parallel to (y, z). More generally, slightly asymmetric configuration may be contemplated, whereby the transducer area  24  is located closer to the apex line than to outer ends of the bearing surface  22   d.    
     A convex curvature  22   d,  such as a cylinder section, with the transducers ideally located at or near the apex line, allows the achievable minimum width of the contact area to be decreased, in cases where the (convex) curvature of the tape is limited by the finite divergence angle of the gas jet or by the bending stiffness of the tape web. 
     Referring back to  FIGS. 2, 3 , the present tape drives  1 - 3  typically comprise tape positioning means  40 ,  40   a.  And such means may notably include a tape positioning mechanism  40 ,  40   a,  designed so as to adequately position S 10  the tape and achieve optimal wrap angles, as discussed earlier. Once the tape is suitably positioned (e.g., retracted from the surface  22 , at an intermediate distance between the gas outlet  30  and the bearing surface  22 , the tape is entrained S 20   a  and the gas flow ejected S 20   b  locally confers a U-shaped profile to the tape above and at the level of the transducer area  24 , in operation. The tape tension may also need be adjusted so as to allow adequate bending of the tape. 
     Note, the positioning mechanism  40 ,  40   a  shall, in practice, typically involve rollers, which can be designed/positioned (and thus guide the tape  10 ) independently from the tape reels (not shown) that are coupled to tape drive motors (not shown), as usual in the art. Again, the optimal tape position can be jointly devised with the gas flow parameters, which may itself be adapted to the tape speed/tension (or any characteristic of the contact between the tape and the tape head, if suitably monitored). More generally, the position of the tape  10  may be dynamically adjusted, as can also the gas flow characteristics. Also, the positioning of the tape needs to take into account the geometry of the gas ejection mechanism  30 . E.g., one may want to slightly retract the tape  10  from the tape head (to obtain a negative lead wrap angle), yet making sure the tape  10  does not collide with the nozzle  30  or other parts of the mechanism  30 ,  35 ,  37 . 
     The above embodiments have been succinctly described in reference to the accompanying drawings and may accommodate a number of variants. Several combinations of the above features may be contemplated. Examples are given in the next section. 
     2. Specific Embodiments—Technical Implementation Details 
     In the particular embodiments discussed here, a specific air bearing configuration ( FIG. 2 ) is relied on, in conjunction with a jet of air impinging against the backside BS of the tape  10 , in order to force the latter locally into contact with the head ( FIG. 3 ). The jet has a form factor, having its longest dimensions in the cross-track direction x to cover the lateral span of the array of transducers (˜3 mm long). The air jet has shorter dimensions in the along-track direction z, so as to localize the pressure imprint to the region  24  of the head surface corresponding to the along-track span of the transducers (typically as small as ˜10 μm). To generate the air jet, a nozzle configuration  30  is used, which compares to nozzles of air knives such as used in industrial settings to blow-dry or cool products during manufacturing. In the present context, however, the nozzle and air jet are scaled to obtain the required jet dimensions and properties. 
       FIG. 6  shows the result of an experiment, in which a commercial air knife was used to bring the tape into local contact with a tape head. The head is a typical commercial tape write head, whose air-skiving function was disabled by beveling the sharp outer edges, such that the tape does not come into (too) close contact with the head at any normal operating wrap angle. 
     More precisely,  FIG. 6  shows experimental results in terms of friction and normal forces, as obtained with the air knife jet on (as shown in  FIG. 3A ), with the tape initially positioned well off contact, i.e., at a large negative lead wrap angle (as in  FIG. 2 ), and the lead wrap angle was gradually increased. The wrap geometry in this experiment was symmetric, i.e. the trailing wrap angle was kept equal in magnitude and with the same sign as the leading wrap angle (in accordance with the angle convention for tape drives). As seen in  FIG. 6 , the air flow from the air knife was not sufficient to push the tape into measurable contact for leading wrap angles substantially more negative than −1.25 degrees. At a lead wrap angle of about −1.25 degrees, an onset of friction and normal force was detected. In addition, magnetic particulate noise could be detected using servo readers of the write head, indicating that the spacing is small enough to read back a magnetic signal. However, quantitative estimates of the magnetic spacing (based on the shape of the measured noise) could not be obtained in this case because of the low cutoff frequency of the servo channel analog front-end, whence the arbitrary units in the plot. 
     As the lead wrap angle was increased (i.e., became less negative), by repositioning S 10  the tape, an increase of friction as well as normal force was observed, as expected due to the fact that the region of contact widens, which, in turn, increases such forces proportionally. Also, above the critical lead wrap angle of −1.25 degree, the estimate of the magnetic spacing remains approximately constant, indicating a robust contact between the head and tape at the position of the servo read elements. 
       FIG. 7  shows results from finite element simulations, wherein the steady-state shape of the moving tape web was computed as results from the equilibrium between the bending stiffness of the tape, momentum of the tape, pressure of the compressed air bearing on the head side of the tape, and pressure from a laterally centered, low-diverging air jet (not shown) that impinges on the back side of the tape. The head surface is denoted by a black line, while the dotted and (long) dashed lines indicate the position of the tape for different imposed values of the (symmetric) wrap angle. The inventors have observed that by varying the wrap angle the contact region changes size, demonstrating that by acting on the geometry it is possible to adjust the width of the contact region to—or towards—an optimal small value. 
     One understands that an optimal wrap angle may be achieved. More generally, by performing tests on and simulating various experimental settings, the present Inventors came to the conclusion that an optimal contact should be obtained for wrap angles in the range of +2.0 to −4.0 degrees and, in particular, in the range of −0.5 to −1.5 degrees. Still, owing to the variety of tape drive designs, other wrap angle values may possibly be contemplated. 
     While the present invention has been described with reference to a limited number of embodiments, variants and the accompanying drawings, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In particular, a feature (device-like or method-like) recited in a given embodiment, variant or shown in a drawing may be combined with or replace another feature in another embodiment, variant or drawing, without departing from the scope of the present invention. Various combinations of the features described in respect of any of the above embodiments or variants may accordingly be contemplated, that remain within the scope of the appended claims. In addition, many minor modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. In addition, many other variants than explicitly touched above can be contemplated.