FIELD OF THE INVENTION 
     The field of the invention is diffusers for seal protection from a velocity fluid flow and more particularly in applications for sliding sleeve valves or chokes in subterranean applications. 
     BACKGROUND OF THE INVENTION 
     Sliding sleeve valves are used to regulate formation flow into a production string or to balance flows from an interval. The housing has a port as does a sliding sleeve that can move axially within the housing. Normally the sleeve has a series of circumferentially spaced slots that travel past an isolation seal to initiate formation flow into the production tubing. The initial flow has to rush past the seal that is uphole from the housing inlet port. High initial velocities can damage the seal so that in the past diffusers have been used to protect the seal by reducing the fluid velocity that reaches the seal. 
     One attempt to slow down the fluid velocity has been to use a non-metallic ring, primarily made of PEEK and place the ring upstream from the seal being protected. The problem with such designs is that the material had service limits and the high velocity gas and temperatures in many applications limited the service life of such designs. Such single rings are illustrated in U.S. Pat. No. 6,722,439 as item 38. They were typically installed in an interference fit to the sliding sleeve on the inside and the valve housing on the exterior side. Other sliding sleeve valve designs that have similar components are U.S. Pat. Nos. 7,363,981; 7,921,915 and 7,575,058. 
     Metal ring diffusers were also used as alternatives to the PEEK designs. The problem with these rings is that they needed too much clearance for mounting purposes and let too much flow at high velocity get to the seal. 
     What is needed and addressed by the present invention is a diffuser assembly that has the durability feature with the ability to slow or stop the incoming high velocity fluid before it can reach the seal assembly and damage the seal. Thus an assembly of rings is provided that is energized by differential pressure to enhance an initial fit that is at least a clearance fit but preferably is an interference fit to the sliding sleeve on the inside and the surrounding housing on the outside. The rings are fabricated with a bias either toward the sleeve or the surrounding housing and are preferably disposed in alternating arrangements. Sloping surfaces are used in conjunction with pressure differential to further bias some rings inwardly and adjacent rings outwardly. In another variation the rings are split and matched in pairs that are biased out alternating with pairs biased to move in. The rings that move in a given direction can be split with the splits offset circumferentially and the relative position of the adjacent rings that move in a given direction prevented from relative rotation using a projection on one ring registering with a depression on an adjacent ring for each pair of rings that are designed to move either inwardly toward the sleeve or outwardly toward the surrounding housing. These and other features of the present invention will be more readily understood by those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be found in the appended claims. 
     SUMMARY OF THE INVENTION 
     A diffuser assembly has pairs of split rings rotationally locked to each other in an alternating array with other pairs of split rings where adjacent pairs are responsive to pressure differential to be biased toward a sliding sleeve or the surrounding housing in an alternating pattern. The split rings are made to have an interference initial fit to the sleeve or housing and the splits on adjacent rings are offset while a relation of a projection to a depression between adjacent rings prevents relative rotation to keep the desired circumferential offset in the splits between adjacent rings. End tapers can bias adjacent pairs in opposed directions responsive to applied differential pressure. The rings are preferably metallic and can have a coating to facilitate relative sliding and enhance durability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a section view of a sliding sleeve valve in the closed position showing the diffuser of the present invention; 
         FIG. 2  is a close up view of the diffuser shown in  FIG. 1 ; and 
         FIG. 3  is an exploded perspective view of the ring array that comprises the diffuser of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates casing  10  which defines an annulus  12  around a valve housing  14  that is connected to production tubing that is not shown. The valve assembly  16  is shown in the closed position. The housing  14  has inlets  18 . Primary seal  20  and backup seal  22  are disposed between the inlets  18  and the slots  24  on the sliding sleeve  26 . Seals  20  and  22  are fixed in the housing  14  so that as the sliding sleeve  26  is moved either mechanically with a shifting tool (not shown) or hydraulically using control lines (not shown) the slots  24  will move past seal  20  so that the fluid can flow from the annulus  12  into inlets  18  and to or past the diffuser assembly  28  and into slots  24  of the sliding sleeve  26  and on up to the surface. The diffuser assembly  28  is axially retained between radial surface  30  on housing  14  and top ring  32 , a part of which can be seen in  FIG. 2 . 
       FIG. 2  is a close up view of the diffuser assembly  28  shown in  FIG. 1 . The assembly  28  is bookended by rings  34  and  36  with each having an exterior radial surface such as  38  shown on ring  34 . Once the slots  24  get past seal  20  pressure in the annulus  12  represented by arrow  40  enters the annular gap between the sliding sleeve  26  and the housing  14 . The force from pressure represented by arrow  40  moves all the illustrated components axially so that initially radial surface  38  abuts an opposing and stationary surface  42  on ring  32 . 
     There are pairs of rings  44  and  46  with sloping end walls  48  and  50  that face away from each other. Rings  44  and  46  are essentially mirror image trapezoidal shapes in section. Adjacent the ring pair  44  and  46  is another ring pair  52  and  54 . Rings  52  and  54  have opposed end surfaces  56  and  58  respectively so that on application of an axial force from pressure represented by arrow  40  the diffuser assembly  28  shifts axially and opposed surfaces  48  and  58  on one side and surfaces  50  and  56  on the other side create a net radial outward force on rings  44  and  46  and a net radial inward reaction force on rings  52  and  54 . Rings  52  and  54  are essentially mirror image trapezoidal shapes in section. It should be noted that rings  44  and  46  are manufactured to preferably be in an interference fit against the housing  14  on assembly although a clearance fit can also be used. The application of pressure represented by arrow  40  simply pushes rings  44  and  46  harder against the housing  14 . Similarly, ring pairs  52  and  54  are fabricated to have an initial interference fit to the sleeve  26  although a clearance fit is also possible. Force created by pressure represented by arrow  40  enhances the contact force to the sleeve  26  for the ring pairs  52  and  54 . Preferably the pattern on rings that are forced toward the housing  14  is alternated with a ring pair that is forced against the sleeve  26 . 
     It should be noted that ring pair  52  and  54  have opposed contacting radial surfaces  60  and  62  that are preferably perpendicular to the axis of the sleeve  26 . Similarly, ring pair  44  and  46  has opposed radial surfaces  64  and  66  that are preferably perpendicular to the axis of the sleeve  26 . The surface pairs  50  and  56  on one side and  48  and  58  on the other side of the pair of rings  44  and  46  are shown at a preferred angle of about 15 degrees to a plane perpendicular to the axis of the sleeve  26  but a range of 0-45 degrees is contemplates. At 0 degrees there is no radial sliding component of force while at 45 degrees such radial force is maximized. The various rings are preferably made of a softer material than the housing  14  or the sleeve  26  to avoid scoring either of those opposing surfaces. The rings can also be coated with a lubricious material to facilitate radial movement and in that case can also be of a material that is harder than the housing  14  or the sleeve  26 . 
       FIG. 3  illustrates ring pairs such as  44  and  46  or  52  and  54  can be rotationally locked to each other using a combination of a projection  68  on ring  52  mating with a depression  70  on the ring  54 . The locking mechanism of projection with depression can be reversed and other types of rotational locks can be used within the spirit of the invention. 
     The rotational locking serves to keep splits  72  and  74  on adjacent rings circumferentially offset. Adjacent splits are preferably kept 180 degrees apart. End rings  34  and  36  are preferably not split but optionally can also have a split. While the figures show rotational locking only between pairs such as  44  and  46  or  52  and  54 , those skilled in the art can appreciate that ring pairs that move toward housing  14  can be optionally rotationally locked to ring pairs that move toward sleeve  26  which in effect locks all the split rings between end rings  34  and  36  together rotationally. 
     As an alternative to having a split  72  or  74  which can incorporate butted ends cut in a plane going through the ring axis or on a skew so that the cut ends overlap, the ring can simply have a flexible portion in a complete ring to achieve the same effect. A part of the ring can have a sinusoidal component or an alternating bend pattern that allows the diameter to increase or decrease without undue resistance. The flexible portions can also be circumferentially offset and maintained in their relative positions in the manner described above. In some respect the locking feature of projection and depression can integrate some diametric flexibility that can allow elimination of the split or use in conjunction with the splits in the rings. If the splits in the rings are eliminated in favor of flexible portions on the rings then the rotational locking can be optionally omitted. 
     As another option the rings can be made of a shape memory alloy which allows rapid assembly but on exposure to well fluids or other heat sources before initially moving the sliding sleeve  26  the rings can revert to an original shape that can have some rings moving toward sleeve  26  and alternating rings moving in an opposite direction toward the housing  14 . In that manner initial clearances on assembly are closed before operation of the sleeve  26 . 
     Those skilled in the art will appreciate that the described diffuser assembly can slow down or stop migrating fluid that can potentially damage the seal in a sliding sleeve valve. The assembly uniquely has multiple components. More specifically the components can be manufactured with a bias toward the sleeve or the housing and preferably in alternating patterns. The bias can either be created in the manufacture of the rings or the shape can change using shape memory material exposed to a temperature above a critical temperature to gain at least a clearance fit but preferably an interference fit before the valve is opened. If the rings are made of shape memory alloy they may not need to have a split but can have a flexible segment. Additionally, ring pairs need not be used as the reconfiguration of each ring can build into that ring movement in the desired direction toward the housing or the sleeve on an alternating basis after the critical temperature is reached. The rings can be shaped to create radial forces toward the sleeve or the housing in response to an axial force created by fluid as the valve is opened. The rings can be split for rapid assembly with the splits circumferentially offset and the relative positions held by a locking feature so that adjacent pairs can be rotationally locked to each other. The split or some flexibility in a whole ring structure also allows the rings to compensate for dimensional tolerances in the moving sleeve during operation of the valve. Optionally all the pairs whether urged toward the sleeve or toward the housing can be rotationally locked to each other or to end rings or an internal housing shoulder on opposed ends of the assembly. Although ring pairs are illustrated as moving radially in a given direction toward the housing or the sleeve one or more rings can be used to move in a given radial direction instead of the pairs illustrated in the FIGS. 
     While the application in which the diffuser assembly is discussed in a sliding sleeve valve, other applications where an annular space is sealed and the seal is exposed to fluid flow that can potentially damage the seal can be also situations where the diffuser assembly can be deployed. 
     The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below:
A diffuser assembly has pairs of split rings rotationally locked to each other in an alternating array with other pairs of split rings where adjacent pairs are responsive to pressure differential to be biased toward a sliding sleeve or the surrounding housing in an alternating pattern. The split rings are made to have an interference initial fit to the sleeve or housing and the splits on adjacent rings are offset while a relation of a projection to a depression between adjacent rings prevents relative rotation to keep the desired circumferential offset in the splits between adjacent rings. End tapers can bias adjacent pairs in opposed directions responsive to applied differential pressure. The rings are preferably metallic and can have a coating to facilitate relative sliding and enhance durability.