X-ray or gamma ray systems or devices – Source – Electron tube
Patent
1983-10-14
1986-11-11
Church, Craig E.
X-ray or gamma ray systems or devices
Source
Electron tube
313 30, 378141, 378144, H01J 3510, H01J 3526
Patent
active
046226877
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention is directed to liquid cooled anode x-ray tubes, and in particular, x-ray tubes having a continuously cooled anode whereby high average power is achieved while still maintaining the high peak powers characteristic of rotating anodes.
BACKGROUND OF THE INVENTION
The need for continuous duty, high power rotating anode x-ray tubes exists in medical radiography, i.e., fluoroscopy and computerized tomography (CT), and in industrial applications such as x-ray diffraction topography and non-destructive testing.
A number of schemes have been proposed in the past to achieve continuous power output at high peak power with a rotating anode x-ray tube. These include direct liquid cooling of the anode, liquid to vapor phase cooling of the anode, as well as other techniques.
Examples of rotating anode x-ray tubes using other than liquid cooling are described in U.S. Pat. No. 4,165,472 issued Aug. 21, 1979 to Wittry (liquid to vapor phase cooling in a sealed anode chamber), U.S. Pat. Nos. 3,959,865 issued to Koncesznski on May 25, 1976, 3,719,847 issued to Webster on Mar. 6, 1973, 4,146,815 issued to Childenc on Mar. 6, 1973 (melting or vaporization of inserts in solid anode), and 3,736,175 issued to Blomgen May 22, 1973 (heat pipe to external heat sink).
Such rotating anode x-ray tubes have proven to be less efficient than direct liquid cooled tubes, and sometimes have a tendency to burst or explode when overheated, rendering such tubes unsafe.
Liquid cooled rotating anode x-ray tubes are, in general, well known. In such x-ray tubes, a hollow anode is disposed so that a rotating portion thereof is irradiated by an energy beam (e.g. electron beam). The irradiated portion of the anode is generally referred to as the electron beam track. Substantially all of the heat generated by irradiation by the energy beam is transmitted to a heat exchange surface, typically the interior wall of the hollow anode underlying the electron beam track and adjacent areas. In other words, the heat exchange surface is generally an area of the interior surface of the anode larger than the electron beam track, centered on and underlying the electron beam track. A flow of liquid coolant is passed into contact with the heat exchange surface to remove the heat therefrom, and thus cool the anode.
The basic cooling mechanism in liquid cooled anodes for use in x-ray tubes is nucleate boiling (or other vapor or gas mechanism). In nucleate boiling, bubbles of vaporized fluid are generated on the anode heat exchange surface. The vapor bubbles break away and are replaced by fresh bubbles, much like a pot of boiling water, thus providing efficient cooling by the removal of heat from the exchange surface to vaporize the liquids.
However, under certain circumstances, the power handling capacity of the system is limited by transformation of the nucleate boiling mechanism into what is known as a destructive film boiling phenomenon (or other vapor or gas blanket). The heated surface becomes surrounded by an insulating vapor blanket, thus causing significantly reduced heat transfer. The primary heat removal mechanism therefore becomes radiation and convection through the vapor.
The heat flux at the transition from nucleate to film boiling is called the critical heat flux. Should this value be exceeded in electrically heated structures such as a liquid cooled x-ray tube anode, the insulating film blanket would cause a rapid rise in temperature, typically resulting in burn out (i.e., melt down) of the structure. In general, burn out occurs very quickly, and the protective means required are extremely elaborate and expensive. Thus adequate protection has not heretofore been practical.
Formation of the boiling film occurs when expanding bubbles are generated faster than they can be carried away. The expanding nucleate bubbles interact and combine ultimately to form an insulating blanket of vapor. Thus, the transition is made from nucleate boiling to film boiling. It is therefore the bubble interaction which controls the heat
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Philips Technical Review, vol. 19, 1957/50, No. 11, pp. 314-317.
A. Taylor, "High-Intensity Rotating Anode X-Ray Tubes", from Mallet et al., Advances in X-Ray Analysis, vol. 9, Plenum Press, N.Y., pp. 194-201.
Queisser, "X-Ray Optics: Applications to Solids", Springer-Verlag, N.Y., 1977, Ch. 2, pp. 9-33.
Iversen Arthur H.
Whitaker Stephen
Church Craig E.
Iversen Arthur H.
Wieland Charles F.
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