According to one embodiment, a heat exchanger includes a pipe through which a fluid flows, and a supply device that supplies the fluid to the pipe. The pipe includes a flexible part deformed by flowing of the fluid, and a constricted part located at a downstream side of the flexible part along a flow direction of the fluid.

According to one embodiment, a heat exchanger includes a pipe through which a fluid flows, and a supply device that supplies the fluid to the pipe. The pipe includes a flexible part deformed by flowing of the fluid, and a constricted part located at a downstream side of the flexible part along a flow direction of the fluid.

A method may produce a heat regenerating material particle, including: preparing a slurry by adding a powder of the heat regenerating substance to an alginic acid aqueous solution and mixing the powder of the heat regenerating substance and the aqueous alginic acid solution; and forming a particle by gelling the slurry by dropping the slurry into a gelling solution. The gelling solution may include a metal element including calcium (Ca), manganese (Mn), magnesium (Mg) beryllium (Be), strontium (Sr), aluminum (Al), iron (Fe), copper (Cu), nickel (Ni), and cobalt (Co). The forming may involve controlling the gelation time so that a concentration of the metal element in a first region of the particle becomes lower than a concentration of the metal element in a second region. The second region may be closer to an outer edge of the particle compared to the first region.

A method may produce a two-stage heat regenerating cryogenic refrigerator including a vacuum vessel, first and second cylinder disposed in the vessel, the second cylinder coaxially connected to the first cylinder, and first and second regenerator respectively disposed in the first and second cylinder. The method may include: accommodating a first heat regenerating material (HRM) in the first regenerator; and filling a plurality of HRM particles in the second regenerator. The HRM particles may be a second HRM, each of the HRM particles including an oxide or oxysulfide heat regenerating substance having a maximum value of specific heat at a temperature of 20 K of 0.3+ J/cm.sup.3.Math.K and Ca, Mn, Mg, Be, Sr, Al, Fe, Cu, Ni, and/or Co. Each of the HRM particles may include a first and second region, the second region being closer to an HRM particle outer edge than the first region.