NOVELTY - The hydrogen generator comprises a holder, and a fuel unit (210). The fuel unit comprises a disc shaped substrate having two opposite planar surfaces. One of the surfaces has a reactant composition (214) that is a solid hydrogen containing a reactant for releasing hydrogen gas by a thermal decomposition reaction when heated to a minimum temperature. The holder comprises a cavity in which the fuel unit is removably contained, and an indexing mechanism. A laser projects a beam of electromagnetic radiation on a portion of the reactant composition to heat the reactant to the minimum temperature.    USE - The hydrogen generator is useful in a fuel cell system (claimed) that is useful as power sources for portable electronic devices.    ADVANTAGE - The portable hydrogen generator: can be readily moved by an individual person without the need of lifting or transporting equipment such as hoist, dolly or lift truck; is capable of economically producing a large total volume of hydrogen gas per unit of mass and per unit of volume of the hydrogen generator, controlling the reaction of the reactant to provide hydrogen without producing an excessive internal pressure, and operating at or below a desired maximum temperature; and has long term durability, reliability, and a user replaceable fuel unit that can be made easily and inexpensively.    DETAILED DESCRIPTION - The hydrogen generator comprises a holder, and a fuel unit (210). The fuel unit comprises a disc shaped substrate having two opposite planar surfaces. One of the surfaces has a reactant composition (214) that is a solid hydrogen containing a reactant for releasing hydrogen gas by a thermal decomposition reaction when heated to a minimum temperature. The holder comprises a cavity in which the fuel unit is removably contained, and an indexing mechanism. A laser projects a beam of electromagnetic radiation onto a portion of the reactant composition to heat the reactant to the minimum temperature. The indexing mechanism aligns the laser beam and an unreacted portion of the reactant composition, and comprises a disc rotating device for rotationally indexing the disc from a first disc position to a second disc position and a laser positioning device for indexing the laser beam from a first laser position to a second laser position. The reactant composition contains additives, admixed with, adjacent to, underlying or covering a portion of the reactant composition. The additives include an electromagnetic energy absorbing medium, a binder, a stabilizing compound, a thermally conductive material and/or an ignition material. The reactant composition is free of catalysts, and is segregated into individual portions. The laser is turned on and off to provide hydrogen gas as needed, is a laser diode, and uses pulsed power. The disc rotating device includes a stepper motor, a ratchet mechanism, a chain drive, a belt drive and a worm drive for indexing the disc from the first disc position to the second disc position. The laser positioning device includes the stepper motor, the ratchet mechanism, the chain drive, the belt drive, the worm drive and mirrors for indexing the laser beam from a first laser position to a second laser position. The mirrors project the laser beam onto a surface of the fuel unit that does not face the laser. The holder further includes a housing that includes portions of one of a fuel cell system and a device with which the fuel cell system or the hydrogen generator is used. The hydrogen generator further comprises: an energy source for providing energy to the laser and the indexing mechanism; and a control system comprising a microprocessor, a microcontroller, digital, analog and hybrid circuitry, solid state and electromechanical switching devices, capacitors and/or sensing instrumentation for controlling operation of the laser, operation of the indexing mechanism and monitoring parameters indicative of a need for hydrogen. The fuel unit is portable. An INDEPENDENT CLAIM is included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross sectional view of a hydrogen generator.    Fuel unit (210)    Reactant composition (214)    Track (222)    Worm gear (224)    Motor. (226)