  We propose a concept for production of high power coherent attosecond pulses
in X-ray range. An approach is based on generation of 8th harmonic of radiation
in a multistage HGHG FEL (high gain high harmonic free electron laser)
configuration starting from shot noise. Single-spike phenomena occurs when
electron bunch is passed through the sequence of four relatively short
undulators. The first stage is a conventional "long" wavelength (0.8 nm) SASE
FEL which operates in the high-gain linear regime. The 0.1 nm wavelength range
is reached by successive multiplication (0.8 nm $\to$ 0.4 nm $\to$ 0.2 nm $\to$
0.1 nm) in a stage sequence. Our study shows that the statistical properties of
the high-harmonic radiation from the SASE FEL, operating in linear regime, can
be used for selection of radiation pulses with a single spike in time domain.
The duration of the spikes is in attosecond range. Selection of single-spike
high-harmonic pulses is achieved by using a special trigger in data acquisition
system. The potential of X-ray SASE FEL at TESLA at DESY for generating
attosecond pulses is demonstrated. Since the design of XFEL laboratory at TESLA
is based on the use of long SASE undulators with tunable gap, no special place
nor additional FEL undulators are required for attophysics experiments. The use
of a 10 GW-level attosecond X-ray pulses at X-ray SASE FEL facility will enable
us to track processes inside atoms.
