## 1 Introduction  
The advent of X-ray free electron laser (XFEL) [1; 2; 3; 4; 5] sources has set a new frontier in X-ray science due to remarkable advances in its characteristics. Synchrotron sources produce insufficient brightness and picosecond-long X-ray pulses, which yield only blurred images of atoms and molecules in motion. XFELs are coherent, ultra-brilliant tunable laser pulses. The current XFEL pulse is characterized by its peak power of 10-50 GW and its pulse duration of a few to about 100 femtoseconds (fs).  
Attosecond science is a new exciting frontier born with the subfemtosecond extreme ultraviolet light pulses via high harmonic generation (HHG) based on femtosecond lasers [6; 7]. This field will enrich even more with the development of isolated attosecond (as) XFEL pulses. XFEL presently exceeds HHG sources [8; 9], with a greater power and a shorter wavelength. It is these properties that still suggest XFEL pulse as a future light source because even a shorter pulse duration and higher power is possible [10]. By the development of such high-intensity attosecond X-ray sources, the realm of ultrafast processes that can be explored will be greatly extended. The imaging of a single molecule [11], the real time tracking of electron distribution around atoms and molecules [12; 13], and the dynamical investigation of X-ray nonlinear processes [14] are a few examples among the investigations that will set milestones. Therefore, the generation of attosecond X-ray pulses has drawn attention. Such high-intensity X-ray pulses cause radiation damage by depositing energy directly into the sample. However, the estimation of radiation damage as a function of photon energy, pulse length, integrated pulse intensity, and sample size shows that experiments using very high X-ray dose rates and ultrashort exposures provide useful structural information before radiation damage destroys the sample [15; 16]. Novel techniques have been proposed to further shorten the pulse length, mostly employing one or more external lasers [17; 18; 19; 20; 21; 22; 23; 24; 25; 26] and generating short electron bunches [27; 28].  
Recently, [PERSON] [29] proposed a scheme to produce high peak powers of up to the terawatt range. His design uses a combination of slotted foil [27], enhanced self-amplified spontaneous emission (SASE) [20], and optical and electron beam delay between undulator sections. In another work, [PERSON] and [PERSON] [30] suggested a simpler scheme for a TW-attosecond free electron laser (FEL) in which a multiple slot foil is used to effectively divide an electron bunch into many different parts by preserving the emittance in some parts. The same authors in another work [31] show a TW-attosecond pulse by introducing a transverse tilt to an electron beam while properly delaying and correcting the trajectory of the electron beam between certain undulator modules. [PERSON] et al. [32] devised a new idea where only one single electron spike is used repeatedly. The radiation amplification is based on the superradiant behavior of short pulses [33; 34], where the power level can significantly exceed the saturation power of an XFEL while shortening its pulse length.  
In this paper, the multiple electron spike scheme [29] is adopted for realistic simulations. For this purpose, the beam parameters of the XFEL in Pohang Accelerator Laboratory (PAL-XFEL) are taken. We studied both cases of hard and soft TW-attosecond XFEL. The role of tapered undulator [35] on the output radiation pulse characteristics is also discussed for both cases. This is the first numerical demonstration for a TW, isolated, attosecond, soft X-ray pulse.