## 1 Introduction  
Since [PERSON] synthesized carbon nanotubes (CNTs) by the arc discharge process in 1991 [1], many CNT synthesis processes have been developed. Notably, the chemical vapor deposition (CVD) process has been one of the most successful methods to make multiwalled carbon nanotubes (MWCNTs) [2; 3; 4; 5]. In order to improve the yield of CNTs, direct current plasma chemical vapor deposition (DC-PECVD) [6], radio-frequency plasma chemical vapor deposition (RF-PECVD) [7; 8], and microwave plasma chemical vapor deposition (MPCVD) [9; 10; 11; 12] were developed based on thermal CVD. Process temperatures for CVD production of CNTs typically lie in the range of 700 to 1200 \({}^{\circ}\)C in the pressure range of 10-10\({}^{5}\) Pa [2; 3; 4; 5]. The typical temperature range for the synthesis of CNTs by PECVD is 520-1000 \({}^{\circ}\)C at the pressure range of 40-3000 Pa [6; 7; 8; 9; 10; 11]. It has also been reported that PECVD has realized CNT synthesis at 340 \({}^{\circ}\)C [12]. Compared with thermal CVD, the CNT growth rates of DC-PECVD, RF-PECVD, and MPCVD are increased by 2-5 times at the same temperatures and pressures. They indicate that the plasma is very significant for the improvement of the growth rate of CNTs due to the plasma's higher activation at the high temperatures on the precursor, which enhances reaction rates.  
So far, all kinds of PECVD processes work under low pressure (\(<\)10\({}^{4}\) Pa). However, the atmospheric pressure microwave plasma has the advantages of higher electron density, electron activity, electron temperature, and stronger molecular decomposition, which are very powerful in the synthesis of nanomaterials. By atmospheric pressure microwave plasma, free-standing CNTs were synthesized with 900-1500 W plasma power and 1000 \({}^{\circ}\)C [13; 14; 15]; the CNT growth rate increased more than ten times by thermal CVD. Most atmospheric pressure microwave plasma nanomaterial synthesis systems use waveguide plasma generators with large volumes and high power. Usually, it needs more than 900 W microwave power to maintain high-power atmospheric pressure waveguide microwave plasma. It has several problems in the nanomaterial synthesis process because of the system structure and the high power: (1) The atmosphere of the system is not closed, making the synthesis area impure; there is residual air in the synthetic environment. (2) The growth area is small, and the flow rate is high, limiting the process in nanomaterial synthesis. Nevertheless, there is still great potential in the synthesis of nanomaterials by atmospheric pressure microwave plasma.  
In this paper, a kind of AMPCVD system that includes a small-scaled 200 W atmospheric pressure 2.45 GHz coaxial microwave plasma generator and a heating device is presented. The combination of atmospheric pressure microwave plasma and a CVD tube furnace makes the technology have a strong precursor decomposition, a pure nanomaterial synthesis atmosphere, a large control range of particles density, and accurate temperature control, which overcomes the pressure limits of PECVD and achieves accurate control of the nanomaterials synthesis process. For now, this is the only study of nanomaterial synthesis by AMPCVD.  
The CNTs are synthesized by using AMPCVD at the temperature of 500-800 \({}^{\circ}\)C. Compared with the CVD and all kinds of PECVD, the CNT growth rate of AMPCVD significantly increases. Compared with the experiments in [13; 14; 15], the decomposition capacity of 200 W atmospheric pressure microwave plasma of the AMPCVD is high enough to decompose the precursors completely.