@article{SchoppRohrbachLangeretal.2024,
  author    = {Schopp, Christoph and Rohrbach, Felix and Langer, Luc and Heuermann, Holger},
  title     = {Detection of welding wire length by active S11 measurement},
  series = {IEEE Transactions on Plasma Science},
  journal   = {IEEE Transactions on Plasma Science},
  number    = {Early Access},
  publisher = {IEEE},
  issn      = {0093-3813 (Print)},
  doi       = {10.1109/TPS.2024.3356659},
  pages     = {1 -- 6},
  year      = {2024},
  abstract  = {A novel method to determine the extruded length of a metallic wire for a directed energy deposition (DED) process using a microwave (MW) plasma jet with a straight-through wire feed is presented. The method is based on the relative comparison of the measured frequency response obtained by the large-signal scattering parameter (Hot-S) technique. In the practical working range, repeatability of less than 6\% for a nonactive plasma and 9\% for the active plasma state is found. Measurements are conducted with a focus on a simple solution to decrease the processing time and reduce the integration time of the process into the existing hardware. It is shown that monitoring a single frequency for magnitude and phase changes is sufficient to achieve good accuracy. A combination of different measurement values to determine the length is possible. The applicability to different diameter of the same material is shown as well as a contact detection of the wire and metallic substrate.},
  language  = {en}
}
@article{TurdumamatovBeldaHeuermann2024,
  author    = {Turdumamatov, Samat and Belda, Aljoscha and Heuermann, Holger},
  title     = {Shaping a decoupled atmospheric pressure microwave plasma with antenna structures, Maxwell's equations, and boundary conditions},
  series = {IEEE Transactions on Plasma Science},
  journal   = {IEEE Transactions on Plasma Science},
  number    = {Early Access},
  publisher = {IEEE},
  issn      = {0093-3813 (Print)},
  doi       = {10.1109/TPS.2024.3383589},
  pages     = {1 -- 9},
  year      = {2024},
  abstract  = {This article addresses the need for an innovative technique in plasma shaping, utilizing antenna structures, Maxwell's laws, and boundary conditions within a shielded environment. The motivation lies in exploring a novel approach to efficiently generate high-energy density plasma with potential applications across various fields. Implemented in an E01 circular cavity resonator, the proposed method involves the use of an impedance and field matching device with a coaxial connector and a specially optimized monopole antenna. This setup feeds a low-loss cavity resonator, resulting in a high-energy density air plasma with a surface temperature exceeding 3500 o C, achieved with a minimal power input of 80 W. The argon plasma, resembling the shape of a simple monopole antenna with modeled complex dielectric values, offers a more energy-efficient alternative compared to traditional, power-intensive plasma shaping methods. Simulations using a commercial electromagnetic (EM) solver validate the design's effectiveness, while experimental validation underscores the method's feasibility and practical implementation. Analyzing various parameters in an argon atmosphere, including hot S -parameters and plasma beam images, the results demonstrate the successful application of this technique, suggesting its potential in coating, furnace technology, fusion, and spectroscopy applications.},
  language  = {en}
}
@article{HeuermannEmmrichBongartz2022,
  author    = {Heuermann, Holger and Emmrich, Thomas and Bongartz, Simon},
  title     = {Microwave spark plug to support ignitions with high compression ratios},
  series = {IEEE Transactions on Plasma Science},
  journal   = {IEEE Transactions on Plasma Science},
  number    = {Early Access},
  publisher = {IEEE},
  issn      = {1939-9375},
  doi       = {10.1109/TPS.2022.3183690},
  pages     = {1 -- 6},
  year      = {2022},
  abstract  = {Upcoming gasoline engines should run with a larger number of fuels beginning from petrol over methanol up to gas by a wide range of compression ratios and a homogeneous charge. In this article, the microwave (MW) spark plug, based on a high-speed frequency hopping system, is introduced as a solution, which can support a nitrogen compression ratio up to 1:39 in a chamber and more. First, an overview of the high-speed frequency hopping MW ignition and operation system as well as the large number of applications are presented. Both gives an understanding of this new base technology for MW plasma generation. Focus of the theoretical part is the explanation of the internal construction of the spark plug, on the achievable of the high voltage generation as well as the high efficiency to hold the plasma. In detail, the development process starting with circuit simulations and ending with the numerical multiphysics field simulations is described. The concept is evaluated with a reference prototype covering the frequency range between 2.40 and 2.48 GHz and working over a large power range from 20 to 200 W. A larger number of different measurements starting by vector hot-S11 measurements and ending by combined working scenarios out of hot temperature, high pressure and charge motion are winding up the article. The limits for the successful pressure tests were given by the pressure chamber. Pressures ranged from 1 to 39 bar and charge motion up to 25 m/s as well as temperatures from 30◦ to 125◦.},
  language  = {en}
}
@article{SchoppBritunVoracetal.2019,
  author    = {Schopp, Christoph and Britun, Nikolay and Vorac, Jan and Synek, Petr and Snyders, Rony and Heuermann, Holger},
  title     = {Thermal and Optical Study on the Frequency Dependence of an Atmospheric Microwave Argon Plasma Jet},
  series = {IEEE Transactions on Plasma Science},
  volume    = {47},
  journal   = {IEEE Transactions on Plasma Science},
  number    = {7},
  publisher = {IEEE},
  address   = {New York},
  issn      = {1939-9375},
  pages     = {3176 -- 3181},
  year      = {2019},
  language  = {en}
}
@article{SchoppDollGraeseretal.2016,
  author    = {Schopp, Christoph and Doll, Timo and Gr{\"a}ser, Ulrich and Harzheim, Thomas and Heuermann, Holger and Kling, Rainer and Marso, Michael},
  title     = {Capacitively Coupled High-Pressure Lamp Using Coaxial Line Networks},
  series = {IEEE Transactions on Microwave Theory and Techniques},
  volume    = {64},
  journal   = {IEEE Transactions on Microwave Theory and Techniques},
  number    = {10},
  publisher = {IEEE},
  address   = {New York, NY},
  issn      = {0018-9480},
  doi       = {10.1109/TMTT.2016.2600326},
  pages     = {3363 -- 3368},
  year      = {2016},
  abstract  = {This paper describes the development of a capacitively coupled high-pressure lamp with input power between 20 and 43 W at 2.45 GHz, using a coaxial line network. Compared with other electrodeless lamp systems, no cavity has to be used and a reduction in the input power is achieved. Therefore, this lamp is an alternative to the halogen incandescent lamp for domestic lighting. To serve the demands of domestic lighting, the filling of the lamp is optimized over all other resulting requirements, such as high efficacy at low induced powers and fast startups. A workflow to develop RF-driven plasma applications is presented, which makes use of the hot S-parameter technique. Descriptions of the fitting process inside a circuit and FEM simulator are given. Results of the combined ignition and operation network from simulations and measurements are compared. An initial prototype is built and measurements of the lamp's lighting properties are presented along with an investigation of the efficacy optimizations using large signal amplitude modulation. With this lamp, an efficacy of 135 lmW -1 is achieved.},
  language  = {en}
}
@article{HoltrupSadeghfamHeuermannetal.2014,
  author    = {Holtrup, S. and Sadeghfam, Arash and Heuermann, Holger and Awakowicz, P.},
  title     = {Characterization and optimization technique for microwave-driven high-intensity discharge lamps using hot S-parameters},
  series = {IEEE transactions on microwave theories and techniques},
  volume    = {62},
  journal   = {IEEE transactions on microwave theories and techniques},
  number    = {10},
  publisher = {IEEE},
  address   = {New York},
  issn      = {0018-9480},
  doi       = {10.1109/TMTT.2014.2342652},
  pages     = {2471 -- 2480},
  year      = {2014},
  abstract  = {High-intensity discharge lamps can be driven by radio-frequency signals in the ISM frequency band at 2.45 GHz, using a matching network to transform the impedance of the plasma to the source impedance. To achieve an optimal operating condition, a good characterization of the lamp in terms of radio frequency equivalent circuits under operating conditions is necessary, enabling the design of an efficient matching network. This paper presents the characterization technique for such lamps and presents the design of the required matching network. For the characterization, a high-intensity discharge lamp was driven by a monofrequent large signal at 2.45 GHz, whereas a frequency sweep over 300 MHz was performed across this signal to measure so-called small-signal hot S-parameters using a vector network analyzer. These parameters are then used as an equivalent load in a circuit simulator to design an appropriate matching network. Using the measured data as a black-box model in the simulation results in a quick and efficient method to simulate and design efficient matching networks in spite of the complex plasma behavior. Furthermore, photometric analysis of high-intensity discharge lamps are carried out, comparing microwave operation to conventional operation.},
  language  = {en}
}
@article{HeuermannSadeghfam2009,
  author    = {Heuermann, Holger and Sadeghfam, A.},
  title     = {Enhanced system architecture for rugged wide band data transmission / Sadeghfam, A. ; Heuermann, H.},
  series = {European Radar Conference, 2009 : EuRAD 2009 ; Sept. 30 - Oct. 2 2009, Rome, Italy ; part of the European Microwave Week (EuMW) / sponsored by EuMA, European Microwave Association},
  journal   = {European Radar Conference, 2009 : EuRAD 2009 ; Sept. 30 - Oct. 2 2009, Rome, Italy ; part of the European Microwave Week (EuMW) / sponsored by EuMA, European Microwave Association},
  publisher = {IEEE},
  address   = {Piscataway, NJ},
  isbn      = {978-2-87487-014-9},
  pages     = {347 -- 350},
  year      = {2009},
  language  = {en}
}
@article{HeuermannHoltrup2009,
  author    = {Heuermann, Holger and Holtrup, Stephan},
  title     = {Fundamentals and ignition of a microplasma at 2.45 GHZ / Holtrup, Stephan ; Heuermann, Holger},
  series = {European Microwave Conference, 2009 : EuMC 2009 ; Sept. 29, 2009 - Oct. 1, 2009, Rome, Italy ; part of European Microwave Week (EuMW) / sponsored by EuMA, European Microwave Association ... Endorsed by IEEE},
  journal   = {European Microwave Conference, 2009 : EuMC 2009 ; Sept. 29, 2009 - Oct. 1, 2009, Rome, Italy ; part of European Microwave Week (EuMW) / sponsored by EuMA, European Microwave Association ... Endorsed by IEEE},
  publisher = {IEEE},
  address   = {Piscataway, NJ},
  isbn      = {978-1-4244-4748-0},
  pages     = {1607 -- 1609},
  year      = {2009},
  language  = {en}
}
@article{HeuermannIbrahim2009,
  author    = {Heuermann, Holger and Ibrahim, Irfan},
  title     = {Improvements in the flicker noise reduction technique for osillator designs / Ibrahim, Irfan ; Heuermann, Holger},
  series = {European Microwave Conference, 2009 : EuMC 2009 ; Sept. 29, 2009 - Oct. 1, 2009, Rome, Italy ; part of European Microwave Week (EuMW) / sponsored by EuMA, European Microwave Association ... Endorsed by IEEE},
  journal   = {European Microwave Conference, 2009 : EuMC 2009 ; Sept. 29, 2009 - Oct. 1, 2009, Rome, Italy ; part of European Microwave Week (EuMW) / sponsored by EuMA, European Microwave Association ... Endorsed by IEEE},
  publisher = {IEEE},
  address   = {Piscataway, NJ},
  isbn      = {978-1-4244-4748-0},
  pages     = {1215 -- 1218},
  year      = {2009},
  language  = {en}
}
@article{HeuermannErkens2007,
  author    = {Heuermann, Holger and Erkens, H.},
  title     = {Mixed-Mode Chain Scattering Parameters: Theory and Verification},
  series = {IEEE Transactions on Microwave Theory and Techniques},
  volume    = {55},
  journal   = {IEEE Transactions on Microwave Theory and Techniques},
  number    = {8},
  publisher = {IEEE},
  address   = {New York},
  isbn      = {0018-9480},
  doi       = {10.1109/TMTT.2007.902587},
  pages     = {1704 -- 1708},
  year      = {2007},
  abstract  = {Chain scattering parameters or T-parameters are a useful tool for calculating cascaded two-ports. With the increasing importance of mixed-mode S-parameters, a need for converting the T-parameters from their unbalanced form into a balanced form emerges for suiting both common and differential mode waves, as well as the mode conversion. This paper presents the derivation of the equations for transformations between mixed-mode S- and T-parameters for a mixed-mode two-port. Although derived in a way very similar to monomode T-parameters, no simplifications were necessary. Measurement results exemplify the quality of the T-parameter transformation under real-life conditions.},
  language  = {en}
}