BEGIN:VCALENDAR VERSION:2.0 PRODID:-//linuxsoftware.nz//NONSGML Joyous v1.4//EN BEGIN:VEVENT SUMMARY:Assessment of reflectometry diagnostics for DEMO DTSTART:20220126T090000Z DTEND:20220126T110000Z DTSTAMP:20260801T074152Z UID:2f55a025-69c4-4dc0-9a91-8476ef652ed6 SEQUENCE:4 CREATED:20220111T145319Z DESCRIPTION:Abstract: DEMO will be the first prototype of a fusion power p lant. Unlike experimental tokamaks\, only the necessary diagnostics for ma chine protection and plasma control will be implemented. One of the fundam ental measurements is the position and shape of the last closed magnetic s urface\, typically measured with the magnetic diagnostics. One of the majo r issues of its implementation in DEMO is the large integration drifts tha t can occur during the operation due to the high levels of radiation. This can lead to a wrong plasma position estimation\, putting the operation at risk.The prime candidate to complement or substitute the magnetic diagnos tics in DEMO is the microwave reflectometry. By sweeping the frequency of the probing beam\, microwave reflectometry is capable of measuring the ele ctron density profile. As the density is directly linked to the magnetic f lux surfaces these measurements give access to the magnetic configuration\ , providing its local radial position. The O-mode propagation is independe nt from the magnetic field\, being ideal for replacing the magnetic diagno stics.The DEMO plasma position reflectometer (DEMO PPR) consists a system of multi­ reflectometers distributed poloidaly along the wall at differen t positions that will provide the separatrix reconstruction. The optimizat ion of such system requires the simulation of the measurement process for different poloidal views\, emitting angles\, antenna assemblies and plasma configurations. The final system must be optimized for the operation scen ario and be stable under the possible deviations to its equilibrium that c an occur during the discharges. This includes the plasma displacement\, tu rbulence or MHD activity. For now\, the DEMO PPR is in an early developmen t stage and there are many questions that need to be investigated before r eaching its final design.In this work we study the process of optimization of PPR systems with a general approach\, taking into account the future c hanges in the geometry and plasma scenario. The important variables of a g eneral multiple reflectometers system were identified and the techniques a nd the procedure to optimize it were developed. The simulation of such sys tems is in general a complex task that requires the definition of several different regions of interest and testing different antenna models and pla smas\, which is a very demanding task from the computational point of view and of necessary time to build the simulation scripts. For this reason\, we develop the structure of a high-level framework for multiple reflectome try simulations that is capable of automatizing all the simulation process of a multiple reflectometers system for the REFMUL* codes\, a family of f ull­ wave FDTD codes that has been used for reflectometry simulations. Th e user defines the configuration files of the system geometry and plasma\, the probing bands and the dependence between the main variables of the pr oblem. A script creates all the necessary models and scripts that to run a ll the simulations in the HPCs.Using the developed framework\, we optimize d the DEMO PPR system using the official DEMO scenario from EUROFUSION. We started by defining 100 different positions around the tokamak and testin g two different configurations. In the first one\, the antennas were align ed perpendicularly with the wall. This configuration has advantages from t he point of view of the implementation of the antennas. However\, the resu lts shown that there are positions in the top of the machine and in the di vertor region that have a very poor measurement performance and in some ca ses the signal is totally lost. In the second configuration\, the antennas were aligned perpendicularly to the separatrix. In this case\, since the direction of the probing beam is approximately parallel to the density gra dient\, a better measurement performance is expected. The results confirme d this principle\, improving the results in several positions. At the dive rtor region\, some of the positions continued to have a poor measurement p erformance\, being necessary to sweep the probing angle to verify if there is an optimized configuration.One of the problems associated with the opt imization is that is necessary to extract the round trip group delay and c alculate the amplitude of the detected signal for many different configura tions. The analysis of the simulation results requires the manual adjustme nt of the data analysis parameters\, as the filter cutoff frequency or the signal delay. Using the principle that a slow varying group delay has a m inimum standard deviation if it is well filtered\, we developed an automat ized version of the the 1/Q detection\, designed as IQA method. With this technique\, it was possible sweeping the probing angle at all the position s in an acceptable time and select the optimized configuration. The maximu m average detected amplitude shown to be a good selection criteria for the optimized configuration. The results show that\, with the exception of so me positions in the divertor region\, there is an optimized configuration with low position error {<\;1 cm) and the power losses minimized.With th e optimized configuration\, the stability of the system was tested for pla sma displacements of 5 (reference case) and 15 cm (limit case). The result s show that\, in contrast to the results of the configuration with the ant ennas perpendicular to the separatrix\, the system is stable for 5 cm plas ma displacements in different directions (0\, 90\, 180 and 270 degrees). F or displacements of 15 cm\, the positions at the top of the machine can re ach errors in the order of the minimum error requirement (1 cm).The effect of turbulence in reflectometry measurements was studied in one gap of the equatorial region\, in the high field side. Due to the lack of informatio n on the turbulence properties of DEMO plasmas\, the fluctuations were def ined with an analytical model. A Kolmogorov-like spectrum was used to gene rate 400 random plasma samples for 16 different levels of amplitude (1-16% )\, compatible with the order of values observed in the experiments. Using the IQA algorithm\, the principal statistical parameters were calculated. The results show that for higher levels of turbulence ($>\;5%$)\, the m ean position error becomes negative due to the change of the effective cut off position. This effect occurs for all the frequencies\, leading to an a ccumulative error that can affect the position measurement in the order of accuracy requirements. In order to prove the reliability of the entire sy stem\, it is necessary to apply the same procedure to the other positions of the system\, which requires a huge amount of computation time on HPCs t o be done.The techniques and the algorithms developed in this work can be applied in other processes which involve the analysis of a high number of reflectometry simulations\, including studies with other reflectometry tec hniques. LAST-MODIFIED:20220118T093813Z LOCATION:Online URL:http://df.vps.tecnico.ulisboa.pt/pt/eventos/assessment-of-reflectometr y-diagnostics-for-demo/ X-ALT-DESC;FMTTYPE=text/html:

Abstract:
DEMO will be the first prototype of a fus ion power plant. Unlike experimental tokamaks\, only the necessary diagnos tics for machine protection and plasma control will be implemented. One of the fundamental measurements is the position and shape of the last closed magnetic surface\, typically measured with the magnetic diagnostics. One of the major issues of its implementation in DEMO is the large integration drifts that can occur during the operation due to the high levels of radi ation. This can lead to a wrong plasma position estimation\, putting the o peration at risk.

The prime candida te to complement or substitute the magnetic diagnostics in DEMO is the mic rowave reflectometry. By sweeping the frequency of the probing beam\, micr owave reflectometry is capable of measuring the electron density profile. As the density is directly linked to the magnetic flux surfaces these meas urements give access to the magnetic configuration\, providing its local r adial position. The O-mode propagation is independent from the magnetic fi eld\, being ideal for replacing the magnetic diagnostics.

The DEMO plasma position reflectometer (DEMO PPR) c onsists a system of multi­ reflectometers distributed poloidaly along the wall at different positions that will provide the separatrix reconstructi on. The optimization of such system requires the simulation of the measure ment process for different poloidal views\, emitting angles\, antenna asse mblies and plasma configurations. The final system must be optimized for t he operation scenario and be stable under the possible deviations to its e quilibrium that can occur during the discharges. This includes the plasma displacement\, turbulence or MHD activity. For now\, the DEMO PPR is in an early development stage and there are many questions that need to be inve stigated before reaching its final design.

In this work we study the process of optimization of PPR systems w ith a general approach\, taking into account the future changes in the geo metry and plasma scenario. The important variables of a general multiple r eflectometers system were identified and the techniques and the procedure to optimize it were developed. The simulation of such systems is in genera l a complex task that requires the definition of several different regions of interest and testing different antenna models and plasmas\, which is a very demanding task from the computational point of view and of necessary time to build the simulation scripts. For this reason\, we develop the st ructure of a high-level framework for multiple reflectometry simulations t hat is capable of automatizing all the simulation process of a multiple re flectometers system for the REFMUL* codes\, a family of full­ wave FDTD c odes that has been used for reflectometry simulations. The user defines th e configuration files of the system geometry and plasma\, the probing band s and the dependence between the main variables of the problem. A script c reates all the necessary models and scripts that to run all the simulation s in the HPCs.

Using the developed framework\, we optimized the DEMO PPR system using the official DEMO scena rio from EUROFUSION. We started by defining 100 different positions around the tokamak and testing two different configurations. In the first one\, the antennas were aligned perpendicularly with the wall. This configuratio n has advantages from the point of view of the implementation of the anten nas. However\, the results shown that there are positions in the top of th e machine and in the divertor region that have a very poor measurement per formance and in some cases the signal is totally lost. In the second confi guration\, the antennas were aligned perpendicularly to the separatrix. In this case\, since the direction of the probing beam is approximately para llel to the density gradient\, a better measurement performance is expecte d. The results confirmed this principle\, improving the results in several positions. At the divertor region\, some of the positions continued to ha ve a poor measurement performance\, being necessary to sweep the probing a ngle to verify if there is an optimized configuration.

One of the problems associated with the optimization i s that is necessary to extract the round trip group delay and calculate th e amplitude of the detected signal for many different configurations. The analysis of the simulation results requires the manual adjustment of the d ata analysis parameters\, as the filter cutoff frequency or the signal del ay. Using the principle that a slow varying group delay has a minimum stan dard deviation if it is well filtered\, we developed an automatized versio n of the the 1/Q detection\, designed as IQA method. With this technique\, it was possible sweeping the probing angle at all the positions in an acc eptable time and select the optimized configuration. The maximum average d etected amplitude shown to be a good selection criteria for the optimized configuration. The results show that\, with the exception of some position s in the divertor region\, there is an optimized configuration with low po sition error {<\;1 cm) and the power losses minimized.

With the optimized configuration\, the stability of the system was tested for plasma displacements of 5 (reference case ) and 15 cm (limit case). The results show that\, in contrast to the resul ts of the configuration with the antennas perpendicular to the separatrix\ , the system is stable for 5 cm plasma displacements in different d irections (0\, 90\, 180 and 270 degrees). For displacements of 15 cm\, the positions at the top of the machine can reach errors in the order of the minimum error requirement (1 cm).

T he effect of turbulence in reflectometry measurements was studied in one g ap of the equatorial region\, in the high field side. Due to the lack of i nformation on the turbulence properties of DEMO plasmas\, the fluctuations were defined with an analytical model. A Kolmogorov-like spectrum was use d to generate 400 random plasma samples for 16 different levels of amplitu de (1-16%)\, compatible with the order of values observed in the experimen ts. Using the IQA algorithm\, the principal statistical parameters were ca lculated. The results show that for higher levels of turbulence ($>\;5%$ )\, the mean position error becomes negative due to the change of the effe ctive cutoff position. This effect occurs for all the frequencies\, leadin g to an accumulative error that can affect the position measurement in the order of accuracy requirements. In order to prove the reliability of the entire system\, it is necessary to apply the same procedure to the other p ositions of the system\, which requires a huge amount of computation time on HPCs to be done.

The techniques and the algorithms developed in this work can be applied in other processe s which involve the analysis of a high number of reflectometry simulations \, including studies with other reflectometry techniques.

END:VEVENT END:VCALENDAR