Gas-puff induced cold pulse propagation in ADITYA-U tokamak

Gas-puff induced Cold Pulse Propagation in ADITYA-U tokamak Tanmay Macwan1,2, Harshita Raj3, Kaushlender Singh1,2, Suman Dolui1,2, Sharvil Patel1,4, Ankit Kumar1,2 , P Gautam1, J Ghosh1,2, R L Tanna1,4, K A Jadeja1,5, K M Patel1, Rohit Kumar1, Suman Aich1, V K Panchal1, Umesh Nagora1,2, M B Chowdhuri1, R Manchanda1, Nandini Yadava1,4, Ritu Dey1,2, Kiran Patel1,2, J. Raval1, S. K. Pathak1, 2, M. K. Gupta1, K. Tahiliani1, P. K. Chattopadhyay1,2, A Sen1,2, Y C Saxena1,2, R Pal7 and ADITYA-U Team1 1 Institute for Plasma Research, Gandhinagar 382 428 Homi Bhabha National Institute (HBNI), Mumbai 400 085 3 EPFL, Route Cantonale, 1015, Lausanne, Switzerland 3 Pandit Deendayal Petroleum University, Gandhinagar 382 007 4 Institute of Science, Nirma University, Ahmedabad 382 481, Gujarat, India 5 Department of Physics, Saurashtra University, Rajkot, Gujarat, India 6 Saha Institute for Nuclear Physics, Kolkata 700 064 2 Abstract Short bursts (1 – 2 ms) of gas, injecting ~ 1017 − 1018 molecules of hydrogen as well as of deuterium, lead to observations of phenomena of cold pulse propagation in hydrogen plasmas of the ADITYA-U tokamak. After every injection of the gas-pulse, a sharp increase in the chord-averaged density followed by an increase in the core temperature has been observed. Simultaneously the edge density and temperature decreases, suggesting a cold pulse propagation due to the gas-pulse application. It has been observed that the increase in the chord-averaged density and the subsequent increase in the core temperature depend on both the value of the chord-averaged plasma density at the instant of gasinjection and the amount of gas injected up to a threshold value. Increasing the amount of gas-puff leads to higher increments in core-density and the core-temperature follow suit, however, the rate of rise of density and temperature remain the same. The gas-puff leads to a fast decrease in the positive radial electric field and a simultaneous rapid increase in the loop-voltage suggesting a reduction in the ion-orbit loss and an increase in Ware-pinch. This may be explaining the sharp density rise, which remains mostly independent of toroidal magnetic field in the experiments. Application of another gas-puff before the effect of previous gas-pulse dies down, leads to increase in the overall discharge density and consequently the energy confinement time (τe). I. Introduction: The term ‘‘cold or heat pulse propagation’’ (CHP) in tokamaks describes the rapid temperature modification in the plasma core due to cooling or heating of the edge plasma. First reported in the TEXT tokamak [1-2], where significant temperature perturbations throughout the plasma are induced with a finite but small injection of carbon into the edge region, the phenomena has been observed in several tokamaks and stellarators [1-24]. The “cold-pulse propagation” experiments are more common than the “heat-pulse propagation” experiments as they are triggered by a much simpler gas or impurity injection in the edge region of the tokamak plasma. The impurity injection mainly by laser blow offs of material targets [3, 16] and gas injection by supersonic molecular beam injection [4-6] leads to a significant drop in the edge temperature. The drop in the edge temperature also occurs simultaneously with the increase in core temperature on a time scale much faster than any known diffusive time scale [7, 8]. The study of ‘‘cold or heat pulse propagation’’ is important from the point of view of understanding the heat and particle transport in tokamaks as the changes in core temperatures are observed in the time-scales much faster than the energy confinement time [7, 8]. And hence this CHP phenomenon is often described as a prime example of ‘non-local’ transport in a tokamak and helical devices. Such phenomena are a hindrance for the predictive capabilities of any transport model for physics validation of future devices including ITER. Based on the experimental observations in the TEXT tokamak [1-2], Gentle et al argued that the reduction in the core electron heat diffusivity and a simultaneous increase in the edge electron heat diffusivity could only be explained only through non-local mechanism [1]. An accidental release of impurities during Ion Cyclotron Resonance Heating (ICRH) experiment leads to observance of cold pulse phenomenon in DIII-D tokamak [10]. Baker et al [10] suggested that the increase in the edge resistance due to cooling results in the peaking of the density profile, which modifies the q profile, which through its relation with temperature, could explain the rapid increase in the core temperature. In the RTP tokamak, formation of large temperature gradient has been observed between 1 < 𝑞 < 2, which acted as a transiently enhanced transport barrier when hydrogen pellets are injected tangentially, cooling the periphery while increasing the core temperature [11]. It has been speculated that small variations in the q (safety-factor)-profile might have enhanced the transport barrier [11]. Similar experiments with similar results have been reported from the Tore Supra tokamak [12]. Cold and heat pulse propagation dynamics has been studied extensively in ASDEX-U tokamak with impurity injection through laser blow-off (LBO) and application of ECR heat pulse respectively [3]. The response of ECR heating of the plasma edge region suggested that the underlying mechanism is very much similar to the cold pulse, although no temperature inversion has been observed [3]. The cold pulse propagation has also been reported from the stellarators like Large Helical Device (LHD) using tracer encapsulated pellets [13-15] in low density plasmas. The observations of CHP in current-less plasmas of helical devices point towards the fact that changes in thermal diffusivity is cannot due to the changes in magnetic shear [7]. In HL-2A tokamak, the CHP has been studied with sequential firing of Supersonic Molecular Beam Injection (SMBI) and the off - axis ECRH switch-off experiments [4]. They have reported a suppression of the central turbulence, measured using the O mode reflectometers [4]. Experiments in the Alcator-C Mod tokamak [16] and the KSTAR tokamak [17] demonstrated that above a critical plasma density, for which the direction of the toroidal rotation reverses, the cold pulse propagation ceases to exist indication a relation between the CHP and toroidal rotation of the plasma. Various theories/models have been proposed for explaining the cold or heat pulse propagation dynamics, although no single theory accounts for all the features of the experimental observations. The non-local transport model (NLTM) invoking diffusivity having a nonlinear dependence on temperature gradient [7, 8, 18] is used to explain several experimental results including the results from cold pulse experiments on JET [19]. Apart from that models based on self-organized criticality (SOC) [20], the radial propagation of the turbulence with toroidal coupling [21] are also proposed to reproduce the prompt temperature changes due to cold-pulse propagation. Hahm et al. [20], proposed the role of mesoscale turbulence, with larger scale lengths compared to the microscale turbulence to describe the fast propagation of cold/heat pulses. Turbulence spreading via ballistic fronts is shown to be responsible for the cold pulse propagation by Hariri et al. [22]. Substantial increase in the core density fluctuations has been reported in the EAST tokamak with SMBI during the cold pulse propagation and hence the fast propagation has been attributed to the effect of turbulence spreading [5]. However, reduction in the density fluctuations in the range of 500 kHz to 2 MHz has been reported from SMBI induced cold pulse experiments the J-TEXT tokamak [6]. Contrary to the above theories, Rodrigues et al.[23], claimed to have resolved the long-standing enigma in plasma transport by modelling of cold-pulse experiments conducted on the Alcator C-Mod tokamak without resorting to the non-local model [23]. Using the trapped gyro-Landau fluid (TGLF) model in heat transport simulations, Rodrigues et al., has shown that the cold pulse dynamics in tokamak can be explained by a quasi-linear turbulent model [9, 23]. Indicating a decrease in the normalized gradient scale lengths of the dominant instabilities like the trapped electron modes (TEM) and ion temperature gradient (ITG) modes, which reduces the core turbulence, several features of experimental observations of the cold-pulse propagation in Alcator-C Mod has been reproduced in the simulation. They include the steady state profiles, the cold pulse rise-time and the existence of density cut-off etc. A successful attempt has been made by Rodrigues et al., to predict first the results of cold-pulse experiments in DIII-D tokamak [24], substantiating their claim of local-effects describing the cold-pulse propagation. For the temperature rise to happen, the Rodrigues model put emphasis on the fast rise of density [24] prior to the temperature rise due to gas or impurity injection, which is always observed in all the cold-pulse experiments in both the tokamaks and stellarators [1-24]. Interestingly, the rise time of the density after gas or impurity injection seems to be of same order in different tokamaks and stellarators. Although the temperature rise during the cold-pulse propagation is thoroughly investigated, the rapid increase in the core density prior to the temperature changes remains mostly unexplained until recently [25]. Angioni et al., [25] concluded that the density is the first to react during a cold-pulse injection in ASDEX upgrade (AUG) using laser impurity ablation and the core-temperature increase is a consequence of density flattening due to impurity injection. They have also set forth that any mechanism, even without any impurity injection, which produces a negative perturbation of the electron temperature at the edge and, concomitantly, a sudden flattening of the electron density profile is predicted to produce similar effects. The cold-pulse propagation in ADITYA-U (AU) Tokamak has been triggered by application of short gas-puff (GP) pulses of hydrogen and deuterium, injecting ~ 1017 − 1018 molecules in the edge region. The effect of short GP pulses is widely explored in various tokamaks like PBX-M [26], ADITYA [27], NSTX [28] and T-10 [29]. They are known to significantly influence the scrape-off-layer (SOL) and edge plasma characteristics like the density, temperature, floating potential and related turbulence in tokamak discharges. Following an injection of the gas-pulse in AU, the edge density and temperature decreases whereas the central chord-averaged density and core temperature increases in a time-scale less than the energy-confinement time, suggesting a cold pulse propagation due to the gas-pulse application. It has been observed that the change in central chord-averaged density always precedes the change in core temperature. These rapid changes in the central chord-averaged density and temperature depend on both the value of the chord-averaged plasma density at the instant of gas-injection and the amount of gas injected up to a threshold value. The amount of gas-puff has been varied to study its effect on the cold pulse propagation in AU. It has been observed that increasing the injection amplitude leads to higher increments in core-density and the increment in core temperature is found to be proportional to the density increment. Although increment in density and temperature increases with increasing the injected gas amount, however, the rate of rise of density and temperature remained almost constant. Furthermore, the variations in edge/core density and temperature with gas injection remained unchanged over a range of plasma currents and toroidal fields within the limits of parameter-range of discharges in AU. The rapid rise in the chord-averaged density is difficult to explain without invoking a pinch against the pre-existing density gradient. Measurements show that the gas injection reduces the positive radial electric field in the edge region in the edge region. Simultaneously, it also increases the loop-voltage significantly. It is quite wellknown that the radial electric field can affect the ion orbits in toroidal geometry [30-32]. The negative (inward) radial electric field always pushes the ions inward and the positive (outward) radial electric field drags them outward. Hence the observed reduction in positive (outward) radial electric with the gas-puff in our experiments, indicate reduction in ion-orbit loss. The increase in loop-voltage also leads to increased Ware pinch [33, 34]. Furthermore, gas-puff also leads to shorter mean free path of electrons than the connection length which in-turn increases the poloidal electric field, Eθ in the edge region. This leads to enhanced radial inward pinch 𝜐𝑟 ≈ 𝐸𝜃 ⁄𝐵𝜙 . A combination of all of the above mechanisms occurring simultaneously after the gas injection in the edge plasma of ADITYA-U may be responsible for the rapid increase in central chord-averaged density due to gas injection in our experiments. It has also been observed that after its sharp increase, the central chord-averaged density decays in the particle confinement time-scale and application of another gas-puff before the density decay leads to an increase in the overall discharge density and consequently the energy confinement time (τe). The paper is organized as follows: The experimental setup is described in section II and the experimental observations are presented in section III. The observations are discussed in section IV and the conclusions are drawn in section V. II. Experimental Setup The experiments of GP pulse induced cold-pulse propagation are performed in ADITYAU tokamak [35], which is a medium sized, air-core tokamak with R = 0.75 m and a = 0.25 m. The results presented in this paper are from Ohmically-heated limiter (toroidal belt limiter) plasmas in of AU. The experimental results are observed for a large range of plasma parameters: Ip ~100 – 200 kA, central chord-averaged density ne ~ 1 – 4 x 1019 m-3, BT ~ 1.0 - 1.44 T and chord-averaged central electron temperature ~ 250 – 500 eV. The base pressure is maintained at ~ 6 – 9 x 10-9 Torr and the hydrogen discharges are produced at pre-fill gas pressure of ~ 1 – 2 x 10-4 Torr. The diagnostic used for the analysis consists of a 100 GHz heterodyne system employing a microwave interferometer for real-time chord-averaged density measurement [36]. The chord-averaged temperature is measured using Soft X-Ray (SXR) tomography camera [37], consisting of two arrays of 16 channel AXUV photodiodes. The temperature is estimated using absorption foil technique, employing two Beryllium foils of thickness 10 µm and 25 µm. Additionally, one surface barrier detector monitors the integrated Soft XRay emission and it is collimated to view emission from core plasma of radius ~8.5 cm in every discharge. Spectroscopic measurements include the temporal evolution of Hα, C III, O II spectral line emissions and visible continuum emission [38]. The radiated power is measured using a single channel collimated camera mounted at one of the top ports, offering a wide view (0.16 sr) [39]. A fast visible imaging video camera is used to observe the plasma-wall interaction. The edge diagnostics consist of a set of Langmuir probes which provides the time evolution and spatial profile of floating potential, density and temperature in the edge region. The loop voltage, plasma current and plasma position are measured using standard magnetic sensors. Two sets of 16 Mirnov coils are placed poloidally at equal separation to measure the MHD oscillations at two toroidal locations. All the signals are acquired with a minimum sampling frequency of 100 kHz. A programmable gas feed system is used to inject multiple pulses of H2 and D2 gas puffs during the plasma discharge [40]. During the current flattop the appropriate prefixed values of gas pulse width and voltages are fed to a piezo-electric valve which is installed at one of the bottom ports of the vacuum vessel. The gas-puffing is controlled in such a way that each gas puff injects about 1 − 10 × 1017 particles, i.e., about 1 − 10 % of the central chord-averaged plasma density, ne. III. Experimental Observations a. Cold pulse characteristics in AU The effect of multiple short GP pulses of both hydrogen (H2, red vertical lines in fig. 1d) and deuterium (D2, magenta vertical lines in fig. 1d) on a typical plasma discharge (#33536) of ADITYA-U tokamak having central chord-averaged density below ~ 1.5 – 2 x Figure 1: The effect of multiple gas puff pulses on a typical ADITYA-U discharge (Shot # 33536); temporal evolution of plasma parameters (a) loop voltage, (b) plasma current, (c) H α line emission (d) H2 (red) and D2 (magenta) gas puffs along with chord averaged electron density (ne) and (e) central electron temperature. 1019 m-3 is shown in figure 1. Only the GP pulses during the plasma current flat-top are analysed for the present study. For this discharge, the frequency of GP pulses is kept at 200 Hz for H2 and between 50 60 Hz for D2. The pulse widths of the GPs for both the gases are fixed at 0.8-1.0 ms injecting ~8 − 10 × 1018 number of particles in the edge region having a density of ~1 − 2 × 1018 𝑚−3. With the gas injection, a rapid rise in the core temperature on a time scale shorter than the energy confinement time has been observed as shown in figure 1d, indicating cold-pulse propagation. Although, the cold-pulse is triggered by laser-blown impurity injection in most of the tokamaks, it has been triggered by fast gas puffs of pure deuterium too in ASDEX Upgrade tokamak. Apart from the rapid changes in the core/edge temperatures, a sharp increase, even faster than the core temperature variation, in the central chord averaged density has been observed as shown in fig. 1e. These modifications in temperature and density with the gas injection repeat themselves after every gas puff as long as the central chord-averaged density remained below ~ 2.5 x 1019 m-3 in the plasma current flat-top. Along with the above-mentioned modifications, the i Figure 2: The effect of multiple gas puff pulses on a typical ADITYA-U discharge (Shot # 33536); temporal evolution of plasma parameters (a) loop voltage (black, solid), plasma current (red, dash), (b) Hα/Dα line emission (c) CIII (black, solid), OII (red, dash) line intensity (d) Visible continuum (e) Radiated Power (f) chord averaged electron density (ne) and (g) edge density (h) central electron temperature (i) edge temperature application of a GP pulse brings in several characteristic changes in loop voltage, plasma current, edge spectral emissions, total radiated power, soft X-ray emission etc. These changes are highlighted in figure 2, where the zoomed time traces of loop voltage, plasma current, Hα/Dα, visible bremsstrahlung, O II and C III spectral emissions, total radiated power, plasma density in the edge and the central chord averaged density, core/edge temperature, and soft X-ray emission intensity during plasma current flat-top, comprising of three GP pulses, are plotted. The vertical dotted lines (blue) lines refer to the timeinstants of the D2 GP pulses. With the gas injection, a little (< 5%) fall in the plasma current has been observed. The loop voltage first increases by ~ 60 – 100 % for a few milliseconds (ms) after the gas injection and then drops below its original value as shown in figure 2a. Exactly same behaviour of loop-voltage after impurity injection has been speculated in TEXT tokamak [1, 2]. Similarly, the Hα/Dα, O+ and C2+ emission intensities rise initially and drop below their respective values before the gas injection as seen from figure 2b and 2c respectively. The initial increase and later drop in O+ and C2+ emission intensities reflect in the increase and drop in power radiated as well (figure 2e). Interestingly, the visible continuum hardly increases and starts decreasing almost instantly with the gas injection even as the Hα/Dα, O+ and C2+ emission intensities keep rising during that time (figure 2d). This indicates that the initial increase in O+ and C2+ emission intensities and in the power radiated is due to cooling of the edge and not due to an increase in Zeff. The edge cooling is further confirmed by the measured fall in the edge temperature after the gas injection as shown in figure 2i. The edge temperature is measured using a fixed single Langmuir probe located just inside the limiter radius at ρ = 0.99 (ρ = r/limiter radius). The fall in edge temperature is further substantiated by spectroscopic measurements using Helium line ratio method after injecting a trace amount of Helium. The core-temperature, shown in figure 2h, starts increasing as soon as the O+ and C2+ emission intensities and power radiated start decreasing, in a time-scale faster than the energy confinement time of ~ 3 - 6 ms. However, a comparatively faster rise in the central chord-averaged density has been observed, starting almost instantaneously with the gas injection (figure 2f). Contrary to the variation observed in other parameters, after reaching to its maximum value in ~ 1 – 1.5 ms of gas injection, the central chord-averaged density remains at its maximum value for much longer time and then starts decaying slowly. In addition to that a sharp fall in the edge density, measured at the limiter radius (r = 25 cm), after a minute initial increase with the gas-injection has been observed (figure 2g), a phenomenon not reported in the cold-pulse literature before. With the gas injection, the central temperature rises by ~ 10-15 %, whereas central chord-averaged density increases by ~ 15-20 % of their initial values before the gas injection. Importantly, a significant reduction in the central chord averaged density fluctuations as well as in the edge density fluctuations has been observed after the gas injection as shown in the frequency versus time plot of above-mentioned fluctuations (figure 3). The broadband (1 – 50 kHz, limited by data sampling) fluctuations are suppressed initially after the gas injection and reoccur thereafter when the mean edge density regains its prior-to-gas-puff value. Furthermore, the fluctuation suppression is more pronounced in the central chordaveraged density as compared to the edge density measured by Langmuir probe. a b Figure 3: The effect of gas puff on the density fluctuations: a) Density fluctuations in the central chord measured with microwave interferometry, b) Density fluctuations measured locally with Langmuir probe placed at 𝜌 = 1. The temperature inversion, i.e., the fast rise in the core temperature, concomitant with edge cooling, confirms the propagation of a ‘cold pulse’ in ADITYA-U tokamak with each gas puff. The cold pulse propagation in ADITYA-U is accompanied by a ‘density inversion’ too, i.e., the fall in the edge density concomitant with the rise in the central chord-averaged density. To characterise further the cold pulse propagation with the gas injection in AU and to understand the association between rise in central chord-averaged density and the rise in core-temperature, a parametric study of cold pulse behaviour has been carried out as described in the following section. b. Parametric variation of Cold pulse characteristics As mentioned earlier, a sharp increase in the central chord-averaged density has been observed, which remained constant at its enhanced value for 3 – 5 ms after the gas injection and then falls of slowly in a particle confinement time-scales of ~ 10 – 15 ms. Bringing another GP pulse before the density enhancement, due to a prior GP pulse, does not die down, leads to an overall increase in the central chord-averaged density of the discharge. However, the cold-pulse propagation, i.e., the temperature reversal ceases to exist when the central chord average density increases above 2.5 x 1019 m-3. The density threshold for cold pulse non-propagation has been reported in several tokamaks [5, 6, 16, 24]. The percentage increment, δne/ne0 in the central chord-averaged density, ne0, is observed to be dependent on the ne0 with the maximum increment has been observed at the ne0 ~ 0.5 x 1019 m-3 as shown in figure 4, where the percentage density increment δne/ne0 is plotted against the ne0. The density increment, δne/ne0 diminishes at ne0 ~ 2.5 x 1019 m-3 in our experiment. To our surprise, this density threshold value matches quite well with the threshold central chord-averaged density necessary for toroidal rotation reversal in the ADITYA-U tokamak [41]. Similar correlation between the cold-pulse propagation and the toroidal rotation reversal has been observed in ALCATOR-C Mod tokamak [16]. The above observations suggest that a density increment is necessary for the temperature inversion to happen. Furthermore, as mentioned earlier, with the application of the GP pulse the causality of the events show that the central chord-averaged density gets modified before the core temperature modification takes place. The above observations prompted to look for a relation between the incremental change in ne0 and incremental change in core temperature, Te0. A strong correlation has been observed between the δne/ne0 and the δTe/Te0 as shown in figure 5, the higher the density increment, the larger the temperature increment. Note the data point having high density increment but no temperature rise, is obtained for above threshold central chord-averaged density. This suggests that observed correlation between the δne/ne0 and the δTe/Te0 remain true below the density threshold. Furthermore, the density increment is found to be proportional to the amount of gas injected as shown in figure 6a where δne/ne0 is plotted with respect to amount of gas injected. However, as can be seen from figure 6b, the rate of rise of central chord-averaged density almost remained constant with amount of gas injected. The incremental change in density, δne/ne0, with fixed amount of gas injection does not vary with the toroidal magnetic field, BT in the range of 1 – 1.44 T and the rate of rise of density, δne/dt also remain almost constant with BT as shown in figure (7a, 7b). These Figure 4: Density increment for fixed gas pulse injection for different prior to gas puff density. The density decrement decreases with increasing density. Figure 5: Temperature increment for varying gas pulse injection plotted with corresponding density increment. Temperature increment is linear with density increment up to ~2.5 × 1019 𝑚−3 Figure 6: a) Variation of density increment with gas puff amount. Increasing the gas puff amount increases the density increment, b) Variation of density rise rate with the gas puff amount. The density rise rate remains constant with the amount of gas puff injected. Figure 7: a) Density decrement b) rate of density rise do not vary substantially with the toroidal field (BT). parameters, δne/ne0 and δne/dt also remained almost constant in discharges having plasma current in the range of ~ 100 – 200 kA for same amount of gas injection. Note here that the main advantage of triggering the cold-pulse by fuel gas (hydrogen or deuterium) injection is that the multiple events of cold-pulse propagation can be triggered in a single discharge. Moreover, it is comparatively easier to obtain a good statistics of cold pulse events by varying the pulse width of the GP pulse, thus varying the amount of gas and also varying the frequency of GP pulses to influence the time evolution of the events triggered by the GP pulses. Hence, each point in the figure showing the statistics of the cold-pulse events with different operational as well as GP pulse parameters is an average of several data points with the variations captured by the error bars. IV. Discussion: A common feature of the almost all cold pulse propagation experiments in several tokamaks [1-25] is a rapid rise of the central chord-averaged density immediately after the impurity/gas/SMB injection well in advance to the core temperature increase. Furthermore, the increment in chord-averaged electron density has always been long-lived as compared to the increment in the core-temperature. Even the successful modelling of Alcator C-Mod coldpulse experiments using local models like TGLF required a density pulse originating at the edge and propagating inwards as an input to their code for reproducing the increase in the core temperature [9,23]. The authors concluded that such a density pulse can lead to the observed temperature behaviour in Alcator C-Mod [16]. Based on a qualitative analysis of the experimental evidence from several tokamaks available from the literature, it can be safely concluded that a rapid increase in the central chord-averaged density is necessary for a cold pulse propagation, i.e., a temperature inversion to happen after an injection of gas/impurity/SMB. However, not much emphasis has been given to understand the mechanism behind this rapid inwardly propagating density peak associated with all the cold pulse experiments. Figure 7: Propagation velocity of the density peak measured with Langmuir probes placed at 1.0 and 1.01 ρ = 0.99, Figure 7 shows the temporal evolution of edge density during and after an application of a GP pulse at three different radial locations of ρ = 0.99, 1.0 and 1.01, measured by Langmuir probes. A density peak propagating inward after the gas injection is clearly seen from the figure. From the time difference between the peaks observed at different radial locations, the velocity of the density peak is estimated to be ~ 100 m/s. The peaking time of central chordaveraged density also gives the similar velocity estimates (70 – 100 m/s). Interestingly, the velocity of particles, for observing a rapid increase in the central chord-averaged density after gas/impurity/SMB injection, hover around ~ 100 m/s in several experiments reported on cold pulse propagation [1, 11, 24] except in Alcator C-Mod [16], again, based on a qualitative analysis of the experimental results from several tokamaks available in literature. Note here that the experiments in Alcator C-Mod are conducted with high toroidal magnetic field of 5 T whereas other experiments are in the range of 1 – 2.5 T. As the rapid rise in the chord-averaged density and subsequent core-temperature increase is obtained by injecting short pulses of fuel gas (hydrogen), the inward particle propagation due to impurity density gradient driven mode seems quite unlikely [25]. The observed changes in the positive (outward) radial electric field and in the loop-voltage suggest an enhanced pinch against the pre-existing density gradient may be responsible for the inward particle propagation. In our experiments, the loop-voltage increases by a factor of ~ 2 – 3 within a millisecond of the application of the gas-puff as shown in figure 2a. The increase in the loopvoltage by a factor of ~ 2 – 3 enhances the Ware pinch by 2 – 3 times. Simultaneously, the positive radial electric field in the edge region decreases by a factor of ~ 2 within a millisecond after the gas injection. The temporal evolution of radial electric field in the edge region in absence and presence of gas-puff pulse is plotted in figure 8. The radial electric field in the edge region has been estimated from the difference in plasma potential (𝑉𝑝 ~ 𝑉𝑓 + 3𝑘𝑇𝑒 ) at two radially separated Langmuir probes. It is quite well-known that the radial electric field can affect the ion orbits in toroidal geometry [30-32]. The negative (inward) radial electric field always pushes the ions inward and the positive (outward) radial electric field drags them outward. The inward ion mobility may sharply slow down the outward ion flow and hence the observed reduction in positive (outward) radial electric with the gas-puff in our experiments, indicate reduction in ion-orbit loss. The reduced ion-loss seems to be contributing to rapid increase in the chord-averaged density in our experiments. Furthermore, gas-puff also leads to shorter mean free path of electrons than the connection length in the extreme edge region. This leads to incomplete charge neutralization on a flux surface along the helical magnetic field lines and an increase in the poloidal electric field, Eθ in the edge region, enhancing the radial inward pinch 𝜐𝑟 ≈ 𝐸𝜃 ⁄𝐵𝜙 [42, 43]. We note that any of the above pinch models does not explain the observed inward density peak movement with velocities ~ 100 m/s quantitatively. However, a combination of all of the above pinch mechanisms occurring simultaneously after the gas injection in the edge plasma of ADITYAU seems to be responsible for the rapid increase in central chord-averaged density. Figure 8: Modulation of radial electric field (Er) with gas puff. Each gas puff reduces E r by ~ 2 kV/m. Therefore, based on the above observations and analysis, it is quite clear that with the application of GP pulse, pinch can cause the rapid rise in central chord averaged density in ADITYA-U tokamak, which in turn triggers the rise in core temperature. The density dependence of the confinement may be one of the possible reasons for the increase in core temperature. However, it has been observed in our experiments that application of GP pulse in the plasma having central chord-averaged density above its threshold value, the density rise still occurs whereas the temperature inversion in not observed. Hence, the confinement may not be cause of core-temperature rise. We observed that with the application of gas injection, the edge density decreases whereas the central chord-averaged density increases, indicating a steepening of the radial density profile. This may affect the current density profile, which may also be supported by the observation of increase in loop-voltage, and hence increase the temperature as argued by Baker et al. [10]. However, again as the density profile steepening and the increase in loop-voltage is also detected above the threshold density, where no core temperature changed in observed, the Baker model does not seem to explain the phenomena of cold-pulse propagation in ADITYA-U. Turbulence spreading [4] also seems inadequate in our case, as the density fluctuations are observed to be suppressed with the gas injection. The Rodrigues model [9] may be explaining the rapid-density-rise induced core-temperature increase as observed in our experiments; however, diagnostics limitations restrict us to provide an experimental verification of the same at present. It should also be mentioned that the increase in core temperature along with the increase in the central chord-averaged density enhances the overall confinement of the ADITYA-U plasma. V. Conclusion: Application of multiple short gas puff pulses of H2/D2 in plasma discharges of ADITYA-U tokamak leads to an enhanced core density and temperature and a decreased edge density and temperature on a time scale faster than the global τe. Along with that the GP pulse brings several characteristics changes in plasma discharge such as, a transient increase in loop voltage followed by a fall below its original level, an enhancement in impurity radiation and the radiated power but a decrease in visible continuum emission confirming the edge cooling. The edge density fluctuations as well as fluctuations in the central chord-averaged density are suppressed. With the gas injection the central chord-averaged density increased rapidly in ~ 1 – 2 ms time which is followed by the increase in core temperature. The increment in core temperature increases with the density increment, which increases with the amount of injected gas molecules. However, above a threshold central chord-averaged density ~ 2.5 × 1019 m-3, although the density increment occurs with gas injection, the core temperature increment is not observed. This indicates that the core temperature increase may not be due to the confinement improvement originating from the increase in the central chord-averaged density. Furthermore, profile modification due to increase and decrease of central chordaveraged density and edge density respectively, also seems not to be causing the core temperature increase as the loop-voltage modification has been observed above the threshold density without any increase in the core temperature. The parametric study shows that the rate of rise of central chord-averaged density does not vary with toroidal magnetic field and plasma current within the limited variations available in ADITYA-U. With the gas injection an inward movement of a density peak with a velocity ~ 100 m/s is observed in the edge region. The observations of a significant reduction in the positive (outward) radial electric field and an increase in the loop voltage with the gas-injection suggest that the increase in inward particle pinch may be causing the sharp density rise in the experiments. The rapid rise in central chord averaged density is followed by a very slow decay in particle confinement time scales and bringing another GP pulse before the density fully decays increase the density further. This cumulative density increases along with temperature increase results in an improve confinement after each gas injection. Acknowledgement and Author Contributions: This work has been carried out in context of the PhD work of the first author, registered in HBNI, Mumbai, who has conceptualized, designed and carried out the experiments, constructed the probes, analysed the results, prepared the original draft and edited the later versions and is the primary contributor of the work. 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