High Frequency Pulse Research Articles

Torque and Discomfort During Electrically Evoked Muscle Contractions in Healthy Young Adults: Influence of Stimulation Current and Pulse Frequency

ObjectiveTo investigate (1) how current and pulse frequency of electrical stimulation (ES) as well as contraction mode (isometric, concentric, and eccentric) influence torque output and discomfort and (2) how familiarization by repeated ES sessions influences ratings of perceived discomfort. DesignAn experimental study, 3 sessions. SettingA university laboratory. ParticipantsEight healthy participants (5 men, 3 women; mean age 25.2 years; N=8). InterventionsParticipants completed 3 trial days, each including 17 electrically evoked thigh muscle contractions. On each trial day, the first 6 contractions consisted of 2 isometric, 2 concentric, and 2 eccentric muscle contractions randomly ordered with a fixed stimulation current and pulse frequency (200 mA, 20 Hz), while the remaining 11 muscle contractions were all isometric with randomly ordered combinations of current (100-250 mA) and pulse frequency (20-100 Hz). Main Outcome MeasuresTorque and perceived discomfort were measured for each ES-evoked contraction. ResultsOverall, the findings revealed that a higher stimulation frequency was associated with an increased torque without increased discomfort, while higher currents were associated with increases of both torque and discomfort. Contraction type did not influence level of discomfort, despite eccentric contractions eliciting higher torque compared with concentric and isometric contractions (P<.001). Finally, a significant familiarization to ES (P<.001) was observed after just 1 of 3 identical stimulation sessions. ConclusionsThe outlined data suggest that to elicit high torque levels while minimizing levels of discomfort in young subjects, eccentric muscle contractions evoked with a low stimulation current, and a high pulse frequency are preferable. Furthermore, a single familiarization session significantly lowers rating of perceived discomfort during ES.

Spatiotemporal control of ERK pulse frequency coordinates fate decisions during mammary acinar morphogenesis.

The signaling events controlling proliferation, survival, and apoptosis during mammary epithelial acinar morphogenesis remain poorly characterized. By imaging single-cell ERK activity dynamics in MCF10A acini, we find that these fates depend on the average frequency of non-periodic ERK pulses. High pulse frequency is observed during initial acinus growth, correlating with rapid cell motility and proliferation. Subsequent decrease in motility correlates with lower ERK pulse frequency and quiescence. Later, during lumen formation, coordinated multicellular ERK waves emerge, correlating with high and low ERK pulse frequencies in outer surviving and inner dying cells, respectively. Optogenetic entrainment of ERK pulses causally connects high ERK pulse frequency with inner cell survival. Acini harboring the PIK3CA H1047R mutation display increased ERK pulse frequency and inner cell survival. Thus, fate decisions during acinar morphogenesis are coordinated by different spatiotemporal modalities of ERK pulse frequency.

Mitigating the Effects of Stray-Current Attack on Non-Machined Surfaces in Electrochemical Machining Through Gas-Shielding in C6H5K3O7 Solution

Electrochemical machining (ECM) is a promising method for processing the leading and trailing edges of the blades that are composed of nickel-based superalloys (Inconel 718), but often leads to unwanted stray corrosion on non-machined surfaces. This study investigates the use of a novel C6H5K3O7 solution to fabricate the leading and trailing edges of the blade that is free of stray corrosion. First, the basic electrochemical dissolution behavior of Inconel 718 in the C6H5K3O7 solution is analyzed in comparison with that in NaNO3 solution by means of a potentiodynamic polarization. The results showed that Inconel 718 in the C6H5K3O7 solution has a lower breakdown potential and the structure of its passive film was looser than that in the NaNO3 solution. Second, the current efficiency of Inconel 718 in the C6H5K3O7 solution exhibited non-linear dissolution behavior, and the material removal rate was much lower than that in the NaNO3 solution. In-situ observations indicated that a stable and continuous insulating gaseous layer was induced surrounding the anode surface through C6H5K3O7 solution to suppress the stray-current attack on the non-machined surfaces. Third, corresponding models and simulations of ECM process in the C6H5K3O7 solution were formulated to examine the effects of gaseous layer on stray corrosion. Fourth, the effects of the parameters of pulse processing on stray corrosion on the non-machined surfaces was analyzed. Experimental results showed that the self-induced gaseous layer could suppress stray machining by insulation effect, and a pulsed current with a short-pulse duration and a high-pulse frequencies was helpful for reducing the zones of stray corrosion. Precise structure of the leading/trailing edges of the twisted blade free of stray corrosion were successfully fabricated in the C6H5K3O7 solution. In comparison with that manufactured in the NaNO3 solution, the zones of stray corrosion was reduced by 8.4 times under the same processing parameters. The results here verify the feasibility of C6H5K3O7 as an electrolyte to reduce the stray-current attack on non-machined surfaces.

TaSe2 nanosheets for harmonic mode-locked fiber laser

• It is the first time to achieved a harmonic mode-locking fiber laser based on TaSe 2 . • 128th harmonic mode-locked pulses at 908.8 MHz repetition rate are stably generated. • The work provides a new solution for generating high repetition frequency pulses. Passively harmonic mode locking of an erbium-doped fiber laser based on TaSe 2 nanosheets saturable absorber (SA) is demonstrated. The TaSe 2 nanosheets are prepared by the liquid phase exfoliation technology and then deposited onto side surface of a D-shaped fiber to form a fiber-compatible TaSe 2 SA. The balanced twin-detector method is utilized to measure nonlinear absorption characterization of the fabricated SA, which gives saturable absorption, nonsaturable absorption, and saturation intensity as 5.49%, 24.6%, and 89.2 MW/cm 2 , respectively. Then, TaSe 2 SA is inserted into a passively mode-locked erbium-doped fiber laser, and the fiber laser is demonstrated with a fundamental frequency of 7.1 MHz. By adjusting the polarization controller and pump power, harmonic mode-locking operation is realized. As the pump power increases to 380 mw, 128th harmonic mode-locked pulses at 908.8 MHz repetition rate are stably generated. The results confirm that TaSe 2 is indeed a very desirable photonic material, which can be used in photonics, fiber optic communication, laser material processing and light modulators.

Bifurcations in the Firing of Neuronal Population Caused by a Small Difference in Pulse Parameters During Sustained Stimulations in Rat Hippocampus in Vivo.

The bifurcation of neuronal firing is one of important nonlinear phenomena in the nervous system and is characterized by a significant change in the rate or temporal pattern of neuronal firing on responding to a small disturbance from external inputs. Previous studies have reported firing bifurcations for individual neurons, not for a population of neurons. We hypothesized that the integrated firing of a neuronal population could also show a bifurcation behavior that should be important in certain situations such as deep brain stimulations. The hypothesis was verified by experiments of rat hippocampus in vivo. Stimulation sequences of paired-pulses with two different inter-pulse-intervals (IPIs) or with two different pulse intensities were applied on the alveus of hippocampal CA1 region in anaesthetized rats. The amplitude and area of antidromic population spike (APS) were used as indices to evaluate the differences in the responses of neuronal population to the different pulses in stimulations. During sustained paired-pulse stimulations with a high mean pulse frequency such as ∼130 Hz, a small difference of only a few percent in the two IPIs or in the two intensities was able to generate a sequence of evoked APSs with a substantial bifurcation in their amplitudes and areas. Small differences in the excitatory inputs can cause nonlinearly enlarged differences in the induced firing of neuronal populations. The novel dynamics and bifurcation of neuronal responses to electrical stimulations provide important clues for developing new paradigms to extend neural stimulations to treat more diseases.

A nearest level control technique for an asymmetric source configuration of multi-level inverter topology

In this paper, an asymmetric source configuration of Multilevel Inverter (MLI) topology has been proposed. It consists of eight unidirectional switches, two bidirectional switches and four isolated DC sources. By considering 1:5 and 1:4 source configurations, the inverter produces 25-level and 21-level outputs respectively with the same switching action. For producing negative voltage levels, there is no requirement of separate backend H-bridge and inherently produces both positive and negative voltage levels. The main advantage of this topology is that in every state, only four switches are in ON mode and else are in OFF state. It also gives less per unit Total Standing Voltage (TSV) and thereby cost requirement of semiconductor devices can become decreases. For generating gate pulses, the simple Nearest Level Control (NLC) has been used by considering the round function. This technique is basically a fundamental switching frequency technique thereby switching losses are greatly reduces as compared with high switching frequency Pulse Width Modulation (PWM) techniques and it is particularly suitable for large number of levels. With this control technique, there is no inrush current has been developed at the input of DC sources. Finally, with step change in Modulation Index (MI) values the proposed topology with two different source configurations have been validated through MATLAB/Simulink platform.

Effect of pulse mode and frequency on microstructure and properties of 2219 aluminum alloy by ultrahigh-frequency pulse Metal-Inert Gas Welding

Based on self-developed ultra high frequency pulse Metal-Inert Gas Welding (UFP-MIG) power source, the ultra-high frequency pulse (UFP) current was studied as an effective way for improving the microstructure and properties of 2219 aluminum alloy joints. The welding pores reduction performance, microstructures, microhardness, and tensile properties of conventional pulsed MIG(CP-MIG) welds and the UFP-MIG welds with different modes and frequencies were investigated. The results indicated that the UFP-MIG process could almost completely eliminate welding porosity in the joints of 2219 aluminum alloy. Compared with CP-MIG welding process, the grain size of the UFP-MIG welding zone was reduced and gradually decreased as the frequency of UFP increased. Similarly, the grain refinement effect also appeared in the heat affected zone (HAZ) near the welds, and the grain size could be reduced by up to 14.85%. Meanwhile, the morphology and uniformity of θ phases and α+θ eutectics along grain boundaries of UFP-MIG welds were also improved. The microhardness in the weld zone and partially melted zone (PMZ) of UFP-MIG weld joints with different modes and frequencies were significantly improved. The highest joint average microhardness occurred at peak pulse mode (PP) with 40 kHz(75.17HV), which was 27.9% higher than that of CP-MIG (58.74HV). The strength and ductility of welded joints were simultaneously improved by UFP-MIG weld process. Fracture analysis results showed that the large reduction of weld pores, the refinement of weld grains, and the lower segregation of Cu element in the eutectic were responsible for the joint tensile properties improvement.

Sensorless Control of Five-Phase Permanent-Magnet Synchronous Motor Based on Third-Harmonic Space

Different from three-phase motor, the decoupled model of five-phase motor contains two orthogonal subspaces. In this article, a sensorless hybrid control strategy of five-phase permanent-magnet synchronous motor based on third-harmonic space with no need of motor parameters for whole speed range operation is proposed. At low-speed range, a high frequency pulse voltage is injected into the third-harmonic space to obtain the rotor position information, and the torque ripple is reduced by controlling the third-harmonic space current. Meanwhile, linear extended state observers are applied to replace the low-pass and band-pass filters for signal processing. At middle- and high-speed ranges, the back electromotive force at third-harmonic space is obtained to estimate the rotor position by controlling the third-harmonic space current. Compared with conventional methods, motor parameters and additional circuits are not needed. Software phase-locked loops are also adopted to estimate the rotor position. During the switch-over area, the two position estimation methods are both enabled for the sensorless control. The experimental results demonstrate that the robustness of the hybrid sensorless control system is well-performed in the whole speed range.

Effects of inter-pulse coupling on nanosecond pulsed high frequency discharge ignition in a flowing mixture

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H2/air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow.

Vibration detection method for core loosening degree of high‐voltage direct current converter valve saturable reactor based on variational mode decomposition‐symmetrized dot pattern and image matching

Saturable reactor is the key equipment in converter valves; it is important to evaluate the core condition. In this paper, a recognition model of saturable reactor core loosening based on variational mode decomposition (VMD)-symmetrized dot pattern (SDP) feature fusion images and scale-invariant feature transform (SIFT) was proposed. Firstly, the saturable reactor vibration test under high frequency pulse excitation was carried out, and the vibration signals in different core loosening degrees were collected. Secondly, the VMD algorithm was used to decompose the broadband vibration signal into multiple narrowband functions, which were used to reflect the characteristics of each frequency band. Thirdly, each function and the original signal were transformed by SDP, the generated spiral arms were fused into a new image, and the typical templates in different loosening degrees were selected. Finally, the improved SIFT algorithm was used to obtain the matching results between test sets and templates. The results show that the recognition accuracy of the proposed model for core loosening is 97.5%, which is better than traditional algorithms. It can find the core loosening defect early and avoid further failures such as water pipe break and discharge, which can provide an important basis for saturable reactor monitoring.

Micro-arc oxidation for improving high-temperature oxidation resistance of additively manufacturing Ti2AlNb

The poor formability and high-temperature oxidation resistance of Ti2AlNb-based intermetallics (for example, Ti–22Al–25Nb at. %, nominal composition) are considered to be the main obstacles that need to be overcome in critical high-temperature applications. In this study, micro-arc oxidation (MAO) was applied to Ti2AlNb additively manufactured by selective laser melting (SLM). SLM offered advanced forming capabilities, while MAO enhanced the mechanical and chemical properties of the materials. The MAO coating comprised an amorphous layer, a nanocrystalline oxide layer, and a continuous TiO2 layer. The multi-layers structure of MAO coating significantly improved the high-temperature oxidation resistance by blocking inward oxygen diffusion. The pulse frequency had significant effects on the microstructure, hardness, and wear resistance of the MAO coatings. Low pulse frequency led to thick MAO coatings with coarse surface roughness, large crater-like pores, and serious surface cracks. High pulse frequency improved the hardness and wear resistance of the MAO coatings. A pulse frequency of 1000Hz deteriorated the wear resistance of MAO coating caused by the poor metallurgical bonding between the substrate and MAO coating. This study demonstrated the possibility of employing MAO to enhance the critical properties of the Ti2AlNb-based intermetallics fabricated by SLM.

Research on Application of High-Frequency Pulse Vibration in Ship Electric Propulsion System

Aiming at the problems existing in the method of using sensors to detect the rotor position of marine main propulsion motor at low speed, a sensorless rotor position identification method based on high-frequency pulse vibration signal is proposed. In this method, the low-pass filter is used to separate the high-frequency signal component, and the current component of d-axis to q-axis is demodulated. The control mode of PI action law is used to phase-locked processing of the demodulated signal, in which purpose is to obtain the rotor position signal. The design method is simulated and verified by MATLAB/Simulink software. The error between the estimated position and the actual position of the rotor in the simulation waveform is small, and the accuracy is high. The simulation results show that this method can achieve good results when applied to the ship electric propulsion system without position sensor, which provides a theoretical reference for the design of marine main propulsion motor control system.

Deep brain stimulation of the nucleus basalis of Meynert in an experimental rat model of dementia: Stimulation parameters and mechanisms

Background/objectiveDeep brain stimulation (DBS) of the nucleus basalis of Meynert (NBM) has gained interest as a potential therapy for treatment-resistant dementia. However, optimal stimulation parameters and mechanisms of action are yet to be elucidated. MethodsFirst, we assessed NBM DBS at different stimulation parameters in a scopolamine-induced rat model of dementia. Rats were tested in the object location task with the following conditions: (i) low and high frequency (20 Hz or 120 Hz), (ii) monophasic or biphasic pulse shape (iii) continuous or intermittent DBS (20s on, 40s off) and 100 μA amplitude. Thereafter, rats were stimulated with the most effective parameter followed by 5-bromo-2′-deoxyuridine (BrdU) administration and perfused 4 weeks later. We then evaluated the effects of NBM DBS on hippocampal neurogenesis, synaptic plasticity, and on cholinergic fibres in the perirhinal and cingulate cortex using immunohistochemistry. We also performed in-vivo microdialysis to assess circuit-wide effects of NBM DBS on hippocampal acetylcholine levels during on and off stimulation. ResultsBiphasic, low frequency and intermittent NBM DBS reversed the memory impairing effects of scopolamine when compared to sham rats. We found that acute stimulation promoted proliferation in the dentate gyrus, increased synaptic plasticity in the CA1 and CA3 subregion of the hippocampus, and increased length of cholinergic fibres in the cingulate gyrus. There was no difference regarding hippocampal acetylcholine levels between the groups. ConclusionThese findings suggest that the potential mechanism of action of the induced memory enhancement through NBM DBS might be due to selective neuroplastic and neurochemical changes.

Experimental study on multi-channel ignition of propane-air by transient repetitive nanosecond surface dielectric barrier discharge

Multi-channel ignition by nanosecond surface dielectric barrier discharge (nSDBD) offers potential for efficiency boost in spark ignition engines. In this paper, transient repetitive nSDBD ignition is proposed using a simple pulse forming circuit to make multi-channel ignition more feasible, considering that nSDBD can be enhanced by high frequency pulses and that convenient discharge energy control can be achieved by adjusting pulse number. Ignition experiments for propane/air mixtures were carried out in a constant volume combustion chamber under stoichiometric conditions at ambient initial temperature and pressure. Discharge channel transformation and flame propagation were captured by a high-speed camera. For 20 kHz negative pulses, discharge energy rose from 4 mJ to 8 mJ as pulse number increases from 1 to 12, and filaments grew from 2 mm to 12 mm. Partially overlapping filaments formed a tree-like discharge pattern with divergent roots and a concentrated middle part. Multiple flame kernels were generated in the middle region of the filaments and then greatly promoted by subsequent discharges, resulting in a petaloid flame. Ignition probability has been tested and the minimum ignition energy of repetitive nSDBD is obtained. The influence of discharge energy on flame propagation speed and combustion characteristics were analyzed.

Seedling responses to soil moisture amount versus pulse frequency in a successfully encroaching semi-arid shrub.

Rainfall timing, frequency, and quantity is rapidly changing in dryland regions, altering dryland plant communities. Understanding dryland plant responses to future rainfall scenarios is crucial for implementing proactive management strategies, particularly in light of land cover changes concurrent with climate change. One such change is woody plant encroachment, an increasing abundance of woody plants in areas formerly dominated by grasslands or savannas. Continued woody plant encroachment will depend, in part, on seedling capacity to establish and thrive under future climate conditions. Seedling performance is primarily impacted by soil moisture conditions governed by precipitation amount (quantity) and frequency. We hypothesized that (H1) seedling performance would be enhanced by both greater soil moisture and pulse frequency, such that seedlings with similar mean soil moisture would perform best under high pulse frequency. Alternatively, (H2) mean soil moisture would have greater influence than pulse frequency, such that a given pulse frequency would have little influence on seedling performance. The hypotheses were tested with Prosopis velutina, a shrubnative to the United States that has encroached throughout its range and is invasive in other continents. Seedlings were grown in a greenhouse under two soil moisture treatments, each which was maintained by two pulse frequency treatments. Contrary to H1, mean soil moisture had greater impact than pulse frequency on seedling growth, photosynthetic gas exchange, leaf chemistry, and biomass allocation. These results indicate that P. velutina seedlings may be more responsive to rainfall amount than frequency, at least within the conditions seedlings experienced in this experimental manipulation.

Modeling of a Flyback Converter Controlled by an IGBT for Generating a High-Frequency Pulse Voltage

With the rapid development of semiconductor devices, researchers have proposed different types of high-frequency pulse voltage generators based on power electronic converters to meet the requirements of microorganism inactivation, water pollution purification, and piezoelectric and dielectric electroactive polymer drives. The transient modeling of these high-voltage converters aids in understanding the electrical characteristics of the devices and optimizing their performances. This article analyzes the electrical behavior of a semiconductor switching device (i.e., an insulated gate bipolar transistor) used during the switching transient of the converter and constructs an electromagnetic transient model of the flyback converter that considers both the parasitic elements and the ferromagnetic losses of the transformer. The accuracy of the proposed model is then verified by comparing calculation results obtained using the model with experimental measurement results.

Numerical investigation on fluidization characteristics of binary particles in supercritical water fluidized bed reactor under pulsed conditions

The fluidization properties of binary particles in a supercritical water fluidized bed reactor (SCWFBR) are numerically investigated based on coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations. The accuracy and effectiveness of the CFD-DEM model is firstly validated via previously published experimental data. Then, the model is adopted to study the effect of flow velocities and sinusoidal pulse on the mixing of the binary particles. Numerical results show that it is effective to promote mixing of binary particles in SCWFBR by introducing external energy pulses, which cannot be achieved by simply increasing the flow velocities without pulses. Compared to high frequency pulses, low frequency pulses are more effective in promoting mixing. Within the parameters considered here, it is also found that the pulse amplitude is almost positively correlated with the effect on the promotion of particle mixing when it is smaller than the fixed flow velocity. In addition, an efficient and economical method based on Fast Fourier Transform (FFT) on the bed pressure drop is given to determine the optimal pulse frequency, which can prevent the waste of manpower and the physical resources from happening. This study can provide technical guidance to achieve optimal mixing of binary particles in SCWFBR and offer theoretical support for the design and optimization.

The Effect of Laser Ablation Pulse Width and Feed Speed on Necrosis and Surface Damage of Cortical Bone

Bone cutting is of importance in orthopaedic surgery but is also challenging due to its nature of brittleness—where severe mechanical and thermal damages can be introduced easily in conventional machining. Laser machining is a new technology that can allow for complex cut geometries whilst minimising surface defects i.e., smearing, which occur in mechanical methods. However, comparative studies on the influence of lasers with different pulse characteristics on necrotic damage and surface integrity have not been reported yet. This paper for the first time investigates the effects of laser type on the necrotic damage and surface integrity in fresh bovine cortical bone after ex-situ laser machining. Three lasers of different pulse widths, i.e., picosecond, nanosecond and continuous wave lasers have been investigated with different feed speeds tested to study the machining efficiency. The cutting temperature, and geometrical outputs have been measured to investigate the thermal influence on the cooling behaviour of the bone samples while high-speed imaging was used to compare the material removal mechanisms between a pulsed and continuous wave laser. Furthermore, an in-depth histological analysis of the subsurface has revealed that the nanosecond laser caused the largest necrotic depth, owing to the high pulse frequency limiting the dissipation of heat. It has also been observed that surface cracks positioned perpendicular to the trench direction were produced after machining by the picosecond laser, indicative of the photomechanical effect induced by plasma explosions. Therefore, the choice of laser type (i.e., in terms of its pulse width and frequency) needs to be critically considered for appropriate application during laser osteotomy with minimum damage and improved healing.

Optimization and sensitivity analysis of high frequency pulse tube cryocooler for aerospace application

The demand for cryogenic cooling systems has been on the rise ever since the development of long-range infrared imaging systems, especially high-frequency stirling-type cryocoolers which have a vast array of applications in the aerospace sector. While there have been many studies reporting analysis of the pulse tube cryocoolers, these lack an optimization approach with a further sensitivity analysis from computational fluid dynamic (CFD) data to obtain better cold end temperatures. Presently, a response surface method (RSM), has been utilized to characterize the influence of the affecting parameters and a global sensitivity analysis has been conducted to put forward the critical parameters that influence the performance of the device. It is observed that an increase in frequency for lower pressure ratios with higher mean pressure results in better cooling performance in smaller pulse tube diameter models. The sensitivity analysis indicates the operating parameters such as frequency and pressure ratios to be more influential than the geometric parameters with an impact of over 80%.

A thermal-coupled/gas-coupled hybrid high-frequency pulse tube cryocooler attaining the liquid-helium temperature

High-frequency pulse tube cryocooler (HPTC) has become an attractive method for space cooling in the liquid-helium temperature range, due to its advantages of potential high efficiency, no moving parts at the cold end, small size, and light weight. However, key problems such as large regenerator losses and insufficient phase-shifting ability challenge it to obtain the liquid-helium temperature. A thermal-coupled/gas-coupled hybrid refrigeration process of HPTC has been designed to obtain the liquid-helium temperature. One HPTC working in the liquid-nitrogen temperature range is used as a pre-cooling stage, and another two-stage gas-coupled structure is employed to obtain liquid-helium temperature. The simulation results show that the lowest no-load temperature is 3.6 K and a cooling power of 84 mW can be provided at 6 K with a total input PV power of 128 W and 10.9 W/77 K pre-cooling power when Helium-4 is chosen as the working gas. An experimental prototype was designed, built, and tested. The simulation and experiment results will be introduced in detail in this paper.