Small Heat Load Research Articles

Experimental investigation on the operating characteristics of a dual compensation chamber loop heat pipe subjected to acceleration field

High power and high local heat flux electronic devices employed in aircraft and spacecraft sustain the high acceleration condition in maneuvers and take-off stage. Loop heat pipe (LHP) are promising in dissipating high heat load to meet the increasing cooling needs. This article presents an experimental investigation on the operating characteristics of a dual compensation chamber loop heat pipe (DCCLHP) under elevated acceleration conditions. A centrifuge with a 2 m-long arm is used to provide the acceleration up to 7 g with four different acceleration directions. The heat load applied on the evaporator ranges from 80 W to 300 W. The typical performances in terrestrial were obtained and the influence of the different acceleration direction and magnitude on the operating characteristics was analyzed. Experimental results show that the change of the vapor–liquid distributions induced by the acceleration force results in some specific operating characteristics of the DCCLHP. The operating temperature becomes lower as the effect of the acceleration force improves the liquid returning. The operation of the DCCLHP demonstrates the sensitive behavior to the acceleration direction at small heat load and insensitive behavior at large heat load. It was also found that the acceleration magnitude can alter the operating mode. A number of unstable phenomena are observed under both terrestrial gravity and elevated acceleration conditions.

He flow rate measurements on the engineering model for the Astro-H Soft X-ray Spectrometer dewar

The sixth X-ray Japanese astronomy satellite, namely Astro-H, will be launched in 2015. The Soft X-ray Spectrometer onboard the Astro-H is a 6×6 X-ray microcalorimeter array and provides us with both a high energy resolution of <7eV at 0.5–10keV and a 3′×3′ modest imaging capability for the first time. To cool the detector down to the operation temperature of 50mK, five cryocoolers, a 30-L superfluid helium cryostat, and a 3-stage adiabatic demagnetization refrigerator are utilized. A very small heat load up to ∼0.9mW on the helium tank is allowable to realize the helium lifetime of >3years, which consequently requires that the vapor flow rate out of the helium tank should be very small <42μg/s. We adopted a porous plug phase separator in combination with a film flow suppression system composed of an orifice, a heat exchanger and knife edge devices to retain the liquid helium under zero gravity and safely vent the small amount of the helium vapor. We measured helium mass flow rates from the helium tank equipped in the engineering model dewar. We tilted the dewar at an angle of 75° so that one side of the porous plug located at the top of the helium tank attaches the liquid helium and the porous plug separates the liquid and vapor helium by thermomechanical effect. Helium mass flow rates were measured at helium tank temperatures of 1.3, 1.5 and 1.9K. We confirmed that resultant mass flow rates are in good agreement within the systematic error or low compared to component test results and achieve all the requirements. The film flow suppression also worked normally. Therefore, we concluded that the SXS helium vent system satisfactorily performs integrated into the dewar.

Effect of non-condensable gas on startup of a loop thermosyphon

Non-condensable gas (NCG) generated inside two-phase heat transfer devices can adversely affect the thermal performance and limit the lifetime of such devices. In this work, experimental investigation of the effect of NCG on the startup of an ammonia–stainless steel loop thermosyphon was conducted. In the experiment, nitrogen was injected into the loop thermosyphon as NCG. The effect of NCG inventory on the startup behavior was investigated by adjusting the injected amount of nitrogen. The experimental results reveal that NCG prolongs the startup time and increases the startup liquid superheat and temperature overshoot; the more NCG exists in the loop thermosyphon, the higher the liquid superheat and temperature overshoot. When NCG is present in the system, boiling usually occurs in the evaporator before startup, but it does not mean the system will start up instantly, which differs from the conditions without NCG. Under all the conditions, increasing the heat load can effectively shorten the startup time but leads to a large temperature overshoot; forced convection cooling of the condenser has almost no effect on shortening the startup time especially for large NCG inventory situations, but it can effectively limit the temperature overshoot. For large NCG inventory situations, the loop thermosyphon can start up at a small heat load (5 W) or even without a heat load when the condenser is cooled by forced convection of ethanol. No failed startups occurred during any of the tests.

Analysis of Competitiveness and Support Instruments for Heat and Electricity Production from Wood Biomass in Latvia

Analysis of Competitiveness and Support Instruments for Heat and Electricity Production from Wood Biomass in Latvia Utilisation of renewable energy sources is one of the key factors in a search for efficient ways of reducing the emissions of greenhouse gases and improving the energy supply security. So far, the district heating supply in Latvia has been based on natural gas, with the wood fuel playing a minor role; the same is true for decentralised combined heat-power (CHP) production. The paper describes a method for evaluation of the economic feasibility of heat and electricity production from wood biomass under the competition between different fuel types and taking into account the electricity market. For the simulation, a cost estimation model is applied. The results demonstrate that wood biomass can successfully be utilised for competitive heat production by boiler houses, while for electricity production by CHP utilities it cannot compete on the market (even despite the low prices on wood biomass fuel) unless particular financial support instruments are applied. The authors evaluate the necessary support level and the impact of two main support instruments - the investment subsidies and the feed-in tariff - on the economic viability of wood-fuelled CHP plants, and show that the feed-in tariff could be considered as an instrument strongly affecting the competitiveness of such type CHP. Regarding the feed-in tariff determination, a compromise should be found between the economy-dictated requirement to develop CHP projects concerning capacities above 5 MWel - on the one hand, and the relatively small heat loads in many Latvian towns - on the other.

HTS Current Leads Prepared by the TFA-MOD Processed YBCO Tapes

HTS current leads have been prepared by the TFA-MOD processed YBCO tapes 5 mm in width and around 120 μm in overall thickness. The YBCO superconducting thin layer with 1.5 μm is formed on the oxide buffer layers deposited on Hastelloy substrate tape. The critical current of the YBCO tapes is around 100 A at 77 K in self-field. A current lead unit is composed of a GFRP board and five YBCO tapes soldered to Cu cap at both ends. The transport current of the ten current lead units ranges from 536 A to 408 A at 77 K in liquid nitrogen. The YBCO current lead is assembled from the ten current lead units in parallel. The transport current of 4,000 A at 77 K was stably carried with no voltage generation on the YBCO tapes of the assembled current lead for ten minutes. Furthermore, the trans port current of 5,000 A was successfully applied with voltage of around 200 μV on YBCO tapes. The heat leakage of the assembled YBCO current lead with 150 mm in length between 77 K and 4.2 K is estimated to be 470 mW. Therefore, the heat load of it at transport current of 4000 A corresponds to 0.12 W/kA, which is one order of magnitude smaller than that of conventional Cu current lead. The small heat load results from high current performance and low thermal conductivity in the present YBCO current lead.

Simulation of tungsten armour cracking due to small ELMs in ITER

Simulations of tungsten armour cracking under small ELM-like plasma heat load, which does not cause surface melting, have been performed using the PEGASUS-3D code. A dedicated series of experiments have been performed in the QSPA-Kh50 facility for measurements of the unknown tungsten thermophysical properties and for verification of the PEGASUS-3D simulation results. The simulations revealed that a cellular crack network with average mesh size Λ ∼ 0.5 mm formed after first ELMs and the pattern does not change further. With increasing number of repetitive ELMs loads, the average crack width Δ( n) has a maximum value Δ m . The ratio of Δ m / Λ is equal to the tungsten thermal expansion at the maximum surface temperature. Δ( n) tends to this value exponentially. The number of ELMs n m needed for Δ stabilization depends on the ELMs energy density and time duration, n m ∼ 300 for the simulated ELMs of 0.45 MJ/m 2 and 0.25 ms duration. The PEGASUS-3D code is prepared for simulations of tungsten armour damage under action of ELMs of various energy deposition and time duration. These parameters of ELMs depend on ITER regimes of operation and on how successful will be the efforts on ELMs mitigation.

Excess heat from kraft pulp mills: Trade-offs between internal and external use in the case of Sweden—Part 2: Results for future energy market scenarios

In this paper the trade-off between internal and external use of excess heat from a kraft pulp mill is investigated for four different future energy market scenarios. The work follows the methodology described in Svensson et al. [2008. Excess heat from kraft pulp mills: trade-offs between internal and external use in the case of Sweden—Part 1: methodology. Energy Policy, submitted for publication], where a systematic approach is proposed for investigating the potential for profitable excess heat cooperation. The trade-off is analyzed by economic optimization of an energy system model consisting of a pulp mill and an energy company (ECO). In the model, investments can be made, which increase the system's energy efficiency by utilization of the mill's excess heat, as well as investments that increase the electricity production. The results show that the trade-off depends on energy market prices, the district heating demand and the type of existing heat production. From an economic point of view, external use of the excess heat is preferred for all investigated energy market scenarios if the mill is studied together with an ECO with a small heat load. For the cases with medium or large district heating loads, the optimal use of excess heat varies with the energy market price scenarios. However, from a CO 2 emissions perspective, external use is preferred, giving the largest reduction of global emissions in most cases.

3He/ 4He dilution refrigerator with high cooling capacity and direct pulse tube pre-cooling

In the article, a 3He/ 4He dilution refrigerator (DR) is described which is pre-cooled by a commercial two-stage pulse tube refrigerator (PTR); cryo-liquids are not necessary with this type of milli-kelvin refrigerator. The simple design of the condensation stage of this so-called dry DR is novel and explained in detail. In most dry DRs the circulating 3He gas is cooled by a two-stage PTR to a temperature of about 4 K. In the next cooling step, the 3He flow is cooled and partially liquefied in a Joule–Thomson circuit, before it is run to the dilution refrigeration unit. The counterflow heat exchanger of the Joule–Thomson circuit is cooled by the cold 3He gas pumped from the still of the DR. In the DR described here, the heat exchanger of the Joule–Thomson stage was omitted entirely; in the present design, the 3He gas is cooled by the PTR in three different heat exchangers, with the first one mounted on the first stage of the PTR, the second one on the regenerator of the second stage, and the third one on the cold end of the second stage. The heat load caused by the 3He flow is mostly absorbed by the first two heat exchangers. Thus the 3He flow presents only a small heat load to the second stage of the PTR, which therefore operates close to its base temperature of 2.5 K at all times. A pre-cooling temperature of 2.5 K of the 3He flow is sufficiently low to run a DR without further pre-cooling. The simplified condensation system allows for a shorter, compacter and more economical design of the DR. Additionally, the pumping speed of the turbo pump is no longer obstructed by the counterflow heat exchanger of the Joule Thomson stage as in our earlier DR design.

Experimental investigation of startup behaviors of a dual compensation chamber loop heat pipe with insufficient fluid inventory

To address the problem of the relative orientation limit between the evaporator and the compensation chambers (CCs), a dual-compensation chamber loop heat pipe (DCCLHP) was developed, and its startup characteristics with small heat loads and insufficient fluid inventory were experimentally investigated in this work. Based on the experimental results, the conclusions below can be drawn: (1) The DCCLHP can start up reliably at small heat loads, but sometimes the temperature overshoot is rather high. (2) The startup performance of the DCCLHP is different when its evaporator and CCs are at different attitudes due to different liquid/vapor composition in the evaporator and heat leak from the evaporator to the CCs. (3) In general, evaporation in the evaporator core is inevitable, which results in a large heat leak from the evaporator to the CCs and generates large temperature overshoot during the startup process. (4) Because the superheat of liquid to initiate nucleate boiling reduces as its temperature rises, nucleate boiling can also occur in the vapor grooves although the temperature of the evaporator rises at the same rate as that of the CCs. (5) The startup performance of LHPs mainly depends on the heat load and thermal resistance between the evaporator and CCs. When the startup heat load is big enough, the DCCLHP has no start-up difficulty.

Identifying the MHD signature and power deposition characteristics associated with type-II ELMs in ASDEX Upgrade

The properties of ASDEX Upgrade's type-II ELM regime, which combines good confinement with benign temporal variation of the power exhaust, make it an attractive candidate for ITER baseline scenario operation. As yet it is not understood what the physics origin of this regime is. In previous type-II ELM studies (Stober J et al 2001 Nucl. Fusion 41 1123, Stober J et al 2005 Nucl. Fusion 45 1213) a whole range of fluctuations have been reported. In this paper the properties of these fluctuations are inspected in more detail, and their relevance for the regime investigated. For the high frequency (∼150–220 kHz) magnetic fluctuation activity, from its absence in certain type-II ELM scenarios and behaviour during regime transitions it is concluded that it is not an essential ingredient of type-II ELM regimes on ASDEX Upgrade. A stronger contender to explain the type-II ELM regime appears to be given by small repetitive electromagnetic bursts, and low frequency (∼5–25 kHz) magnetic precursor activity associated with these bursts. It is demonstrated that these bursts give rise to small but frequent heat loads of the right order to influence ELM cycles. These are best detected in the vicinity of the inner strike point in the lower divertor through infrared thermography and Langmuir probes. Their properties further suggest that the bursts are neither small type-I ELMs nor type-III ELMs. However, it remains to be shown that the losses associated with these bursts are high enough to replace the type-I ELM losses. Other enhanced density fluctuation activity at low frequency (10–30 kHz) is detected by reflectometry near the plasma boundary. The information available about this mode is more limited, but so far a causal implication of this mode in the type-II ELM regime cannot be excluded. Overall it can be concluded that, of the three potential dissipation mechanisms, the low frequency electromagnetic burst behaviour seems to be the most promising candidate to account for type-II ELMs and their energy losses.

ITER transient consequences for material damage: modelling versus experiments

Carbon-fibre composite (CFC) and tungsten macrobrush armours are foreseen as PFC for the ITER divertor. In ITER the main mechanisms of metallic armour damage remain surface melting and melt motion erosion. In the case of CFC armour, due to rather different heat conductivities of CFC fibres a noticeable erosion of the PAN bundles may occur at rather small heat loads. Experiments carried out in the plasma gun facilities QSPA-T for the ITER like edge localized mode (ELM) heat load also demonstrated significant erosion of the frontal and lateral brush edges. Numerical simulations of the CFC and tungsten (W) macrobrush target damage accounting for the heat loads at the face and lateral brush edges were carried out for QSPA-T conditions using the three-dimensional (3D) code PHEMOBRID. The modelling results of CFC damage are in a good qualitative and quantitative agreement with the experiments. Estimation of the droplet splashing caused by the Kelvin–Helmholtz (KH) instability was performed.

拡散法で作製したBi2212酸化物超伝導体電流リードの組織と通電特性の評価

Bi2212 HTS conical tubular conductors have been prepared by diffusion process for current lead. The Bi2212 HTS layers are synthesized through the diffusion reaction between a Sr-Ca-Cu oxide substrate and a Bi-Cu oxide coating with Ag addition. The HTS diffusion layers about 150 μm in thickness are formed around both outside and inside of the conical tubes 34/29 mm in outside/inside diameter at the larger end, 24/19 mm in outside/inside diameter at the smaller end, and 100 mm in length. The Ag added to the coating enhances the diffusion reaction, and precipitates on the surface of the specimen decreasing its contact resistance. The transport current properties were evaluated by measuring system using two cryocoolers at Railway Technical Research Institute. The critical temperature of transport current of 1000 A at 0.5 T for the specimen is 57.4 K, which corresponds to the current density of 33 A/mm2 for the Bi2212 layer. The transport current decreases with increasing temperature at the warm end of the conical specimen, and is about 800 A at 60.2 K and 600 A at 62.5 K. The joint voltage of 20 μV at cold end was generated at 50 K and 0.5 T with transport current of 1000 A, which corresponds to the small heat load of 20 mW resulted from the Joule heating. Present Bi2212 conical tubes seem to be promising as current leads with small heat loads for superconducting magnets.

F234 固体粒子添加による自励振動ヒートパイプの性能向上(ヒートパイプ)

The performance of pulsating heat pipe (PHP) was investigated by using 12 turn copper tubes with water, ethanol and R141b as the working fluids. In the case of small heat load, R141b is the best performance because of its higher vapor presser and lower viscosity. On the other hand, in large heat load, water is the best because of large thermal conductivity and large specific heat. In order to improve performance of R141b in large heat load region, two sizes of diamond particles, that is, large one (8-20μm) and small one (2-4μm), were added. It was found that the larger particle is more efficient than the smaller one, and an optimum concentration exists. The heat transport performance of PHP using R141b with diamond particle is superior to that with water over the whole range of heat load for all orientations.

Bi2212 HTS bulk tubes prepared by the diffusion process for current lead application

Bi2212 HTS tubular bulk conductors have been prepared by the diffusion process for current lead application. The Bi2212 HTS layers are synthesized through the diffusion reaction between a Sr–Ca–Cu oxide substrate and a Bi–Cu oxide coating with Ag addition. Two different shaped substrates were prepared. The former substrates are formed into the cylindrical tubes 30 mm in outside diameter and 200 mm in length by cold isostatic pressing, and then sintered. The latter ones are the conical tubes with larger end of 30–40 mm and smaller end of 20 mm in diameter. The transport current of the specimen depends on the cross-sectional area of Bi2212 layers, and varies from 4000 to 8000 A at 4.2 K, which corresponds to the current density of about 2–3 × 10 4 A/cm 2 for the Bi2212 layer. The transport current decreases with increasing temperature at the warm end of the specimen, which is, for example, about 3200 A at 40 K or 2000 A at 50 K. The total heat loads composed of heat leakage conducted through the tube between the warm joint and the cold joint at 4.2 K, and of Joule heating at the cold joint are estimated. Present Pb-free Bi2212 HTS bulk tubes seem to be promising for large transport current leads with small heat loads for superconducting magnets.

Bi2212 HTS Conical Tubes Prepared by the Diffusion Process for Current Lead Application

Bi2212 HTS conical tubular conductors have been prepared by the diffusion process for current lead application. The Bi2212 HTS layers are synthesized through the diffusion reaction between a Sr-Ca-Cu oxide substrate and a Bi-Cu oxide coating with Ag addition. The HTS diffusion layers about 150 micrometer in thickness are formed around both outside and inside of the conical tubes 37/29 mm in outside/inside diameter at the larger end, 27/19 mm in outside/inside diameter at the smaller end, and 200 mm in length. The Ag added to the coating enhances the diffusion reaction, and precipitates on the surface of the specimen decreasing its contact resistance. The transport current of the specimen exceeds 7,000 A at 4.2 K and self-field, which corresponds to the current density of 32,000 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for the Bi2212 layer. The transport current decreases with increasing temperature at the warm end of the conical specimen, and is about 4,600 A at 30 K, 3,200 A at 40 K or 2,000 A at 50 K. The total heat loads composed of heat leakage conducted through the conical tube and Joule heating at the joint are estimated to be about 320 mW at 3,000 A between the warm end of 42 K and the cold end of 4.2 K, and is 400 mW at 2,000 A between 50 K and 4.2 K, respectively. Present Pb-free Bi2212 conical tubes seem to be promising as large transport current leads with small heat loads for superconducting magnets

Development of a YBa 2Cu 3O 7 filament leads unit

In order to reduce thermal inputs through many measuring copper leads, we intended to fabricate a measuring leads unit using very fine YBa 2Cu 3O 7 filaments. The long YBa 2Cu 3O 7 filaments were prepared by a textile fiber spinning method for the oxide precursor. The 60 filaments with a size of 100 μm in diameter and 60 mm in length were attached onto three fiber-reinforced polymers, and were covered with epoxy resin. Both ends of the YBa 2Cu 3O 7 filament were soldered with pin connectors through copper leads with 0.1 mm in diameter. The YBa 2Cu 3O 7 measuring leads unit of 60 terminals was cooled by a pulse tube cryocooler. The newly developed YBa 2Cu 3O 7 measuring leads unit demonstrates the extremely small heat loads of 1/10 in comparison with conventional copper measuring leads.

Superconducting superferric dipole magnet with cold iron core for the VLHC

Magnetic system of the stage I Very Large Hadron Collider (VLHC) is based on 2 Tesla superconducting magnets with combined functions. These magnets have a room temperature iron yoke with two 20 mm air gaps. Magnetic field in both horizontally separated air gaps is generated by a single, 100 kA superconducting transmission line. An alternative design with a cold iron yoke, horizontally or vertically separated air gaps is under investigation. The cold iron option with horizontally separated air gaps reduces the amount of iron, which is one of the major cost drivers for the 233-km magnet system of future accelerator. The vertical beam separation decreases the superconductor volume, heat load from the synchrotron radiation and eliminates fringe field from the return bus. Nevertheless, the horizontal beam separation provides lowest volume of the iron yoke and, therefore, smaller heat load on the cryogenic system during cooling down. All these options are discussed and compared in the paper. Superconducting correction system combined with the magnet that allows increasing the maximum field is also discussed. Preliminary cost analysis is performed for all these options.

Extended JT-60U plasma regimes for high integrated performance

With the main aim of providing the physics basis for ITER and steady state tokamak reactors, JT-60U has been optimizing operational concepts and extending discharge regimes towards simultaneous sustainment of high confinement, high βN, high bootstrap fraction, full non-inductive current drive, and efficient heat and particle exhaust utilizing a variety of heating, current drive, torque input and particle control capabilities. In the two advanced operation regimes (reversed magnetic shear (RS mode) and weak magnetic shear (high βp mode)), ELMy H mode discharges characterized by both internal and edge transport barriers and by high bootstrap current fraction fBS have been sustained near the steady state current profile solutions under full non-inductive current drive with appropriate driven current profiles (high βp: HHy2 ≈ 1.4 and βN ≈ 2.5 with negative ion based neutral beams (N-NBs); RS: HHy2 ≈ 2.2 and βN ≈ 2with fBS ≈ 80%). Multiple pellet injection has extended the density region with high confinement. These operational modes have been extended to the reactor relevant regime with small values of collisionality and normalized gyroradius, and Te∼Ti. In the RS regime, QDTeq = 0.5 has been sustained for 0.8 s. Stability has been improved in the high βp mode regime by suppression of the neoclassical tearing mode with local ECCD and in the RS mode regime by wall stabilization. The internal transport barrier structure has been controlled by toroidal rotation profile modification, and transport studies have revealed a semiglobal dynamics of the internal transport barrier structure. Simultaneous pumping at the inner and the outer divertor leg has enhanced He exhaust by ∼40%. Argon puff experiments have improved confinement at high density with detached divertor owing to high pedestal temperature Tiped. In H modes, the core confinement degraded with decreasing Tiped, suggesting stiff core profiles. An operational region of grassy ELMs with small divertor heat load has been established at high triangularity, high q95 and high βp. A record value of the neutral beam current drive efficiency of 1.55 × 1019A m-2 W-1 has been demonstrated using N-NB. Abrupt large amplitude events causing a drop in neutron emission have been discovered with frequency inside the TAE gap. Disruption studies have clarified that runaway current is terminated by MHD fluctuations when the surface q decreases to 2-3.

F217 自励振動ヒートパイプの熱伝達特性(オーガナイズドセッション22 : 熱輸送デバイス)

This paper describes heat transfer characteristics of an oscillating heat pipe with check valves, particularly on the operation range. It is found in the experiments that the thermal performance is excellent and the feature of operating limit is drastically different from that of conventional wicked heat pipes. That is, the definite limit of heat load is not observed in the experiments but there exist the operating limit temperature. Over the temperature, oscillating heat pipes can not work even at small heat load, however, so far as the operating temperature is below the limit temperature, very large heat transport capability can be obtained.

The feasibility of a tokamak reactor with a very high aspect ratio plasma

Feasibility of a very high aspect ratio tokamak reactor is investigated. Here, the aspect ratio A is the ratio of plasma major radius R P to the minor radius a P. Over a wide range of A (=2.7∼8), the plasma size, current drive power P CD, divertor heat load W DIV are calculated for the steady-state plasma when the fusion power P FUS (=3 GW) and the maximum toroidal field B TMAX (=18 T) are fixed. Here, the inboard shield thickness δ TF=1.4 m, the normalized beta β N=3, plasma elongation κ=1.4, plasma temperature T=15 keV and other parameters are similar to those of ITER. It is clarified that the acceptable current-drive power ( P CD=81 MW), large fusion gain ( Q=37) and small divertor heat load ( W DIV∼10 MW m −2) are achieved in a very high- A plasma with R P/ a P=12/1.5 m. Radial build is optimized with keeping the field ripple ratio constant. When the number of TF-coil (=20) and ripple level (=1%) are fixed, the inboard shield thickness δ TF is increased by 0.8 m and outboard shield thickness by 1.4 m as A increases from 2.7 to 8. In this case, the internal diameter of TF-coil increases by about 0.4 m since the plasma minor radius a P is decreased by 0.9 m. In the future power plant using SiC structural material, the very high- A reactor is greatly superior to the low- A reactor since the large shielding region is available without degrading plasma performance. These results are dependent on the plasma elongation κ and normalized beta β N. When κ is increased from 1.4 to 1.7, the required B TMAX is reduced from 18 to 15 T. If an advanced operation ( β N∼5) with reversed shear plasma is available, the required B TMAX is reduced from 18 to 14.5 T.