Characteristics of the pressure profile in the accelerator on the RF negative ion source at ASIPP

Mingshan Wu,Luxiang Xu, Yanbo Zhou,,2 Lizhen Liang,and Yelong Zheng

AFFILIATIONS 1 School of Fundamental Physics and Mathematical Sciences,Key Laboratory of Gravitational Wave Precision Measurement of Zhejiang Province,Taiji Laboratory for Gravitational Wave Universe,Hangzhou Institute for Advanced Study,University of Chinese Academy of Sciences,Hangzhou 310024,China

2University of Chinese Academy of Sciences,Beijing 100049,China

3Institute of Plasma Physics,Chinese Academy of Sciences,Hefei 230031,China

4State Key Laboratory of Precision Measuring Technology and Instruments,Tianjin University,Tianjin 300072,China

ABSTRACT Neutral beam injection(NBI)systems based on negative hydrogen ion sources—rather than the positive ion sources that have typically been used to date—will be used in the future magnetically confine nuclear fusion experiments to heat the plasma.The collisions between the fast negative ions and neutral background gas result in a significan number of high-energy positive ions being produced in the acceleration area,and for the high-power long-pulse operation of NBI systems,this acceleration of positive ions back to the ion source creates heat load and material sputtering on the source backplate.This difficult cannot be ignored,with the neutral gas density in the acceleration region having a significan impact on the flu density of the backstreaming positive ions.In the work reported here,the pressure gradient in the acceleration region was estimated using an ionization gauge and a straightforward 1D computation,and it was found that once gas traveled through the acceleration region,the pressure dropped by nearly one order of magnitude,with the largest pressure drop occurring at the plasma grid.The computation also revealed that the pressure drop in the grid gaps was substantially smaller than that in the grid apertures.

KEYWORDS Pressure gradient,Negative ion source,Aperture conductance,Neutral beam injection

I.INTRODUCTION

Auxiliary heating systems are needed to deliver more energy for lengthy pulses in large tokamak devices such as EAST(Experimental and Advanced Superconducting Tokamak)and ITER(International Thermonuclear Experimental Reactor).1,2For plasma heating and current driving,EAST has been equipped with neutral beam injection (NBI) systems based on positive ion sources.However,future massive fusion reactors such as ITER,3,4DEMO,5and CFETR6require high beam energy,and negative ions have a substantially better neutralization efficienc than do positive ions.

As higher-energy and longer-pulse neutral beams are generated by ion sources,the backplate of the arc chamber and acceleration grids will collect more backstreaming particles,and the production processes and behavior of the backstreaming electrons and positive ions have already been well studied.7–9According to the estimation by Huet al.,the particle flu of the firs category accounts for 13.88% of the total beam current and the power flu of the firs category accounts for～6.5% of the total beam power at EAST-NBI.10Those studies were all based on estimating the pressure gradient in the acceleration region of an RF negative ion source,and herein we describe an ionization-gauge experiment and a specifi computation for estimating the acceleration pressure.

FIG.1.Image of HUNTER with an RF source.

As shown in Fig.1,HUNTER (Hefei Utility Negative ions Test Equipment with RF source)has been designed and developed at the Institute of Plasma Physics,Chinese Academy of Sciences(known as ASIPP,from Anhui Institute of Plasma Physics) to explore RF negative ion sources for NBI applications.11,12The negative ion accelerator is a single-stage acceleration system comprising three grids,i.e.,a plasma grid (PG),an extraction grid (EG),and a ground grid (GG),each made of copper and divided into four identical segments.In the firs research phase,only the middle two groups of segments were used for extraction to match the single RF driver.Each segment consisted of 6 × 5 apertures,the aperture spacing was 22 × 20 mm2,and the aperture geometry of each grid is shown in Fig.2.13,14The firs gap (PG-EG) and the second gap(EG-GG)were used for beam extraction and acceleration,respectively,and to suppress the leakage of secondary electrons and steer the negative ion beam,a molybdenum grid [electron suppression grid (ESG)] was attached to the EG at the same electric potential.

II.A STRAIGHTFORWARD 1D CALCULATION

A straightforward 1D calculation was used to estimate the pressure gradient in the RF negative ion source accelerator.During the test,only ten channels on the PG participated in the extraction and the rest were closed,and the ESG was not installed in the negative ion source.Considering the symmetry,one of the beam extraction channels was selected to calculate the channel diameter.

A.Simplification of accelerator structure

Figure 3 shows a simplifie structural model of the negative ion source accelerator.The surface of the PG facing the arc chamber is taken as thex-axis zero point,and the beam extraction direction is the positive direction of thex-axis.This allows the diameter function of a single beam channel to be described as

FIG.2.Left:3D diagram of negative ion source.Right:detailed geometries of grid apertures(units:mm).

However,region I is no longer an independent channel because of the overlapping holes,and the 30 holes are merged into a channel that for calculation convenience is reduced to a cuboid channel with a cross-section of 0.11×0.12 m2and a length of 0.0028 m.The electrode plates are integrated,so regions IV and VII are cuboid channels with a cross-section of 0.253×0.266 m2and lengths of 0.006 and 0.018 m,respectively.Therefore,the modifie function can be described as

B.Specific calculation

FIG.3.Simplified structure of negative ion source accelerator.

Generally,the nature of the flui flo in the channel is assessed using the productof(the average pressure of the gas in the channel)andd(the diameter of the channel):15it is viscous flo ford＞0.67 Pa·m,molecular flo ford＜0.02 Pa·m,and transition flo for 0.02 Pa·m ＜pd＜0.67 Pa·m.According to the current operating parameters of HUNTER,the operating gas is hydrogen,the arc chamber pressure is 0.5 Pa,the gas flo rate is 0.2 Pa·m3/s,and the vacuum pumping rate is 2 m3/s.Considering that there is no discharge plasma in the chamber and the temperature is room temperature,for convenience we assume a constant temperature of 300 K for the gas and the components.Because the gas pressure does not change much in region I,we assume an average gas pressure of 0.5 Pa.Therefore,the product ofanddcan be expressed asd≈pmd=0.5×0.11=0.55 Pa·m ∈(0.02,0.67) Pa·m,and so the flo in region I is transition flow

The molecular mean free path is

whereT(K)is the temperature,p(Pa)is the gas pressure,andσ(m)is the molecular diameter.TakingT=300 K,p≈pm=0.5 Pa,andσ=2.75×10-10m,we obtain≈0.024 650 6 m.

The transition flo conductance of the cuboid channel can be expressed as

whereUn(m3/s)is the viscous flo conductivity of the cuboid channel,Kjis the shape coefficien of the cuboid channel for transition flo (see Fig.4),15Uf(m3/s) is the molecular flo conductance of the cuboid channel,anda(m)is the length of the short side of the cross section.TakingKj≈1.04 anda=0.11 m,the conductance of region I can be simplifie as

The viscous flo conductance of the cuboid channel can be expressed as

whereaandb(m)are the side lengths of the cross section,η(Pa·s)is the viscosity coefficientL(m) is the channel length,(Pa) is the average gas pressure in the channel,andψis the shape coeffi cient of the cuboid channel for viscous flow which is related to the side lengths of the cross section as shown in Fig.5;15here,we takeψ=0.38.

Considering the influenc of temperature on the viscosity coefficient the latter can be expressed as

whereM(kg/mol)is the gas molar mass andC(K)is the Sutherland constant,which for hydrogen isC=76 K.Therefore,the viscosity coefficien isη≈7.031 61×10-6Pa s at 300 K.

Substituting the various parameters into Eq.(3),we obtain

TABLE I.Shape coefficient Kj of cuboid channel for molecular flow.

FIG.4.Shape coefficient Kj of cuboid channel for transition flow.

FIG.5.Shape coefficient ψ of cuboid channel for viscous flow.

and the molecular flo conductance of the cuboid channel can be expressed as

whereKjis the shape coefficien of the cuboid channel for molecular flo (see Table I);15here,we takeKj=1.11.Substituting the various parameters into Eq.(6),we obtain

Substituting Eqs.(5)and(7)into Eq.(2),we obtain

and from the definitio of conductance,we have

whereQ(Pa·m3/s) is the intake air volume.Considering that each grid is composed of two segments,there are two beam extraction channels in region I.UsingQ=0.2 Pa·m3/s andP0=p(x)∣x=0=0.5 Pa and substituting Eq.(8)into Eq.(9),we obtain

By applying the same method,the gas pressure in the acceleration region can be expressed as

and substituting the various parameters into Eq.(13),we obtain the pressure at special positions as follows:

With a vacuum pumping rate of 2 m3/s and a gas flo rate of 0.2 Pa·m3/s,the ultimate gas pressure is 0.1 Pa,and the calculation shows thatP8is larger than 0.1 Pa,which meets the actual operating conditions.From the above calculation,the gas pressure profil in the accelerator is obtained as shown in Fig.6,and we fin that the main pressure-decreasing trend is in the aperture rather than the gird gaps,with the largest pressure drop occurring at the extraction grid.

As shown in Fig.7,Schieskoet al.calculated the pressure profil in a negative ion source accelerator to estimate the flu density of the backstreaming positive ions.9The difference was that their calculation was for a source pressure of 0.4 Pa,which is close to that required in ITER;their profil for MANITU (Multi Ampere Negative Ion Test Unit)and our profil for HUNTER were both calculated from the source pressure and the conductance of the grid apertures.Schieskoet al.also tested using their computation for the ITER NBI extraction system,and the pressure profil that was produced agreed to within 20%with a thorough Monte Carlo model.9Krylov and Hemsworth used simple models that could be verifie by comparison with“classical”cases in which the Knudsen formula for gas conductance is applicable,and then they used a Monte Carlo model for the complex geometry of the beam source,demonstrating agreement to within 5%.16Straightforward 1D computation is used frequently in the industry,and the acceptable difference between the calculated results and the real values offers help in designing ion source accelerators.

III.EXPERIMENT RESULTS

The experiments were performed at HUNTER,and with no plasma present,they were operated solely with gas input.A slim pipe containing a vacuum gauge was extended to the accelerator’s assigned location through the reserved flang on the back plate of the arc chamber;this pipe had a length and inner diameter of～1 and 0.01 m,respectively,and because it had very low gas flo conductance,a sufficientl long gas ingress time was required to ensure that the pressure recorded by the vacuum gauge accurately reflecte the local environment.Two campaigns were conducted,one with single turbopump operation and the other with double turbopump operation,and for each campaign the air intake was set at 50–130 Pa·L/s for different source pressures.

FIG.6.Gas pressure profile in negative ion source accelerator.

FIG.7.Pressure and electrostatic potential in accelerator of NBI testbed MANITU.

The pressure profil with single turbopump operation is shown in Fig.8.In this case,the pressure decreased exponentially at each gas flo rate;it decreased rapidly in the firs grid aperture,and the largest pressure drop occurred at the plasma grid,which differs from the computational prediction.On the whole in the experiment,the gas pressure decreased by an order of magnitude after passing through the accelerator and did not decrease after the GG.By contrast,the calculation predicts significantl lower pressure drops in the grid gaps than those in the grid apertures.

The pressure profil with double turbopump operation is shown in Fig.9,and these pressure characteristics are similar to those with single turbopump operation.Increasing the pumping speed led to less-accurate results,and the pressure after the second gap differed less with variation in the gas flo rate.Under each gas flo rate,the pressure at the arc chamber was less than that at the beginning of the PG,and the cause of this phenomenon is still being analyzed.

FIG.8.Pressure profile with single turbopump operation.

FIG.9.Pressure profile with double turbopump operation.

FIG.10.Pressure profiles with gas flow rate of 90 Pa·L/s.

In Fig.10,comparing the pressures at the same gas flo rate and different pumping speeds shows that the pressure difference in the firs gap was less than that in the second gap.At every position in the accelerator,the pressure was twice as high under double turbopump operation as it was under single turbopump operation.

IV.CONCLUSIONS

In this study,a straightforward 1D calculation was used to predict the pressure gradient in a high-power negative ion source accelerator,and after two gaps,it was found that the pressure was reduced by almost half.However,in experiments,the pressure was observed to decrease more rapidly with near-exponential decrease,and it was logical for the pressure gradient to be influence signifi cantly by the pumping speed.In the calculation,the main pressure decreases were in the apertures rather than in the gird gaps as observed experimentally;the pressure decreased linearly because the pumping speed was neglected,whereas the experimental results indicated that it decreased exponentially.Future work will include simulations involving the gas flo status.

ACKNOWLEDGMENTS

This work was supported by the National Key Research and Development Program of China(Grant No.2021YFC2202700).

AUTHOR DECLARATIONS

Conflict of Interest

The authors have no conflict to disclose.

DATA AVAILABILITY

The data that support the finding of this study are available from the corresponding author upon reasonable request.