Overview of MAST Neutral Beam System Performance D. A. Homfray, D. Ciric, V. Dunkley, R. King, D. Payne, M. R. Simmonds, B. Stevens, P. Stevenson, C. Tame, S. E. V. Warder, A. M. Whitehead and D. Young EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, OX14 3DB, UK. [email protected] Abstract — This paper gives an overview of the MAST Neutral Beam System detailing both control and diagnostic capabilities. The operational experience of commissioning the beam line and the performance and characterization of the beams is described. Additional protection systems have been introduced and their design and operation are covered. Finally, future implementations to provide greater and more efficient performance are proposed, that will allow greater flexibility and configurability in the use of NBI in the MAST experimental program. Keywords — MAST; neutral beam; plasma heating; diagnostic; interlocks

I.

INTRODUCTION

As part of the MAST (Mega Amp Spherical Tokamak) [1] Long Pulse Neutral Beam Upgrade, two JET style Positive Ion Neutral Injectors (PINIs) [2] replaced the original Oak Ridge National Laboratory (ORNL) duopigatron injectors [3]. The MAST PINIs are specified as 75kV, 65A tetrodes, each providing 2.5MW of neutral beam power to the MAST plasma for durations up to 5 seconds. This represents a ten fold increase in injected neutral beam energy compared to the ORNL injectors and has required significant modifications to the Beamline and associated power [4] and control systems. New actively cooled calorimeters and residual ion dumps (RID) containing hypervapotrons with embedded thermocouples and water cooling channels have also been installed. The new PINIs operate at high electrical stress (32% above dc Kilpatrick limit in grid 2 – grid 3 gap) and as part of the commissioning process, they must be “conditioned” by slowly increasing the beam energy and current. Conditioning of one PINI to 65kV was completed in 2007 and it has since been in routine operation on MAST, injecting 1.9MW of neutral beam power. Conditioning of the second PINI to 65kV was completed by March 2009 and both injectors presently provide around 3.6MW of neutral beam power. II.

SYSTEM OVERVIEW

The NBI beamline is maintained at a background pressure of around 1x10-7 mbar using a combination of turbomolecular and titanium sublimation pumps [5] providing both high vacuum and large pumping speed (figure.1). The PINIs use filament driven discharges usually operating in deuterium to produce 65A of extracted beam current from

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Figure. 1 – Schematic of MAST Neutral Beam System 262 apertures. This current is accelerated by a 4 grid (tetrode) assembly, which is also used to focus the individual beamlets via off-set aperture steering and by inclination of the two grid halves. This combination of electrical and geometric steering gives a vertical and horizontal focal length of 6m and 14m respectively. The accelerated ions are passed through a gas target called the neutraliser (figure 2) giving a neutralisation efficiency of ~60%. The neutral beam can be stopped on an actively cooled instrumented calorimeter (ASYNC mode) or fired in to the MAST tokamak (SYNC mode) through a valve (DN400) and a duct that mechanically isolates the injector from the MAST vessel. The remaining ions are deflected using a bending magnet on to a Residual Ion Dump (RID). During a neutral beam pulse, gas is introduced via the source and neutraliser needle valves and the surplus gas is removed at high pumping speed by the getter pumps in order to minimise re-ionisation of the neutral beam. Figure 2 shows a simple schematic of a neutral beam line with associated power supplies, controls, data acquisition (shown in light grey) and the main pulse based interlock system (shown in bold lettering) that is a part of a larger protection system used to safeguard personnel and plant. The power supplies to the injector have their own separate safety system, interlocking if certain key parameters go out of prescribed ranges such as PINI grid overcurrents or the loss of voltage on grid 3. The pulse based, Fast Beam Interlock System (FBIS) protects against potential damage to components due to large power loadings from the injector. This provides in-pulse protection from the following scenarios:

Pulse based data is collected using two main systems, fast electrical waveform data providing information on the PINI performance and slow thermal data using the embedded thermocouples in the calorimeter and RID providing information on the PINI optics and power density profiles. The shine-through beam dump thermocouples will be routinely added to these data in SYNC toward the end of 2009. III.

OPERATIONAL EXPERIENCE

The new PINIs operate at high electrical stress and as part of the commissioning process, must be “conditioned” by slowly increasing the beam voltage, perveance (relationship between beam voltage and current) and gas flow, during repetitive pulsing (typical pulse duration ~ 1s), until HV breakdowns are rare. This process can take 2-3 months of operations in order to slowly increase the beam voltage. Figure. 2 – Subsystem schematic of MAST Neutral Beam System •

High beamline pressure (ASYNC/SYNC) – High tank pressure would cause re-ionisation of the neutral beam depositing power potentially on to non-cooled components.

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Bending magnet error (ASYNC/SYNC) – The bend magnet current is proportional to the energy of the residual ions. If the current is not set correctly there is a potential for the residual ions to miss the RID, depositing energy on to non-cooled components.

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Absence of neutraliser gas (ASYNC/SYNC) – A failure to open the neutraliser gas valve could result in no neutralisation allowing the total beam power to be deflected on to the RID causing possible damage.

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High duct pressure (SYNC) – High duct pressure would cause re-ionisation of the beam which would be deflected by the tokamak’s magnetic field, into the duct wall. This uncooled component could then be subjected to high power density, causing failure.

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Low plasma density (SYNC) – If the plasma density is low the beam is insufficiently attenuated by the plasma (shine-through) and deposits high power density on to the carbon tiled beam dump inside the MAST vessel.

The MAST neutral beam system is controlled from a PC situated in the MAST control room. This SCADA (Supervisory Control And Data Acquisition) PC accesses both the Programmable Logic Controllers (that operate and monitor plant) and the TimerScope control PC that runs the fast timer and electrical waveform data collection software (described as Control PC and PLC in figure 2). The control room PC displays both pulse based and system specific data. During the setup of a neutral beam pulse the set points input to the control PC are sent to the power supplies and TimerScope PC and after checking plant health, a trigger from either MAST or locally acts on the fast timers, creating a neutral beam for the duration requested.

The first PINI was installed and commissioning started in March 2006 and progress initially was reasonable, although breakdown free pulses were rare and it was impossible to operate the PINI at high perveance. It became clear that the main reason for limited conditioning progress was the absence of the arc current feedback control and arc voltage step control. Although modification to the power supplies’ controls was therefore required, it was decided to start with SYNC operation at low power (<1 MW) in support of the MAST experimental programme during August 2006. Beam commissioning resumed in December 2006 following the necessary modifications to the arc and filament power supply and progress is charted in figure 3. Conditioning to 65kV was completed in 2007 and the PINI has been in routine operation on MAST injecting 1.9MW of neutral beam power. Figure 3 shows the beam voltage, beam on time as a percentage of requested pulse length and injected neutral beam power during beamline commissioning and operation in the first half of 2007. It can be seen that during the initial commissioning phase, the percentage beam on time has a large spread and this improves as the injector becomes more conditioned. On the figure it can also be seen that SYNC operations started in earnest after pulse 8500 and this data set shows a significant spread of the percentage beam on time.

Figure. 3 – South PINI commissioning/operation Dec 2006 – May 2007 open blue symbols – ASYNC, solid red symbols - SYNC.

This is due to the MAST plasma terminating before the scheduled end of the neutral beam, generating an early termination through FBIS and therefore skewing the data. Some breakdowns cause serious de-conditioning events that can require a reduction in the operating voltage and may require reduction in either perveance and/or gas flow until once again HV breakdowns are rare. After recovery, gas flow/perveance can be moved back to their optimal values and conditioning by increasing beam voltage can continue. Such an event can be seen in figure 3 around pulse number 7950 With the experience from the first PINI both in terms of conditioning and power supply modifications, conditioning of the second PINI started in October 2008 operation at 65kV was established by March 2009. IV.

BEAM PERFORMANCE AND CHARACTERISATION

During the PINI conditioning phase it is possible to characterize the performance both in terms of electrical power and beam optics in a number of ways, this includes the relationship between the arc current and the filament set point (filament model), the arc current and beam current (arc model) and the efficiency of the neutralisation. These relationships allow the user to input the desired voltage and perveance and the control software calculates all the input parameters needed by the power supplies required to give both reliability (low number of breakdowns) and high power. A series of arc only pulses (no HV applied to the acceleration grids) was completed to determine the filament model (figure 4) confirming the relationship between the filament set point (an arbitrary value that is defined by the power supply) and the measured arc current. The neutral beam current (and therefore power) is controlled by the arc current and therefore an adequate arc calibration model is important to provide known neutral beam powers. A series of HV pulses was completed in ASYNC in which the set arc current was changed and the beam current measured at grid 1. The theoretical model of the PINI predicted that for a beam current of 65A an arc power supply providing around 1500A would be required and these tests provided

Figure. 4 – Arc current dependence on filament set point – Filament model

Figure. 5 – Beam current dependence on arc current – Arc model verification (figure 5). Parameters such as neutraliser gas flow rate and neutraliser gas introduction timing are important for efficient neutralisation of the accelerated ion beam and neutralisation efficiency experiments provide the quantitative analysis. The experiment compares the beam profiles (figure 6) at the calorimeter for the composite and neutral deuterium beam (bending magnet off and on). The difference between the two profiles is the un-neutralised power lost on the residual ion dump. Figure 7 shows the agreement between predicted values (line target density of 5×1016m-2) and measured values (source and neutraliser gas flows of 14 and 17 mbar×l/s). V.

FUTURE ENHANCEMENTS

For each ASYNC pulse a 2D power density profile of the beam is recorded using the instrumented calorimeter, providing data on the focusing and homogeneity of the beam. The beam divergence depends upon the ratio of the beam perveance, which is a measure of the relationship between the beam current and voltage and the geometric perveance of the accelerator. Optimal perveance occurs at minimum divergence as shown in figure 8. The figure demonstrates that changing

Figure. 6 – Comparison between thermocouple temperature rise of composite and neutral beam profiles

Figure. 7 – Measured and predicted neutralisation efficiency of the MAST PINI

Figure. 9 – Beam power variation during perveance scan at fixed voltage

the perveance by 20% has only small scale effects (~10%) on the beam footprint allowing the possibility to modulate beam power (figure 9) during the pulse by changing the arc current in real time, without the need of changing beam voltage. This facility to alter arc current is currently under design and should allow the MAST experimental campaign the ability to quickly ramp power up and down throughout the plasma experiment.

The two injectors are presently delivering a combined power of 3.6 MW. The upgrade to the injectors required significant modifications to many systems including power supplies, interlocks, control, instrumentation and actively cooled components. A conditioning phase gradually increasing the operational voltages of the PINIs up to 65kV has been completed for both injectors.

Work to upgrade the MAST neutral beam control and instrumentation is also in the design phase and is due for implementation during the last quarter of 2009 and the first quarter of 2010. This will upgrade the user interface software, SCADA software, thermocouple data acquisition hardware, data storage and manipulation software and PLC network which control plant and provide for future enhancements or upgrades.

The short term aim is to take both injectors to their specified 75kV (5MW of NB power) and complete a number of enhancements which will provide both reliable and more configurable beams (power waveforms, notching of the beams for diagnostics etc.). ACKNOWLEDGMENT This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by the European Communities under the contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

REFERENCES [1] [2]

[3]

[4]

Figure. 8 – Perveance scan curve of deuterium beam at 51kV (Vertical beam profile) VI.

CONCLUSION

The MAST Long Pulse Neutral Beam Upgrade will provide reliable long pulse, high power injection for the MAST experimental program.

[5]

M. Cox, MAST Team, The mega amp spherical tokamak, Fusion Eng. Des. 46 (1999) 397–404. S.J. Gee, R. Baldwin, A. Borthwick, D. Ciric, G. Crawford, L. Hackett, D.A. Homfray, et al., MAST neutral beam long pulse upgrade, Fusion Eng. Des. 56 74 (2005) 403–407 M.P.S. Nightingale, G.W. Crawford, S.J. Gee, D.J. Hurford, D. Martin, M.R. Simmonds, et al., The MAST neutral beam injection system, Fusion Eng. Des. 56 (2001) 529–532. P. Stevenson, R.E.Baldwin, V.P.Dunkley, A.P.Vadgama, S.E.V.Warder, Design and commissioning of the MAST neutral beam power supplies for the long pulse beam upgrade, 23rd Symposium on Fusion Engineering, 2009. J. H. Feist, et al., Large scale titanium pumps for the ASDEX-upgrade neutral beam injectors, 17th Symposium of Fusion Technology (SOFT) (1992) 262