Power Neutral Beam on RFX-mod: Design and Mission

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Power Neutral Beam on RFX-modDesign and Mission

• M Valisa, M Gobbin, L Grando, L Marrelli, S
  Martini, R Piovan, A Rizzolo, P. Zaccaria and
  the RFX team Consorzio RFX
• Y. Hirano, S. Kiyama, H. Sakakita AIST Tsukuba

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OUTLINE
Outline

• - Mission to study physics of energetic
  particles on RFX
• - Design status
• Issues to be solved to connect the AIST beam on
  to RFX-mod
• Diagnostics to be implemented for the beam
  exploitment

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Motivation

• Advanced scenarios have been reached in Tokamaks
  
• by controlling the current profile. Methods
  include use of
• neutral beams and radio frequency injection
• As in Tokamkas, auxiliary power can be used in
  RFPs to optimize plasma control and performance
  and widen the physics studies.
• MST ( Madison, Wisconsin) is testing
• - Lower Hybrid Berstein waves Neutral Beam
  Injection for current drive and heating
  (25keVx40A)
• TPE-RX (Japan) has developed a 1.2 MW neutral
  beam injector.

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The TPE-RX beam is available
TPE-RX () has been dismantled (Jan 2008)
. Unique opportunity to loan their Neutral
Beam. Beam characteristics Positive Ion
source (filament type) Energy 25 keV Ion
Current 50 A Pulse Duration 30 ms
() AIST Tsukuba Japan
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The TPE-RX beam
Strong focussing 4 cm of minimum waist Suitable
to fit the existing RFX-mod ports( 100 - 150 mm)
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The TPE RX Neutral Beam Injector
Source
Expansion chamber
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The TPE RX Neutral Beam Injector
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The power supply
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Scientific objectives

• Plasma heating
• Momentum injection
• Current drive
• Control of MHD dynamo and tearing
• activity.
• Fast particle confinement, beta and
  instabilities. ( Alfven mode
• excitation by fast ions. In particular, mediation
  of energy
• transfer to thermal ions)
• Core particle source

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On RFX only radial injection possible
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Consequences of the radial injection

• The initial pitch of the particle is zero (but
  for the beam divergence) the initial velocity is
  mainly perpendicular to the field ( apart from
  the large divergence term).
• Particles ionized on the Low Field side are
  deeply trapped.
• Particles ionized beyond the axis are passing.
• Current and momentum drive is low
• Detailed simulation of the experiment to be done
  shortly
• evaluate best position for diagnostics

Z, cm
R, cm
(ORBIT simulation by M. Gobbin)
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Beam attenuation
Because of the 25 keV, shine-through issues are
not severe for the RFX densities
(ADAS data / flat density profile)
In RFX a(minor radius) 0.459 m
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Heating

• 1 MW of injected power in standard discharges is
  marginal to be sizeable on top of the typical
  RFX input power gt 10 MW.
• Slowing down time is 40-60 ms, against a
  confinement time of approx 3 ms, so that heat
  release is diluted in time. Heating efficiency
  will be low
• Perhaps some effect during enhanced confinement
  regimes (QSH, PPCD, OPCD..) with reuced input
  power .

From P. Innocente et al RFP workshop 2008,
Stockholm
2 MA
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Fast Ion Physics
Fast Ion Physics
Relevance Sufficient confinement of fast ions
(MeV) is a mandatory requisite of burning
plasmas. Collective modes excited by fast ions
can enhance energy and momentum losses and cause
wall damage Long time-scale non linear
processes are expected from interaction of
collective modes and fast ion dynamics as well
as between drift waves and turbulent transport
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Fast Ion Physics
- In MST 25 keV particles have exp. confinement
times ( 30 ms)! Fiksel G. et al 2005 Phys. Rev.
Lett. 95 125001
MST
gtNeutron emission in MST after beam short pulse
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Fast Ion Physics
Estimated Slowing down time in RFX-mod
40-60 ms (Simulations in progress including main
losses mechanisms, i.e. mg perturbations,
ripple, CX etc.) Gobbin et al Nucl. Fusion 48
(2008) 075002
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Installation on RFX not trivial

• Design and construction of a new mechanical
  interface
• (expansion chamber and stand).
• Is a Residual Ion Dump necessary?
• Design of a new magnetic shield for the source
  and the neutralizer
• (higher stray fields on RFX than on TPE)
• Design of the power supply layout and connection
  to the beam
• Procurement and installation of ancillary
  equipment (cooling, gas valve PS etc)
• Integration in the RFX timing sequence
• Personnel training (Collaboration from TPE RX
  expected )
• Development of kinetic models for simulation and
  interpretation
• (Colalboration with MST would be helpful)
• Development of diagnostics

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Screening mg fields

• Max filed tolerated in
• ion source 0.5mT
• neutralizer 1mT

Source
neutralizer
ongoing work
Filaments are ON 30 sec before pulse Therefore
must stand the field generated by the
magnetizing windings (60 mT) during charging
time( few sec.s) before the RFX-mod
discharge Screen must not perturbe the RFX field
.
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NBI diagnostics
Diagnostics

• An NPA diagnostic presently being procured for
  general Ti measurements on RFX-mod will also be
  used to study fast ions produced by the power NBI
  
• Development/ procurement of a Fast Ion Losses
  Diagnostic (FILD) to directly study the losses
  due to instabilities.
• Neutron diagnostic only if D operation will be
  permitted
• CERS will be expanded to the power beam.

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NPA

• Currently discussing about the use of the old
  NPA of RFX ( originally built for JET in the
  early 80s) and the loan of a ACORD model
  (IOFFE Institute) from Greiswald (IPP
  COLLABORATION) in view of (possibly) the
  procurement of a Compact NPA ( IOFFE)

IOFFE INSTITUTE http//www.ioffe.rssi.ru/ACPL/npd
/npahtml/00/00.htm
Old JET NPA efficiency vs neutr. pressure ( high
top and low bottom energies)
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Fast Ion Losses Diagnostic

• FILD allows to directly study the losses due to
  instabilities.
• Scheme as in JET and ADEX-U magnetic
  spectrometer disperses fast ions onto a
  scintillator, the strike point depending on
  gyroradius and pitch angle.
• Detector head 3-dimensional ion collimator,
  stainless steel plate coated with scintillator
  powder and a graphite cap.
• Collimator geometry to be optimized by simulating
  typical particle trajectories of interest.

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Alternatively Faraday cup collector

• Alternatively FILD realized with Faraday foil
  collectors
• Charged particles penetrate a stack of conducting
  foils, separated by insulating foils.
• Number of foils passed before being stopped and
  generating a current signal depends on their
  energy .

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Timeline
Timeline (Temptative

• Delivery Autumn 2008
• Installation Winter 2008 /Spring 2009
• (
  various steps during RFX ordinary
  maintainance
  periods)
• Commissioning/connection to RFX Summer/Autumn
  2009
• Installation of related diagnostics
  Autumn/Winter 2009
• Operation Winter 2009 / early 2010