Overview of NDCX-II Physics Design and Comments on final beam-lines for a driver *

* This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Security, LLC, Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, by LBNL under Contract DE-AC02-05CH11231, and by PPPL under Contract DE-AC02-76CH03073.

The Heavy Ion Fusion Science Virtual National Laboratory

Alex FriedmanFusion Energy Program, LLNL

andHeavy Ion Fusion Science Virtual National Laboratory

Workshop on Accelerators for Heavy Ion Inertial Fusion

LBNL, May 23-26, 2011

Overview of NDCX-II Physics Design and

Comments on final beam-lines for a driver*

Slide 2The Heavy Ion Fusion Science

Virtual National Laboratory

Overview of NDCX-II Physics Design

Beam traversing an acceleration gap



The drift compression process is used to shorten an ion bunch

• Induction cells impart a head-to-tail velocity gradient (“tilt”) to the beam

• The beam shortens as it “drifts” down the beam line

• In non-neutral drift compression, the space charge force opposes (“stagnates”) the inward flow, leading to a nearly mono-energetic compressed pulse:

• In neutralized drift compression, the space charge force is eliminated, resulting in a shorter pulse but a larger velocity spread:

vz

z

vz

z

vz

z

vz

z

(in beam frame)



Drift compression is used twice in NDCX-II

Initial non-neutral pre-bunching for:• better use of induction-core Volt-seconds• early use of 70-ns 250-kV Blumlein power supplies from ATA

injectapply

tiltdrift accelerate

apply tilt

neutral-ized drift

target

Final neutralized drift compression onto the target• Electrons in plasma move so as to cancel

the beam’s electric field

• Require nplasma > nbeam for this to work well



The baseline hardware configuration is as presented during the April 2010 DOE Project Review

• 27 lattice periods after the injector• 12 active induction cells• Beam charge ~50 nano-Coulombs• FWHM < 1 ns• Kinetic energy ~ 1.2 MeV



12-cell NDCX-II baseline layout

ATA induction cells with pulsed 2.5 T solenoids

Li+ ion injector

NDCX-II final-focus solenoid and target

chamber

neutralized drift-compression line

with plasma sources

ATA Blumlein voltage sources

oil-filled ATA transmission lines

long-pulse voltage sources





Li+ ion injector


chamber


with plasma sources




12 active cells





Li+ ion injector


chamber


with plasma sources




9 inactive cells





Li+ ion injector


chamber


with plasma sources




6 diagnostic cells



Simulations enabled development of the NDCX-II physics design

• ASP is a purpose-built, fast 1-D (z) particle-in-cellcode to develop acceleration schedules

– 1-D Poisson solver, with radial-geometry correction

– realistic variation of acceleration-gap fields with z

– several optimization options

• Warp is our full-physics beam simulation code

– 1, 2, and 3-D ES and EM field solvers

– first-principles & approximate models of lattice elements

– space-charge-limited and current-limited injection

– cut-cell boundaries for internal conductors in ES solver

– Adaptive Mesh Refinement (AMR)

– large Δt algorithms (implicit electrostatic, large ωcΔt)

– emission, ionization, secondaries, Coulomb collisions...

– parallel processing

A. Friedman, et al., Phys. Plasmas 17, 056704 (2010).



Steps in development of the NDCX-II physics design …

0

10 cm

40g-12

extractor117 kV

emitter130 kV

accel20 kV

second, carry out a time-dependent r-z simulation from the source with Warp

1 mA/cm2 Li+ ion source

z (m)

V (kV)

50

100

00 1

10

0

r (m

)

first, use Warp steady-flow “gun” mode to design the injector for a nearly laminar flow




perveance beam length

beam

length

(m

)

center of mass z position (m)

third, iterate with ASP to find an acceleration schedule that delivers a beam with an acceptable final phase-space distribution

neutral

- maximum- average





200 kV “ramp” measured waveformfrom test stand

“shaped” to equalize beam energy after injection

“shaped” for initial bunch compression (scaled from measured waveforms)

250 kV “flat-top” measured waveformfrom test stand

40g.002-12

fourth, pass the waveforms back to Warp and verify with time-dependent r-z simulation



- time for entire beam to cross a plane at fixed z

* time for a single particle at mean energy to cross finite-length gap

+ time for entire beam to cross finite-length gap


Pulse duration vs. z: the finite length of the gap field folds in

40g.002-12




40g.002-12

fifth, adjust transverse focusing to maintain nearly constant radius


edge

radi

us (c

m)

3

2

1

00 2 4 6 8



Snapshots from a Warp (r,z) simulation

Beam appears long because we plot many particles …

… but current profile shows that it is short

compressing approaching maximum compression

exiting at focus

40g-12



3-D Warp simulation with perfectly aligned solenoids

40ga24-12 simulation and movie from D P Grote




40g-12 with random timing shifts in acceleration voltage pulses

voltage jitter

2-nsnominal

jitter

sixth, test sensitivity to random timing error in acceleration waveforms




40g-12 with random offsets to both ends of each solenoid

solenoid alignment

0.5-mmnominal

misalignment

seventh, test sensitivity to random solenoid misalignments

Beam “steering” via dipole magnets will center beam and minimize “corkscrew” distortion.



Warp runs illustrate effects of solenoid alignment errors

plots show beam deposition for three ensembles of solenoid offsets

maximum offset for each case is 0.5 mm

red circles include half of deposited energy

smaller circles indicate hot spots

x (mm)

y (m

m)

J/cm2

x (mm)

y (m

m)

J/cm2

x (mm)

y (m

m)

J/cm2

ASP and Warp runs show that steering can improve intensity and stabilize spot location

see Y-J Chen, et al., Nucl. Inst. Meth. in Phys. Res. A 292, 455 (1990)



A “zero-dimensional” Python code (essentially, a spreadsheet)captures the essence of the NDCX-II acceleration schedule

• Computes energy jumps of nominal head and tail particles at gaps

• Space-charge-induced energy increments between gaps via a “g-factor” model

0-D ASP

head 923 1100

tail 1082 1300

• The final head and tail energies (keV) are off; the g-factor model does not accurately push the head and tail outward:

• But – not bad, for a main loop of 16 lines.

ASP 0-D



Things we need to measure, and the diagnostics we’ll use

Non-intercepting (in multiple locations):• Accelerating voltages: voltage dividers on cells• Beam transverse position: four-quadrant electrostatic capacitive probes• Beam line charge density: capacitive probes • Beam mean kinetic energy: time-of-flight to capacitive probes

Intercepting (in two special “inter-cell” sections):• Beam current: Faraday cup• Beam emittance: two-slit or slit-scintillator scanner• Beam profile: scintillator-based optical imaging • Beam kinetic energy profile: time-of-flight to Faraday cup• Beam energy distribution: electrostatic energy analyzer

(Underlined items will be available at commissioning)



“Physics risks” concern beam intensity on target, not project completion or risk to the machine due to beam impact

• Alignment errors exceeding nominal 0.5 mm– Machine usable with larger errors with intensity degradation– Beam “steering,” using dipoles in diagnostic cells, can mitigate “corkscrew”

deformation of beam– Off-center beam, if reproducible, is not a significant issue

• Jitter of spark-gap firing times exceeding nominal 2 ns– Slow degradation of performance with jitter expected, per simulations– Similar slow degradation as waveform fidelity decreases

• Source emission non-uniform, or with density less than nominal 1 mA/cm2

– Simulations show a usable beam at 0.5 mA/cm2

– Will run in this mode initially, to maximize source lifetime – Space-charge-limited emission mode offers best uniformity

• Imperfect neutralization because final-focus solenoid not filled with plasma– Build and use a larger-radius solenoid at modest cost to program

–



NDCX-II, when mature, should be far more capable than NDCX-I

NDCX-I (typical bunched beam)

NDCX-II 12-cell (ideal*)

Ion species K+ (A=39) Li+ (A=7)

Total charge 15 nC 50 nC

Ion kinetic energy 0.3 MeV 1.25 MeV

Focal radius (containing 50% of beam) 2 mm 0.6 mm

Bunch duration (FWHM) 2 ns 0.6 ns

Peak current 3 A 38 A

Peak fluence (time integrated) 0.03 J/cm2 8.6 J/cm2

Fluence within a 0.1 mm diameter spot 0.03 J/cm2 (50 ns window)

5.3 J/cm2 (0.57 ns window)

Fluence within 50% focal radius and

FWHM duration (Ekinetic x I x t / area)

0.014 J/cm2 1.0 J/cm2

* NDCX-II estimates of ideal performance are from (r,z) Warp runs (no misalignments), and assume uniform 1 mA/cm2 ion emission, no timing or voltage jitter in acceleration pulses, no jitter in solenoid excitation, and perfect beam neutralization; they also assume no fine energy correction (e.g., tuning the final tilt waveforms)


Virtual National LaboratoryHeavy Ion Fusion Science Virtual National Laboratory

NDCX-II will be a unique user facility for HIF-relevant physics.



Comments on final beam-lines

for a driver



Schematic of final beamlines for ion indirect drive

finalfocus

(only representative beamlines are shown)

axis

fromaccelerator



Schematic of final beamlines for ion direct drive

fromaccelerator

finalfocus

(only representative beamlines are shown)

axis



With a single linac, arcs transport the beams to the two sides of the target (for most target concepts)

• In the final section of the driver, the beams are separated so that they may converge onto the target in an appropriate pattern.

• They also undergo non-neutral drift-compression, and ultimately “stagnate” to nearly-uniform energy, and pass through the final focusing optic.

• In the scenario examined by Dave Judd (1998), the arcs are ~ 600 m long, while the drift distance should be < 240 m.

• Thus, the velocity “tilt” must be imposed in the arcs, or upon exit from the arcs.

• To maintain a quiescent beam, “ear fields” are needed in the arcs.

• For pulse-shaping, the arcs may represent an opportunity to pre-configure the beams before final compression.

or

indirectdrive

directdrive



If a foot pulse of lower K.E. is needed, those beams are “traditionally” extracted from the linac early and routed via shorter arcs

David L. Judd, “A Conceptual Design of Transport Lines for a Heavy-Ion Inertial-Fusion Power Plant” (1998)



Delay between foot and main pulses can be inserted in a nearly linear system

This concept may be useful …• if two linacs are used, one from each side • with a single linac, for a single-sided target• with a single linac, for a two-sided target (see next slide)

accel to intermediate boost foot beams drift footkinetic energy speed at higher speed beams of foot arrive beams first

main pulse beams boost main beams targetdrift at lower speed (delay) to final energy

main

foot

z = 0 z1 z2 z3 z4



z2

z3

A single linac with common arcs could drive a 2-sided target

footmain

z1z = 0

acceleration

drift (with ears, corrections

apply tiltrearrangedrift-compress

z4



Example: for an indirect-drive target requiring two beam energies

Aion = 208.980 amu

Accelgradient = 3.0 MV/m

Int. Vz = 48.046 m/us, beta = 0.1603

Foot Vz = 52.632 m/us, beta = 0.1756

Main Vz = 60.774 m/us, beta = 0.2027

Int. Ek = 2.5 GeV

Foot Ek = 3.0 GeV

Main Ek = 4.0 GeV

t1foot = 3310.884 ns

t1main = 3468.888 ns

t2foot = 10435.840 ns

t2main = 11273.886 ns

t3foot = 19935.780 ns

t3main = 20463.353 ns

t4foot = 23735.757 ns

t4main = 23754.229 ns

delay = 18.473 ns



main

foot

z = 0 z1 z2 z3 z4

z1 = 0.167 km

z2 = 0.542 km

z3 = 1.042 km

z4 = 1.242 km



Example: for an X-target requiring a single beam energy

Aion = 84.910 amu

Accelgradient = 3.0 MV/m

Int. Vz = 165.140 m/us, beta = 0.5509

Foot Vz = 171.883 m/us, beta = 0.5733

Main Vz = 171.883 m/us, beta = 0.5733

Int. Ek = 12.0 GeV

Foot Ek = 13.0 GeV

Main Ek = 13.0 GeV

t1foot = 1978.104 ns

t1main = 2018.490 ns

t2foot = 2559.895 ns

t2main = 2624.038 ns

t3foot = 4499.198 ns

t3main = 4602.142 ns

t4foot = 6244.571 ns

t4main = 6347.515 ns

delay = 102.944 ns



main

foot

z = 0 z1 z2 z3 z4

z1 = 0.333 km

z2 = 0.433 km

z3 = 0.767 km

z4 = 1.067 km



The drift compression process is used to shorten an ion bunch

• Induction cells impart a head-to-tail velocity gradient (“tilt”) to the beam

• The beam shortens as it “drifts” down the beam line

• In non-neutral drift compression, the space charge force opposes (“stagnates”) the inward flow, leading to a nearly mono-energetic compressed pulse:

• In neutralized drift compression, the space charge force is eliminated, resulting in a shorter pulse but a larger velocity spread:

vz

z

vz

z

vz

z

vz

z

(in beam frame)



Experiments on NDCX-II can explore non-neutral compression, bending, and focusing of beams in driver-like geometry

NDCX-II w/ optional new non-neutral drift new final target programmable match line w/ quadrupoles focus induction cell (and dipoles for bend)

fromaccelerator

finalfocus

target

non-neutral drift compression line(magnetic quads & dipoles)In a driver …

On NDCX-II, two configurations to test …



HIF-motivated beam experiments on NDCX-II can study …• How well can space charge

“stagnate” the compression to produce a “mono-energetic”beam at the final focus?

• How well can we pulse-shape a beam during drift compression (vs. the Robust Point Design’s “building blocks”)?

• How well can we compress a beam while bending it?:

– “achromatic” design, so that particles with all energies exit bend similarly

– or, leave some chromatic effect in for radial zooming

– emittance growth due to dispersion in the bend

• Are there any issues with final focus using a set of quadrupole magnets?

Most dimensionless parameters (perveance, “tune depression,” compression ratio, etc.) will be similar to, or more aggressive than, those in a driver.

Initial vz profile Final line-charge profile

J. W-K. Mark, et al., AIP Conf. Proc 152, 227 (1986)



EXTRAS – NDCX-II misc

NDCX-II performance for typical cases in 12-21 cell configurations

NDCX-I(bunched

beam)

NDCX-II

12-cell 15-cell 18-cell 21-cell

Ion species K+ (A=39) Li+ (A=7) Li+ (A=7) Li+ (A=7) Li+ (A=7)

Charge 15 nC 50 nC total25 2xFWHM

50 nC total25 2xFWHM



Ion kinetic energy 0.3 MeV 1.2 MeV 1.7 MeV 2.4 MeV 3.1 MeV

Focal radius (50% of beam) 2 mm 0.6 mm 0.6 mm 0.6 mm 0.7 mm

Duration (bi-parabolic measure = √2 FWHM) 2.8 ns 0.9 ns 0.4 ns 0.3 ns 0.4 ns

Peak current 3 A 36 A 73 A 93 A 86 A

Peak fluence (time integrated) 0.03 J/cm2 13 J/cm2 19 J/cm2 14 J/cm2 22 J/cm2

Fluence w/in 0.1 mm diameter, w/in duration 8 J/cm2 11 J/cm2 10 J/cm2 17 J/cm2

Max. central pressure in Al target 0.07 Mbar 0.18 Mbar 0.17 Mbar 0.23 Mbar

Max. central pressure in Au target 0.18 Mbar 0.48 Mbar 0.48 Mbar 0.64 Mbar

NDCX-II estimates are from (r,z) Warp runs (no misalignments), and assume uniform 1 mA/cm2 emission, high-fidelity acceleration pulses and solenoid excitation, perfect neutralization in the drift line, and an 8-T final-focus solenoid; they also employ no fine energy correction (e.g., tuning the final tilt waveforms)



EXTRAS – ASP code



1-D PIC code ASP (“Acceleration Schedule Program”)

• Follows (z,vz) phase space using a few hundred particles (“slices”)

• Accumulates line charge density (z) on a grid via particle-in-cell• Space-charge field via Poisson equation with finite-radius correction term

Here, α is between 0 (incompressible beam) and ½ (constant radius beam) • Acceleration gaps with longitudinally-extended fringing field

– Idealized waveforms– Circuit models including passive elements in “comp boxes”– Measured waveforms

• Centroid tracking for studying misalignment effects, steering• Optimization loops for waveforms & timings, dipole strengths (steering)• Interactive (Python language with Fortran for intensive parts)



The field model in ASP yields the correct long-wavelength limit

• For hard-edged beam of radius rb in pipe of radius rw , 1-D (radial) Poisson eqn gives:

• The axial electric field within the beam is:

• For a space-charge-dominated beam in a uniform transport line, /rb2 ≈ const.; find:

• For an emittance-dominated beam rb ≈ const.; average over beam cross-section, find:

• The ASP field equation limits to such a “g-factor” model when the k⊥2 term dominates

• In NDCX-II we have a space-charge-dominated beam, but we adjust the solenoid strengths to keep rb more nearly constant;

• In practice we tune α to obtain agreement with Warp results



EXTRAS – Warp code

Adaptive Mesh Refinement

Z

R

beam

quad

Novel e- mover

The HIF program has developed advanced methods to enable efficient simulation of beam and plasma systems

With new electron mover and mesh refinement, run time in an electron cloud problem was reduced from 3 processor-months to 3 processor-days

e- density (cm-3)

Plasma injection in NDCX

Warp simulates beam injector using “cut cell” boundaries



6-MHz oscillations were seen first in simulations; then they were sought and measured at station (c) in experiments.

6-MHz oscillations were seen first in simulations; then they were sought and measured at station (c) in experiments.

The Warp code includes e-cloud & gas models; here, we modeled and tested deliberate e-cloud generation on HCX

WARP-3DT = 4.65s

Beam ions hit end plate

(a) (b) (c)

HCX beam line

Q4Q3Q2Q1

200mA K+ Beam

electronbunching oscillations

(c)

0. 2. time (s) 6.

Simulation Experiment0.

-20.

-40.

I (m

A)



Warp: a parallel framework combining features of plasma (Particle-In-Cell) and accelerator codes

• Geometry: 3D (x,y,z), 2-1/2D (x,y), (x,z) or axisym. (r,z)

• Python and Fortran: “steerable,” input decks are programs

• Field solvers: Electrostatic - FFT, multigrid; implicit; AMR Electromagnetic - Yee, Cole-Kark.; PML; AMR

• Boundaries: “cut-cell” --- no restriction to “Legos”

• Applied fields: magnets, electrodes, acceleration gaps, user-set

• Bends: “warped” coordinates; no “reference orbit”

• Particle movers: Energy- or momentum-conserving; Boris, large time step “drift-Lorentz”, novel relativistic Leapfrog

• Surface/volume physics: secondary e- & photo-e- emission, gas emission/tracking/ionization, time-dependent space-charge-limited emission

• Parallel: MPI (1, 2 and 3D domain decomposition)

Warp 3D EM/PIC on Franklin

32,768 cores

Z (m)

R (

m)



Time and length scales span a wide range

Length scales:

-11-12 -10 -9 -8 -7 -6 -5 -4

electron cyclotron in magnet

pulse

electron drift out of magnet

beam residence

pb

latticeperiod

betatrondepressed betatron

transit thru

fringe fields

log (seconds)

machine length

Time scales:

-5 -4 -3 -2 -1 0 1 2 3

log (meters)

electron gyroradius in magnet D,beam

beam radius

large t=5./c

Standard Boris mover

(fails in this regime)

Problem: Electron gyro timescale

<< other timescales of interest

brute-force integration very slow due to small t

Solution*: Interpolation between full-particle dynamics (“Boris mover”) and drift kinetics (motion along B plus drifts)

correct gyroradius with

New “Drift-Lorentz” mover relaxes the problem of short electron timescales in magnetic field*

Magnetic quadrupole

Sample electron motion in a quad

beam

quad

*R. Cohen et. al., Phys. Plasmas, May 2005

small t=0.25/c

Standard Boris mover

(reference case)

large t=5./c

New interpolated mover

Test: Magnetized two-stream instability



Electrostatic AMR simulation of ion source with the PIC code Warp: speedup x10

Run Grid size Nb particles

Low res. 56x640 ~1M

Medium res. 112x1280 ~4M

High res. 224x2560 ~16M

Low res. + AMR 56x640 ~1M

R (

m)

Z (m) Z (m)Refinement of gradients: emitting area,

beam edge and front.

Z (m)

R (

m)

zoom

0.0 0.1 0.2 0.3 0.40.2

0.4

0.6

0.8

1.0 Low res. Medium res. High res. Low res. + AMR

Em

itta

nce

(mm

.mra

d)

Z(m)



delta-f, continuum Vlasov, EM PIC

electrostatic / magnetoinductive PIC EM PIC rad. hydro

delta-f

“main sequence” tracks beam ions consistently along entire system

instabilities, halo, electrons, ... are studied via coupled detailed models

Approach to end-to-end simulation of a fusion system



51

Warp• Warp is a state-of-the-art 3-D parallel multi-physics code and framework

– modeling of beams in accelerators, plasmas, laser-plasma systems, non-neutral plasma traps, sources, etc.

– unique features: ES/EM solvers, cut-cells, AMR, particles pushers, python interface, etc.

• Contribution to projects– HIFS-VNL (LBNL,LLNL,PPPL): work-horse code; design and support expts.– VENUS ion source (LBNL): modeling of beam transport– LOASIS (LBNL ): modeling of LWFA in a boosted frame– FEL/CSR (LBNL) : modeling of free e- lasers & coherent synch. radiation in boosted frame– Anti H- trap (LBNL/U. Berkeley): simulation of model of anti H- trap– U. Maryland: modeling of UMER sources and beam transport; teaching– Ferroelectric plasma source (Technion, U. Maryland): modeling of source– Fast ignition (LLNL): modeling physics of filamentation – E-cloud for HEP (LHC, SPS, ILC, Cesr-TA, FNAL-MI): see slide on Warp-Posinst– Laser Isotope Separation (LLNL): now defunct– PLIA (CU Hong Kong): modeling of beam transport in pulsed line ion accelerator– Laser driven ions source (TU Darmstadt): modeling of source

• Benchmarking– Heavily benchmarked against various experiments: MBE4, ESQ ion source, HCX,

multibeamlet ion source, UMER, NDCXI, etc.; codes: IGUN, LSP; theory: beam transport and plasma analytic theory



Slides from October 2003

w/ UMER group



The high energy part of a driver consists mostly of accelerating modules (“gaps”)



Multiple beams in driver introduce significant new physics

• Transverse deflections arise from self-fields in accelerating gaps– Can shield transverse electric field via plates-with-holes– But plates allow cavity modes to develop; use wires (?)

– Magnetic forces may be comparable for large Nbeams

• Longitudinal waves obey vwave = 1/2 g1/2p (a0b0)1/2; g = 1/(40)log(rw

2/a0b0)

• They can be driven unstable by module impedance (“resistive wall”)– Convective growth, head-to-tail– Inductive field in multi-beam system

slows space-charge waves on beamsnear center of cluster (destabilizing)

– But spread in wave speed amongbeams is probably stabilizing

– Also stabilize by capacitance, feedforward– May have to avoid g < 0 on any beam

obs

erv

ing

sta

t ion

time in beam frame



Considerations for a scaled multi-beam experiment using electrons

• Goal would be to explore transverse deflections & wave propagation in a regime where magnetic and inductive effects are significant

• UMER is 10 kV ( = 0.2), 100 mA, a ~ 0.5 cm, 32 cm LP, 40 ns = 2.4 m, 36 LP’s total; could go up to = 0.4 w/ upgrade

• Magnetic forces are down from electric forces by 2 but are not shielded by plates-with-holes; so are comparable when N2 ~ 1

• This implies need for ~12 to 25 UMER beams• Waves propagate ~ 1 beam diameter / period; could shorten beam, so to

propagate ~ 1 m would require ~ 3x UMER length• Vacuum of 10-8 needed to avoid poisoning K challenging pumping• Resolving 10 cm wavelength (~ tip?) implies that diagnostics need

~ 2ns time resolution• Crude cost estimate; if UMER was ~ $3M, then cost might be:

$3M x (15 beams) x (multibeam savings 1/3) x (length factor 3) $45M with very large error bars



Further considerations

• Emittance

– Transverse emittance (tune depression) is OK; beams resemble UMER

– In a driver, longitudinal thermal pressure is << space charge force.

In a scaled electron experiment, T|| may be too high (what is it in UMER?); but this may not matter much, since the interesting inductive Ez is of opposite sign to the electrostatic and pressure terms

• Small total current ~1-2 A would not offer beam loading of accel modules; and module impedance effect is unlikely to be “driver-like”

– May be able to use smart electronics to simulate these effects

– Could scale the beams to somewhat larger a and for higher current; but the aspect ratio would become too “fat” well before the kA range

• There are also possible “tech” experiments

– A “real” driver-scale induction module, for measuring impedances

– It might be “tickled” and/or “loaded” by pulsed electron beams (with head, noise); a wire won't work because it shorts the cavity

– The gap could have removable plates-with-holes (or “chicken-wire-with-holes” or “wires-parallel-to-x-axis,” to minimize “rf cavities”)

Documents

Overview of NDCX-II Physics Design and Comments on final beam-lines for a driver *