Preliminary  Experiment  on  Plasma  Wave  Guide  for  PWFA  

Tatsuo  Shoji                                        Nagoya  University,  Japan  

Wtaru  Mizutani  Mitsutoshi  Aramaki  Daiyu    Hayashi                                　Eindhoven  University,  Nederland  

Supported  by  KEK,    K.  Nakajima  and  M.  Yoshida

Tokyo Workshop IZEST 2013.11.20

Demands for plasma channel for 100Gev laser

plasma wake-field accelerator experiment

Discharge plasma with neutral structure + Laser discharge method is developed

・Stable long (~meter scale) plasma channel with narrow diameter (less than mm)

• ne=1015~17cm-3 electron density with hollow radial profile (Δne/ne~0.2)

•  Fully ionized

Impossible for conventional discharge plasmas

New  method  to  make  plasma  wave  guide

ne

ne

n0

r

■ 1st step Efficient low density plasma production by helicon discharge to make hollow neutral density profile

ne=1013-14cm-3

■ 2nd step Laser ionization of neutrals to make fully ionized hollow plasma density profile

Partially ionized plasma

Short pulse laser ne=1015-16cm-3

Subjects  need  to  be  developed  for  helicon  discharge  with  small  diameter  (~  ρion)

•  Develop antennas for helicon waves (m= 1) coupled to small diameter (<1mm) and long (~1m module) plasma tube. ( ion Larmor radiuns ~plasma radius ) Helical antenna could efficiently produced only for >20mm Φ　[1]

• Simple magnetic coils system -> pulse small magnetic coils, permanent magnets array.

• Measure hollow neutral density profile made by helicon discharge.

hollow neutral density was observed in ~50mmΦ　helicon discharge

[1] T.Shoji, et. al., PIasma Sources Sci. Technol. 2 (1993)5-10

Helicon  plasma  as  an  efficient  high  density  rf  discharge  with  external  magne<c  field

■ne(r), R(wall radius) , ω, B0, m -> (1)->kz -> antenna design

m=1　mode  

l

　　　　wave length λ(cm) [1] T.Shoji, et. al., PIasma Sources Sci. Technol. 2 (1993)5-10

ne0

z B0 R

[1]

R=0.2mm

m=1  helicon  antenna  for  small  diameter  plasma

z θ

x

y

y

xz

m=1　mode  

■m=1 helicon antenna

h Brf

Irf

B0 Ez l

Irf

Helicon wave B field

Experimental  set-­‐up

Spectrometer PC

Maching  Box

Φ4mm,  50cm  glass  tube

magneRc  coil

13.56MHz,  5kW  duty  0.03  pluse  width  5ms

antenna

B0  ～1.3kG

RF

CCD camera

probe

Optical filter

HeI(492nm)

（ne）

Electron  density  measurement

x

yy

xz

probe

plasma

Iis(x)

mi: ion mass

Te: electron temperature

nc: electron density

Integrated ion saturation current

Abel Transformation

0.2Φ, 32mm long（W）

Δ (>>λd ) : probe diameter

Δ

ne(r) €

Iis(x) = Δ−y0

y0∫ 0.61e nc (x)CS dy

€

CS = Te /Mi

y0

-y0

Laser  Thomson  scaFering  for  Ne(r),  Te(r)  measurement

Scattered signal

He0  (neutral)  density  measurement  by    Laser  absorp<on

Isolator

FPI

Fabry-Perot

ECDL 707nm PD or CCD

Absorption Spectrum

PD

Wavemeter

PD：<He0*>ex  -­‐>(CR  model)-­‐>  <He0>    grand  state  CCD：He0*  (r)    -­‐>  (CR  model)-­‐>He0  (r)    

Emission  spectrum  of  He  plasma  

HeI(492nm)

492nm HI

388

395 402

446

471 486

501

492

587

667

656 706

727

775

Wave length

Light  emission  measurement

Emission intensity

ne : electron density n0 : neutral density

HeI（492nm）

Emission profile by using band pass filter

σex: excitation cross section

X(mm)

Emission intensity profile in x dirextion

x B0

1kW

2kW

2.5kW

n e [a

.u.]

~5x1013cm-3

Prf

B=1.0kG、p[He]=19mTorr

3cm

13

Plasma  and  neutral  profile

14

1.0kW 2.5kW

Increasing ne

Δn/n0=0.4 Δn/n0=0.65

Rf  power  dependence  on  n0(r)

Δne

2.5　

2.5kW 2kW

1kW

Glass tube radius=2mm

Neutral density profile Dependence of antenna pitch on Δn/n0

l=2cm 5cm

3cm

fast electrons by E acceleration of

l

Dependence  of  antenna  pitch  on  the  depth  of  neutral  profile  on  axsis

ne

16

l

Summary

• Precise neutral density profile measurement by He line ratio and laser absorption method

• Obtain scaling low for the plasma diameter, length for the antenna, density dip ratio, rf power, B0 etc.

• Antenna design for 1m compact plasma wave guide with Permanent magnet.

•  New method of making plasma wave guide has been developed and first experiment to produce small diameter (4mmΦ）plasma by helicon discharge (?) for PWFA has been performed .

• ・Develop m=1 helicon antenna for small plasma

•  Hollow neutral density profile was observed (needs more careful measurement by LIF)

Need to be developed further

Thank you for Attention. Please wait till

we get good plasma.

T.Shoji