High-brightness electron injectors: A review Page: 4 of 7
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Injectors 1'sing Third Harmonic Cavities
<• = 5.0 x 10' 1 .) - mm tnhul. with J m Acm-
The corresponding normalized peak brightness is limned in
B. = 2Lr = r> 2 \ llH.i imps Yin rad1;
lTie oiiTeiU density from a dispenser c.ilhndc is typically
not more than 20 A eni-'. therefore. ilic maximum achievable
brightness is 1.(1 x U);"
■Semiconductor phoineimi t ns have an clfective
temperature of 0.2 oV lief. b*» The electron thermal
temperature is not simply the dillerence between the incident
photon energy and t he semiconductor band gap i a difference
of 0.7 eV because of phonon scattering in ihe semiconductor
crystal lalt ’e. Semiconductor cathodes are capable of
delivering'' o'er 000 A cm- giving a brightness of
2.5 x 10;- Aim rad '■
The brightness of the source normally does not limit the
final brightness of the beam Instead, the acceleration
process and transport through a beamline can decrease the
beam brightness by several orders of magiul tide.
DC Injectors
I'he following information on verv long pulse i >» 1 nsi
and dc injectors is a summnrv of a paixT by W ! lemnnnnsfeldt.
These types of guns are well suited for two applications: first,
for electron cooling of ion beams and, second, for electrostatic
free electmr. lasers FF. 1.
The design of a dc gun must include the effects of space
charge, hi the gun. the space-charge sell-force in „he beam
must bo cancelled out with a carefully designed Incusing
electrode ■ at the Fierce angle"'" . I Ims maintaining a uniform
current density Also, the exit energy oft.be beam from the
gun should he as high as possible to minimi/e further spa tv
charge defocusing downstream Irom the glut. If the beam
maintains a uniform profile up >" relativistic energies, then
the beam emit lance can lie near I be I hernial i emperat lire of
I lie beam as iL was emit I ed Iro in I he cat bode The emit lance
of llie beam caused hv thermal olfecls is discussi d in the
preceding sect ion
A gun designed b\ I lernnannsleldl loi i he I'l 'Sl( FKl,
and described m a paper- hv Ivlin- and liamum is shown in
Kig 2.
m 3Bna3R jjj hi ii i ttti im?mi37/«v ,v
IOUIPPTFNTIAI
FLLCMVON 1HAJCCT0RY
<» n
A*IAL PUyniiH
I 1\ i1 t ■ . • ! • • 1 • i . .! ' • i l ' I . • it i 11 i
Hunches accelerated with a dc field do nut sutler lin-
en ul lance grow t h t bat is due to t rile \ ary mg olfecls l y pi calk
found in rfaeeelerau fs 1 lanmuucs can be u.-ed to eliminate
this source af emit lance growth A design' - I hat corrects fur
the tune varying fields til a radio frei|Uoiicy rfi acre1.‘rater
uses cavities that operate at the t bird harmonic of the mam
hnac frequency Two conditn as must lie met to approximate
a dc accelerating field during pulse transit, first, tlie amplitude
of the third harmonic is set to nine tunes the fundamental
frequency s amplitude Second, the phase of the third harmonic
is chosen to decelerate the bunch at the peak acceleration of
the fundamental. The amplitude is fiat to within 0 TJ over
U7 cf the rf. However, the resulting two-frequency cavity
will have increased phase and amplitude control complexity.
For relativistic beams, the harmonic component may lie
added with separate cavities, consid oahly reducing cavity
design and control complexity Improved accelerator
performance using separated cavities lor the first and third
harmonic has been verified using I'AKMKI A by Todd Smith.1
After initial acceleration to several McV with a long pulse i to
minimize space-charge eifectsi, the peak current is then
increased using magnetic cumpri ion. A schematic of the
design is given m fig. ,'l.
.k)(i kt’V
^IllAIIMoNli'
11 \IIM. 'Nl*
M,MINK lie
\
KI-KiTHON
111 Nil 1 Hit
HnUUJ5_
: ,\r« Iv! I'llAl'
'll | lU'Ni'IIKK
KNKHUY
:uh) krV
100 ki‘V
2 . MeV
1 r. MeV
lll'N'Cll
I.KNCTM
1’KAIx
l.'IM ps
11)0 |1H
1011 jus
' I'S
t'l'HKKNT
:i A
10 A
pi A .’oil A
Kig. !l. Iiiii'i'liir drsigni d Iiv 1‘ Sunih So 11 n> Ni.mluril It gli Riiiu'gv
I’liy-ics l.iilioriiliirv s Sii|i<'minil<irLing An ■•IitiiI nr t 'a In i In leu '‘iiiill.Mii't<
iisiiif,' I'AIIM h I .\ is '.til n non in i in I
I’hoU.injectors
A pliolximjectoc is a phnlnciilissive elect i on source placed
directly in ail rf cavil v. 'The pl\olniii|eclor design depends on
the electron Inmch produced from a pilot ocal bode being
rapidly accelerated to i"Iniivistic eiiergies m a single rf
cat tt v. lienee elimma' mg t he Cm,von! ouial hmuiimg process
entirely The emillauce growth of tin- electron beam
reduced been Use elect run he. ini ! ran spin I at f iw cun pies has
bei ii significantly reduced
I .os Alamos lcx|)ci‘imenl
Kiperiniciit iM-sigll. file I .■.. Alamos expi-rmirlil n-,r.
a III sol' ill I \ I'll pilot III It I lode idol I I oil sol II i r .11 I l.ll ei| on axis
m (III* fil'd rfeaviU ii....., no poise . I ia ........a,lb
I a l loi ed hi I ml 11 lime and pate Ip a ppi up' i a I el v ilia pi i ig I lie
mci deni laser pulse l"ln*-rm i ligi 11 a I uni ol I oe evpin iiioiit i
show n in fig i I'lii1 Imai lias | u ii I llli I M 11 ,■ 11 i a\ il m . w il Ii
mile pin nleiil aioplil isle and pli.i ,e . on I • ol . I Ini 11 11 i i\ il n-,
lull e loop , I o measm i I be plia ,e ai id ampin i nie ol i lie 11 I nid .
pie sent III tile laville, f < i| 11 u\ l n I ■ llie .is 111 n I nil .i'i- llie
dia g I lost 11 s loi I nun 11 ilia i' ;r, I ns i m ei ir i ip ii!ill.in i 11 o I
temporal niolilo I lie dil.nl n! llie il .r il\ , I ■ n n -
pie -el, 11 s I ii ,i ■ W 1 o ■ l i -
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Sheffield, R.L. High-brightness electron injectors: A review, article, January 1, 1989; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1097785/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.